summaryrefslogtreecommitdiff
path: root/src/mongo/s/d_migrate.cpp
blob: 6054a8cc32fb7f410d20a801a6605b7f03c7cb1e (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
2863
2864
2865
2866
2867
2868
2869
2870
2871
2872
2873
2874
2875
2876
2877
2878
2879
2880
2881
2882
2883
2884
2885
2886
2887
2888
2889
2890
2891
2892
2893
2894
2895
2896
2897
2898
2899
2900
2901
2902
/**
 *    Copyright (C) 2008-2015 MongoDB Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    GNU Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the GNU Affero General Public License in all respects
 *    for all of the code used other than as permitted herein. If you modify
 *    file(s) with this exception, you may extend this exception to your
 *    version of the file(s), but you are not obligated to do so. If you do not
 *    wish to do so, delete this exception statement from your version. If you
 *    delete this exception statement from all source files in the program,
 *    then also delete it in the license file.
 */

#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kSharding

#include "mongo/platform/basic.h"

#include <algorithm>
#include <chrono>
#include <condition_variable>
#include <map>
#include <mutex>
#include <string>
#include <thread>
#include <vector>

#include "mongo/client/connpool.h"
#include "mongo/db/auth/action_set.h"
#include "mongo/db/auth/action_type.h"
#include "mongo/db/auth/authorization_manager.h"
#include "mongo/db/auth/authorization_manager_global.h"
#include "mongo/db/auth/authorization_session.h"
#include "mongo/db/auth/privilege.h"
#include "mongo/db/catalog/document_validation.h"
#include "mongo/db/catalog/index_create.h"
#include "mongo/db/clientcursor.h"
#include "mongo/db/commands.h"
#include "mongo/db/concurrency/lock_state.h"
#include "mongo/db/db_raii.h"
#include "mongo/db/dbhelpers.h"
#include "mongo/db/exec/plan_stage.h"
#include "mongo/db/field_parser.h"
#include "mongo/db/service_context.h"
#include "mongo/db/hasher.h"
#include "mongo/db/jsobj.h"
#include "mongo/db/namespace_string.h"
#include "mongo/db/op_observer.h"
#include "mongo/db/operation_context_impl.h"
#include "mongo/db/ops/delete.h"
#include "mongo/db/query/internal_plans.h"
#include "mongo/db/query/query_knobs.h"
#include "mongo/db/range_deleter_service.h"
#include "mongo/db/repl/repl_client_info.h"
#include "mongo/db/repl/replication_coordinator_global.h"
#include "mongo/db/storage/mmap_v1/dur.h"
#include "mongo/db/write_concern.h"
#include "mongo/logger/ramlog.h"
#include "mongo/s/catalog/catalog_manager.h"
#include "mongo/s/catalog/type_chunk.h"
#include "mongo/s/chunk.h"
#include "mongo/s/chunk_version.h"
#include "mongo/s/config.h"
#include "mongo/s/d_state.h"
#include "mongo/s/catalog/dist_lock_manager.h"
#include "mongo/s/client/shard.h"
#include "mongo/s/client/shard_registry.h"
#include "mongo/s/grid.h"
#include "mongo/s/shard_key_pattern.h"
#include "mongo/stdx/memory.h"
#include "mongo/util/assert_util.h"
#include "mongo/util/elapsed_tracker.h"
#include "mongo/util/exit.h"
#include "mongo/util/fail_point_service.h"
#include "mongo/util/log.h"
#include "mongo/util/processinfo.h"
#include "mongo/util/startup_test.h"

// Pause while a fail point is enabled.
#define MONGO_FP_PAUSE_WHILE(symbol)   \
    while (MONGO_FAIL_POINT(symbol)) { \
        sleepmillis(100);              \
    }

namespace mongo {

using namespace std::chrono;
using std::list;
using std::set;
using std::string;
using std::unique_ptr;
using std::vector;

namespace {

const int kDefaultWTimeoutMs = 60 * 1000;
const WriteConcernOptions DefaultWriteConcern(2, WriteConcernOptions::NONE, kDefaultWTimeoutMs);

/**
 * Returns the default write concern for migration cleanup (at donor shard) and
 * cloning documents (at recipient shard).
 */
WriteConcernOptions getDefaultWriteConcern() {
    repl::ReplicationCoordinator* replCoordinator = repl::getGlobalReplicationCoordinator();
    if (replCoordinator->getReplicationMode() == mongo::repl::ReplicationCoordinator::modeReplSet) {
        Status status = replCoordinator->checkIfWriteConcernCanBeSatisfied(DefaultWriteConcern);
        if (status.isOK()) {
            return DefaultWriteConcern;
        }
    }

    return WriteConcernOptions(1, WriteConcernOptions::NONE, 0);
}

}  // namespace

MONGO_FP_DECLARE(failMigrationCommit);
MONGO_FP_DECLARE(failMigrationConfigWritePrepare);
MONGO_FP_DECLARE(failMigrationApplyOps);

Tee* migrateLog = RamLog::get("migrate");

class MoveTimingHelper {
public:
    MoveTimingHelper(OperationContext* txn,
                     const string& where,
                     const string& ns,
                     BSONObj min,
                     BSONObj max,
                     int total,
                     string* cmdErrmsg,
                     string toShard,
                     string fromShard)
        : _txn(txn),
          _where(where),
          _ns(ns),
          _to(toShard),
          _from(fromShard),
          _next(0),
          _total(total),
          _cmdErrmsg(cmdErrmsg) {
        _b.append("min", min);
        _b.append("max", max);
    }

    ~MoveTimingHelper() {
        // even if logChange doesn't throw, bson does
        // sigh
        try {
            if (!_to.empty()) {
                _b.append("to", _to);
            }
            if (!_from.empty()) {
                _b.append("from", _from);
            }
            if (_next != _total) {
                _b.append("note", "aborted");
            } else {
                _b.append("note", "success");
            }
            if (!_cmdErrmsg->empty()) {
                _b.append("errmsg", *_cmdErrmsg);
            }

            grid.catalogManager()->logChange(_txn->getClient()->clientAddress(true),
                                             (string) "moveChunk." + _where,
                                             _ns,
                                             _b.obj());
        } catch (const std::exception& e) {
            warning() << "couldn't record timing for moveChunk '" << _where << "': " << e.what()
                      << migrateLog;
        }
    }

    void done(int step) {
        invariant(step == ++_next);
        invariant(step <= _total);

        const string s = str::stream() << "step " << step << " of " << _total;

        CurOp* op = CurOp::get(_txn);
        {
            std::lock_guard<Client> lk(*_txn->getClient());
            op->setMessage_inlock(s.c_str());
        }

        _b.appendNumber(s, _t.millis());
        _t.reset();
    }

private:
    OperationContext* const _txn;
    Timer _t;

    string _where;
    string _ns;
    string _to;
    string _from;

    int _next;
    int _total;  // expected # of steps

    const string* _cmdErrmsg;

    BSONObjBuilder _b;
};

class ChunkCommandHelper : public Command {
public:
    ChunkCommandHelper(const char* name) : Command(name) {}

    virtual void help(std::stringstream& help) const {
        help << "internal - should not be called directly";
    }
    virtual bool slaveOk() const {
        return false;
    }
    virtual bool adminOnly() const {
        return true;
    }
    virtual bool isWriteCommandForConfigServer() const {
        return false;
    }
};

bool isInRange(const BSONObj& obj,
               const BSONObj& min,
               const BSONObj& max,
               const BSONObj& shardKeyPattern) {
    ShardKeyPattern shardKey(shardKeyPattern);
    BSONObj k = shardKey.extractShardKeyFromDoc(obj);
    return k.woCompare(min) >= 0 && k.woCompare(max) < 0;
}

class MigrateFromStatus {
public:
    MigrateFromStatus() : _inCriticalSection(false), _memoryUsed(0), _active(false) {}

    /**
     * @return false if cannot start. One of the reason for not being able to
     *     start is there is already an existing migration in progress.
     */
    bool start(OperationContext* txn,
               const std::string& ns,
               const BSONObj& min,
               const BSONObj& max,
               const BSONObj& shardKeyPattern) {
        verify(!min.isEmpty());
        verify(!max.isEmpty());
        verify(!ns.empty());

        // Get global shared to synchronize with logOp. Also see comments in the class
        // members declaration for more details.
        Lock::GlobalRead globalShared(txn->lockState());
        std::lock_guard<std::mutex> lk(_mutex);

        if (_active) {
            return false;
        }

        _ns = ns;
        _min = min;
        _max = max;
        _shardKeyPattern = shardKeyPattern;

        verify(_deleted.size() == 0);
        verify(_reload.size() == 0);
        verify(_memoryUsed == 0);

        _active = true;

        std::lock_guard<std::mutex> tLock(_cloneLocsMutex);
        verify(_cloneLocs.size() == 0);

        return true;
    }

    void done(OperationContext* txn) {
        log() << "MigrateFromStatus::done About to acquire global lock to exit critical "
                 "section";

        // Get global shared to synchronize with logOp. Also see comments in the class
        // members declaration for more details.
        Lock::GlobalRead globalShared(txn->lockState());
        std::lock_guard<std::mutex> lk(_mutex);

        _active = false;
        _deleteNotifyExec.reset(NULL);
        _inCriticalSection = false;
        _inCriticalSectionCV.notify_all();

        _deleted.clear();
        _reload.clear();
        _memoryUsed = 0;

        std::lock_guard<std::mutex> cloneLock(_cloneLocsMutex);
        _cloneLocs.clear();
    }

    void logOp(OperationContext* txn,
               const char* opstr,
               const char* ns,
               const BSONObj& obj,
               BSONObj* patt,
               bool notInActiveChunk) {
        ensureShardVersionOKOrThrow(txn->getClient(), ns);

        const char op = opstr[0];

        if (notInActiveChunk) {
            // Ignore writes that came from the migration process like cleanup so they
            // won't be transferred to the recipient shard. Also ignore ops from
            // _migrateClone and _transferMods since it is impossible to move a chunk
            // to self.
            return;
        }

        dassert(txn->lockState()->isWriteLocked());  // Must have Global IX.

        if (!_active)
            return;

        if (_ns != ns)
            return;

        // no need to log if this is not an insertion, an update, or an actual deletion
        // note: opstr 'db' isn't a deletion but a mention that a database exists
        // (for replication machinery mostly).
        if (op == 'n' || op == 'c' || (op == 'd' && opstr[1] == 'b'))
            return;

        BSONElement ide;
        if (patt)
            ide = patt->getField("_id");
        else
            ide = obj["_id"];

        if (ide.eoo()) {
            warning() << "logOpForSharding got mod with no _id, ignoring  obj: " << obj
                      << migrateLog;
            return;
        }

        if (op == 'i' && (!isInRange(obj, _min, _max, _shardKeyPattern))) {
            return;
        }

        BSONObj idObj(ide.wrap());

        if (op == 'u') {
            BSONObj fullDoc;
            OldClientContext ctx(txn, _ns, false);
            if (!Helpers::findById(txn, ctx.db(), _ns.c_str(), idObj, fullDoc)) {
                warning() << "logOpForSharding couldn't find: " << idObj
                          << " even though should have" << migrateLog;
                dassert(false);  // TODO: Abort the migration.
                return;
            }

            if (!isInRange(fullDoc, _min, _max, _shardKeyPattern)) {
                return;
            }
        }

        // Note: can't check if delete is in active chunk since the document is gone!

        txn->recoveryUnit()->registerChange(new LogOpForShardingHandler(this, idObj, op));
    }

    /**
     * Insert items from docIdList to a new array with the given fieldName in the given
     * builder. If explode is true, the inserted object will be the full version of the
     * document. Note that the whenever an item from the docList is inserted to the array,
     * it will also be removed from docList.
     *
     * Should be holding the collection lock for ns if explode is true.
     */
    void xfer(OperationContext* txn,
              const string& ns,
              Database* db,
              list<BSONObj>* docIdList,
              BSONObjBuilder& builder,
              const char* fieldName,
              long long& size,
              bool explode) {
        const long long maxSize = 1024 * 1024;

        if (docIdList->size() == 0 || size > maxSize)
            return;

        BSONArrayBuilder arr(builder.subarrayStart(fieldName));

        list<BSONObj>::iterator docIdIter = docIdList->begin();
        while (docIdIter != docIdList->end() && size < maxSize) {
            BSONObj idDoc = *docIdIter;
            if (explode) {
                BSONObj fullDoc;
                if (Helpers::findById(txn, db, ns.c_str(), idDoc, fullDoc)) {
                    arr.append(fullDoc);
                    size += fullDoc.objsize();
                }
            } else {
                arr.append(idDoc);
                size += idDoc.objsize();
            }

            docIdIter = docIdList->erase(docIdIter);
        }

        arr.done();
    }

    /**
     * called from the dest of a migrate
     * transfers mods from src to dest
     */
    bool transferMods(OperationContext* txn, string& errmsg, BSONObjBuilder& b) {
        long long size = 0;

        {
            AutoGetCollectionForRead ctx(txn, getNS());

            std::lock_guard<std::mutex> sl(_mutex);
            if (!_active) {
                errmsg = "no active migration!";
                return false;
            }

            // TODO: fix SERVER-16540 race
            xfer(txn, _ns, ctx.getDb(), &_deleted, b, "deleted", size, false);
            xfer(txn, _ns, ctx.getDb(), &_reload, b, "reload", size, true);
        }

        b.append("size", size);

        return true;
    }

    /**
     * Get the disklocs that belong to the chunk migrated and sort them in _cloneLocs
     * (to avoid seeking disk later).
     *
     * @param maxChunkSize number of bytes beyond which a chunk's base data (no indices)
     *                     is considered too large to move.
     * @param errmsg filled with textual description of error if this call return false.
     * @return false if approximate chunk size is too big to move or true otherwise.
     */
    bool storeCurrentLocs(OperationContext* txn,
                          long long maxChunkSize,
                          string& errmsg,
                          BSONObjBuilder& result) {
        AutoGetCollectionForRead ctx(txn, getNS());
        Collection* collection = ctx.getCollection();
        if (!collection) {
            errmsg = "ns not found, should be impossible";
            return false;
        }

        // Allow multiKey based on the invariant that shard keys must be single-valued.
        // Therefore, any multi-key index prefixed by shard key cannot be multikey over
        // the shard key fields.
        IndexDescriptor* idx =
            collection->getIndexCatalog()->findShardKeyPrefixedIndex(txn,
                                                                     _shardKeyPattern,
                                                                     false);  // requireSingleKey

        if (idx == NULL) {
            errmsg = str::stream() << "can't find index with prefix " << _shardKeyPattern
                                   << " in storeCurrentLocs for " << _ns;
            return false;
        }

        // Assume both min and max non-empty, append MinKey's to make them fit chosen index
        BSONObj min;
        BSONObj max;
        KeyPattern kp(idx->keyPattern());

        {
            // It's alright not to lock _mutex all the way through based on the assumption
            // that this is only called by the main thread that drives the migration and
            // only it can start and stop the current migration.
            std::lock_guard<std::mutex> sl(_mutex);

            invariant(_deleteNotifyExec.get() == NULL);
            unique_ptr<WorkingSet> ws = stdx::make_unique<WorkingSet>();
            unique_ptr<DeleteNotificationStage> dns = stdx::make_unique<DeleteNotificationStage>();
            // Takes ownership of 'ws' and 'dns'.
            auto statusWithPlanExecutor = PlanExecutor::make(
                txn, std::move(ws), std::move(dns), collection, PlanExecutor::YIELD_MANUAL);
            invariant(statusWithPlanExecutor.isOK());
            _deleteNotifyExec = std::move(statusWithPlanExecutor.getValue());
            _deleteNotifyExec->registerExec();

            min = Helpers::toKeyFormat(kp.extendRangeBound(_min, false));
            max = Helpers::toKeyFormat(kp.extendRangeBound(_max, false));
        }

        unique_ptr<PlanExecutor> exec(
            InternalPlanner::indexScan(txn, collection, idx, min, max, false));
        // We can afford to yield here because any change to the base data that we might
        // miss is already being queued and will migrate in the 'transferMods' stage.
        exec->setYieldPolicy(PlanExecutor::YIELD_AUTO);

        // use the average object size to estimate how many objects a full chunk would carry
        // do that while traversing the chunk's range using the sharding index, below
        // there's a fair amount of slack before we determine a chunk is too large because object
        // sizes will vary
        unsigned long long maxRecsWhenFull;
        long long avgRecSize;
        const long long totalRecs = collection->numRecords(txn);
        if (totalRecs > 0) {
            avgRecSize = collection->dataSize(txn) / totalRecs;
            maxRecsWhenFull = maxChunkSize / avgRecSize;
            maxRecsWhenFull = std::min((unsigned long long)(Chunk::MaxObjectPerChunk + 1),
                                       130 * maxRecsWhenFull / 100 /* slack */);
        } else {
            avgRecSize = 0;
            maxRecsWhenFull = Chunk::MaxObjectPerChunk + 1;
        }

        // do a full traversal of the chunk and don't stop even if we think it is a large chunk
        // we want the number of records to better report, in that case
        bool isLargeChunk = false;
        unsigned long long recCount = 0;
        ;
        RecordId dl;
        while (PlanExecutor::ADVANCED == exec->getNext(NULL, &dl)) {
            if (!isLargeChunk) {
                std::lock_guard<std::mutex> lk(_cloneLocsMutex);
                _cloneLocs.insert(dl);
            }

            if (++recCount > maxRecsWhenFull) {
                isLargeChunk = true;
                // continue on despite knowing that it will fail,
                // just to get the correct value for recCount.
            }
        }
        exec.reset();

        if (isLargeChunk) {
            std::lock_guard<std::mutex> sl(_mutex);
            warning() << "cannot move chunk: the maximum number of documents for a chunk is "
                      << maxRecsWhenFull << " , the maximum chunk size is " << maxChunkSize
                      << " , average document size is " << avgRecSize << ". Found " << recCount
                      << " documents in chunk "
                      << " ns: " << _ns << " " << _min << " -> " << _max << migrateLog;

            result.appendBool("chunkTooBig", true);
            result.appendNumber("estimatedChunkSize", (long long)(recCount * avgRecSize));
            errmsg = "chunk too big to move";
            return false;
        }

        log() << "moveChunk number of documents: " << cloneLocsRemaining() << migrateLog;

        txn->recoveryUnit()->abandonSnapshot();
        return true;
    }

    bool clone(OperationContext* txn, string& errmsg, BSONObjBuilder& result) {
        ElapsedTracker tracker(internalQueryExecYieldIterations, internalQueryExecYieldPeriodMS);

        int allocSize = 0;
        {
            AutoGetCollectionForRead ctx(txn, getNS());

            std::lock_guard<std::mutex> sl(_mutex);
            if (!_active) {
                errmsg = "not active";
                return false;
            }

            Collection* collection = ctx.getCollection();
            if (!collection) {
                errmsg = str::stream() << "collection " << _ns << " does not exist";
                return false;
            }

            allocSize = std::min(
                BSONObjMaxUserSize,
                static_cast<int>((12 + collection->averageObjectSize(txn)) * cloneLocsRemaining()));
        }

        bool isBufferFilled = false;
        BSONArrayBuilder clonedDocsArrayBuilder(allocSize);
        while (!isBufferFilled) {
            AutoGetCollectionForRead ctx(txn, getNS());

            std::lock_guard<std::mutex> sl(_mutex);
            if (!_active) {
                errmsg = "not active";
                return false;
            }

            // TODO: fix SERVER-16540 race

            Collection* collection = ctx.getCollection();

            if (!collection) {
                errmsg = str::stream() << "collection " << _ns << " does not exist";
                return false;
            }

            std::lock_guard<std::mutex> lk(_cloneLocsMutex);
            set<RecordId>::iterator cloneLocsIter = _cloneLocs.begin();
            for (; cloneLocsIter != _cloneLocs.end(); ++cloneLocsIter) {
                if (tracker.intervalHasElapsed())  // should I yield?
                    break;

                RecordId dl = *cloneLocsIter;
                Snapshotted<BSONObj> doc;
                if (!collection->findDoc(txn, dl, &doc)) {
                    // doc was deleted
                    continue;
                }

                // Use the builder size instead of accumulating 'doc's size so that we take
                // into consideration the overhead of BSONArray indices, and *always*
                // append one doc.
                if (clonedDocsArrayBuilder.arrSize() != 0 &&
                    (clonedDocsArrayBuilder.len() + doc.value().objsize() + 1024) >
                        BSONObjMaxUserSize) {
                    isBufferFilled = true;  // break out of outer while loop
                    break;
                }

                clonedDocsArrayBuilder.append(doc.value());
            }

            _cloneLocs.erase(_cloneLocs.begin(), cloneLocsIter);

            // Note: must be holding _cloneLocsMutex, don't move this inside while condition!
            if (_cloneLocs.empty()) {
                break;
            }
        }

        result.appendArray("objects", clonedDocsArrayBuilder.arr());
        return true;
    }

    void aboutToDelete(const RecordId& dl) {
        // Even though above we call findDoc to check for existance
        // that check only works for non-mmapv1 engines, and this is needed
        // for mmapv1.

        std::lock_guard<std::mutex> lk(_cloneLocsMutex);
        _cloneLocs.erase(dl);
    }

    std::size_t cloneLocsRemaining() {
        std::lock_guard<std::mutex> lk(_cloneLocsMutex);
        return _cloneLocs.size();
    }

    long long mbUsed() const {
        std::lock_guard<std::mutex> lk(_mutex);
        return _memoryUsed / (1024 * 1024);
    }

    bool getInCriticalSection() const {
        std::lock_guard<std::mutex> lk(_mutex);
        return _inCriticalSection;
    }

    void setInCriticalSection(bool b) {
        std::lock_guard<std::mutex> lk(_mutex);
        _inCriticalSection = b;
        _inCriticalSectionCV.notify_all();
    }

    std::string getNS() const {
        std::lock_guard<std::mutex> sl(_mutex);
        return _ns;
    }

    /**
     * @return true if we are NOT in the critical section
     */
    bool waitTillNotInCriticalSection(int maxSecondsToWait) {
        const auto deadline = system_clock::now() + seconds(maxSecondsToWait);
        std::unique_lock<std::mutex> lk(_mutex);
        while (_inCriticalSection) {
            if (std::cv_status::timeout == _inCriticalSectionCV.wait_until(lk, deadline))
                return false;
        }

        return true;
    }

    bool isActive() const {
        return _getActive();
    }

private:
    bool _getActive() const {
        std::lock_guard<std::mutex> lk(_mutex);
        return _active;
    }
    void _setActive(bool b) {
        std::lock_guard<std::mutex> lk(_mutex);
        _active = b;
    }

    /**
     * Used to commit work for LogOpForSharding. Used to keep track of changes in documents
     * that are part of a chunk being migrated.
     */
    class LogOpForShardingHandler : public RecoveryUnit::Change {
    public:
        /**
         * Invariant: idObj should belong to a document that is part of the active chunk
         * being migrated.
         */
        LogOpForShardingHandler(MigrateFromStatus* migrateFromStatus,
                                const BSONObj& idObj,
                                const char op)
            : _migrateFromStatus(migrateFromStatus), _idObj(idObj.getOwned()), _op(op) {}

        virtual void commit() {
            switch (_op) {
                case 'd': {
                    std::lock_guard<std::mutex> sl(_migrateFromStatus->_mutex);
                    _migrateFromStatus->_deleted.push_back(_idObj);
                    _migrateFromStatus->_memoryUsed += _idObj.firstElement().size() + 5;
                    break;
                }

                case 'i':
                case 'u': {
                    std::lock_guard<std::mutex> sl(_migrateFromStatus->_mutex);
                    _migrateFromStatus->_reload.push_back(_idObj);
                    _migrateFromStatus->_memoryUsed += _idObj.firstElement().size() + 5;
                    break;
                }

                default:
                    invariant(false);
            }
        }

        virtual void rollback() {}

    private:
        MigrateFromStatus* _migrateFromStatus;
        const BSONObj _idObj;
        const char _op;
    };

    /**
     * Used to receive invalidation notifications.
     *
     * XXX: move to the exec/ directory.
     */
    class DeleteNotificationStage : public PlanStage {
    public:
        virtual void invalidate(OperationContext* txn, const RecordId& dl, InvalidationType type);

        virtual StageState work(WorkingSetID* out) {
            invariant(false);
        }
        virtual bool isEOF() {
            invariant(false);
            return false;
        }
        virtual void kill() {}
        virtual void saveState() {
            invariant(false);
        }
        virtual void restoreState(OperationContext* opCtx) {
            invariant(false);
        }
        virtual unique_ptr<PlanStageStats> getStats() {
            invariant(false);
            return NULL;
        }
        virtual CommonStats* getCommonStats() const {
            invariant(false);
            return NULL;
        }
        virtual SpecificStats* getSpecificStats() const {
            invariant(false);
            return NULL;
        }
        virtual std::vector<PlanStage*> getChildren() const {
            vector<PlanStage*> empty;
            return empty;
        }
        virtual StageType stageType() const {
            return STAGE_NOTIFY_DELETE;
        }
    };

    //
    // All member variables are labeled with one of the following codes indicating the
    // synchronization rules for accessing them.
    //
    // (M)  Must hold _mutex for access.
    // (MG) For reads, _mutex *OR* Global IX Lock must be held.
    //      For writes, the _mutex *AND* (Global Shared or Exclusive Lock) must be held.
    // (C)  Must hold _cloneLocsMutex for access.
    //
    // Locking order:
    //
    // Global Lock -> _mutex -> _cloneLocsMutex

    mutable std::mutex _mutex;

    std::condition_variable _inCriticalSectionCV;  // (M)

    // Is migration currently in critical section. This can be used to block new writes.
    bool _inCriticalSection;  // (M)

    unique_ptr<PlanExecutor> _deleteNotifyExec;  // (M)

    // List of _id of documents that were modified that must be re-cloned.
    list<BSONObj> _reload;  // (M)

    // List of _id of documents that were deleted during clone that should be deleted later.
    list<BSONObj> _deleted;  // (M)

    // bytes in _reload + _deleted
    long long _memoryUsed;  // (M)

    // If a migration is currently active.
    bool _active;  // (MG)

    string _ns;                // (MG)
    BSONObj _min;              // (MG)
    BSONObj _max;              // (MG)
    BSONObj _shardKeyPattern;  // (MG)

    mutable std::mutex _cloneLocsMutex;

    // List of record id that needs to be transferred from here to the other side.
    set<RecordId> _cloneLocs;  // (C)

} migrateFromStatus;

void MigrateFromStatus::DeleteNotificationStage::invalidate(OperationContext* txn,
                                                            const RecordId& dl,
                                                            InvalidationType type) {
    if (type == INVALIDATION_DELETION) {
        migrateFromStatus.aboutToDelete(dl);
    }
}

struct MigrateStatusHolder {
    MigrateStatusHolder(OperationContext* txn,
                        const std::string& ns,
                        const BSONObj& min,
                        const BSONObj& max,
                        const BSONObj& shardKeyPattern)
        : _txn(txn) {
        _isAnotherMigrationActive = !migrateFromStatus.start(txn, ns, min, max, shardKeyPattern);
    }
    ~MigrateStatusHolder() {
        if (!_isAnotherMigrationActive) {
            migrateFromStatus.done(_txn);
        }
    }

    bool isAnotherMigrationActive() const {
        return _isAnotherMigrationActive;
    }

private:
    OperationContext* _txn;
    bool _isAnotherMigrationActive;
};

void logOpForSharding(OperationContext* txn,
                      const char* opstr,
                      const char* ns,
                      const BSONObj& obj,
                      BSONObj* patt,
                      bool notInActiveChunk) {
    migrateFromStatus.logOp(txn, opstr, ns, obj, patt, notInActiveChunk);
}

class TransferModsCommand : public ChunkCommandHelper {
public:
    void help(std::stringstream& h) const {
        h << "internal";
    }
    TransferModsCommand() : ChunkCommandHelper("_transferMods") {}
    virtual void addRequiredPrivileges(const std::string& dbname,
                                       const BSONObj& cmdObj,
                                       std::vector<Privilege>* out) {
        ActionSet actions;
        actions.addAction(ActionType::internal);
        out->push_back(Privilege(ResourcePattern::forClusterResource(), actions));
    }
    bool run(OperationContext* txn,
             const string&,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        return migrateFromStatus.transferMods(txn, errmsg, result);
    }
} transferModsCommand;


class InitialCloneCommand : public ChunkCommandHelper {
public:
    void help(std::stringstream& h) const {
        h << "internal";
    }
    InitialCloneCommand() : ChunkCommandHelper("_migrateClone") {}
    virtual void addRequiredPrivileges(const std::string& dbname,
                                       const BSONObj& cmdObj,
                                       std::vector<Privilege>* out) {
        ActionSet actions;
        actions.addAction(ActionType::internal);
        out->push_back(Privilege(ResourcePattern::forClusterResource(), actions));
    }
    bool run(OperationContext* txn,
             const string&,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        return migrateFromStatus.clone(txn, errmsg, result);
    }
} initialCloneCommand;

// Tests can pause / resume moveChunk's progress at each step by enabling / disabling each fail
// point.
MONGO_FP_DECLARE(moveChunkHangAtStep1);
MONGO_FP_DECLARE(moveChunkHangAtStep2);
MONGO_FP_DECLARE(moveChunkHangAtStep3);
MONGO_FP_DECLARE(moveChunkHangAtStep4);
MONGO_FP_DECLARE(moveChunkHangAtStep5);
MONGO_FP_DECLARE(moveChunkHangAtStep6);

/**
 * this is the main entry for moveChunk
 * called to initial a move
 * usually by a mongos
 * this is called on the "from" side
 *
 * Format:
 * {
 *   moveChunk: "namespace",
 *   from: "hostAndPort",
 *   fromShard: "shardName",
 *   to: "hostAndPort",
 *   toShard: "shardName",
 *   min: {},
 *   max: {},
 *   maxChunkBytes: numeric,
 *   configdb: "hostAndPort",
 *
 *   // optional
 *   secondaryThrottle: bool, //defaults to true.
 *   writeConcern: {} // applies to individual writes.
 * }
 */
class MoveChunkCommand : public Command {
public:
    MoveChunkCommand() : Command("moveChunk") {}
    virtual void help(std::stringstream& help) const {
        help << "should not be calling this directly";
    }

    virtual bool slaveOk() const {
        return false;
    }
    virtual bool adminOnly() const {
        return true;
    }
    virtual bool isWriteCommandForConfigServer() const {
        return false;
    }
    virtual Status checkAuthForCommand(ClientBasic* client,
                                       const std::string& dbname,
                                       const BSONObj& cmdObj) {
        if (!AuthorizationSession::get(client)->isAuthorizedForActionsOnResource(
                ResourcePattern::forExactNamespace(NamespaceString(parseNs(dbname, cmdObj))),
                ActionType::moveChunk)) {
            return Status(ErrorCodes::Unauthorized, "Unauthorized");
        }
        return Status::OK();
    }
    virtual std::string parseNs(const std::string& dbname, const BSONObj& cmdObj) const {
        return parseNsFullyQualified(dbname, cmdObj);
    }

    bool run(OperationContext* txn,
             const string& dbname,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        // 1. Parse options
        // 2. Make sure my view is complete and lock the distributed lock to ensure shard
        //    metadata stability.
        // 3. Migration
        //    Retrieve all RecordIds, which need to be migrated in order to do as little seeking
        //    as possible during transfer. Retrieval of the RecordIds happens under a collection
        //    lock, but then the collection lock is dropped. This opens up an opportunity for
        //    repair or compact to invalidate these RecordIds, because these commands do not
        //    synchronized with migration. Note that data modifications are not a problem,
        //    because we are registered for change notifications.
        //
        // 4. pause till migrate caught up
        // 5. LOCK
        //    a) update my config, essentially locking
        //    b) finish migrate
        //    c) update config server
        //    d) logChange to config server
        // 6. wait for all current cursors to expire
        // 7. remove data locally

        // -------------------------------

        // 1.
        string ns = parseNs(dbname, cmdObj);

        // The shard addresses, redundant, but allows for validation
        string toShardHost = cmdObj["to"].str();
        string fromShardHost = cmdObj["from"].str();

        // The shard names
        string toShardName = cmdObj["toShard"].str();
        string fromShardName = cmdObj["fromShard"].str();

        // Process secondary throttle settings and assign defaults if necessary.
        BSONObj secThrottleObj;
        WriteConcernOptions writeConcern;
        Status status = writeConcern.parseSecondaryThrottle(cmdObj, &secThrottleObj);

        if (!status.isOK()) {
            if (status.code() != ErrorCodes::WriteConcernNotDefined) {
                warning() << status.toString();
                return appendCommandStatus(result, status);
            }

            writeConcern = getDefaultWriteConcern();
        } else {
            repl::ReplicationCoordinator* replCoordinator = repl::getGlobalReplicationCoordinator();

            if (replCoordinator->getReplicationMode() ==
                    repl::ReplicationCoordinator::modeMasterSlave &&
                writeConcern.shouldWaitForOtherNodes()) {
                warning() << "moveChunk cannot check if secondary throttle setting "
                          << writeConcern.toBSON()
                          << " can be enforced in a master slave configuration";
            }

            Status status = replCoordinator->checkIfWriteConcernCanBeSatisfied(writeConcern);
            if (!status.isOK() && status != ErrorCodes::NoReplicationEnabled) {
                warning() << status.toString();
                return appendCommandStatus(result, status);
            }
        }

        if (writeConcern.shouldWaitForOtherNodes() &&
            writeConcern.wTimeout == WriteConcernOptions::kNoTimeout) {
            // Don't allow no timeout.
            writeConcern.wTimeout = kDefaultWTimeoutMs;
        }

        // Do inline deletion
        bool waitForDelete = cmdObj["waitForDelete"].trueValue();
        if (waitForDelete) {
            log() << "moveChunk waiting for full cleanup after move";
        }

        BSONObj min = cmdObj["min"].Obj();
        BSONObj max = cmdObj["max"].Obj();
        BSONElement maxSizeElem = cmdObj["maxChunkSizeBytes"];

        if (ns.empty()) {
            errmsg = "need to specify namespace in command";
            return false;
        }

        if (toShardName.empty()) {
            errmsg = "need to specify shard to move chunk to";
            return false;
        }
        if (fromShardName.empty()) {
            errmsg = "need to specify shard to move chunk from";
            return false;
        }

        if (min.isEmpty()) {
            errmsg = "need to specify a min";
            return false;
        }

        if (max.isEmpty()) {
            errmsg = "need to specify a max";
            return false;
        }

        if (maxSizeElem.eoo() || !maxSizeElem.isNumber()) {
            errmsg = "need to specify maxChunkSizeBytes";
            return false;
        }
        const long long maxChunkSize = maxSizeElem.numberLong();  // in bytes

        // This could be the first call that enables sharding - make sure we initialize the
        // sharding state for this shard.
        if (!shardingState.enabled()) {
            if (cmdObj["configdb"].type() != String) {
                errmsg = "sharding not enabled";
                warning() << errmsg;
                return false;
            }
            string configdb = cmdObj["configdb"].String();
            ShardingState::initialize(configdb);
        }

        // Initialize our current shard name in the shard state if needed
        shardingState.gotShardName(fromShardName);

        // Make sure we're as up-to-date as possible with shard information
        // This catches the case where we had to previously changed a shard's host by
        // removing/adding a shard with the same name
        Shard::reloadShardInfo();

        ConnectionString configLoc =
            ConnectionString::parse(shardingState.getConfigServer(), errmsg);
        if (!configLoc.isValid()) {
            warning() << errmsg;
            return false;
        }

        MoveTimingHelper timing(
            txn, "from", ns, min, max, 6 /* steps */, &errmsg, toShardName, fromShardName);

        log() << "received moveChunk request: " << cmdObj << migrateLog;

        timing.done(1);
        MONGO_FP_PAUSE_WHILE(moveChunkHangAtStep1);

        // 2.

        if (migrateFromStatus.isActive()) {
            errmsg = "migration already in progress";
            warning() << errmsg;
            return false;
        }

        //
        // Get the distributed lock
        //

        string whyMessage(str::stream() << "migrating chunk [" << minKey << ", " << maxKey
                                        << ") in " << ns);
        auto scopedDistLock = grid.catalogManager()->getDistLockManager()->lock(ns, whyMessage);

        if (!scopedDistLock.isOK()) {
            errmsg = stream() << "could not acquire collection lock for " << ns
                              << " to migrate chunk [" << minKey << "," << maxKey << ")"
                              << causedBy(scopedDistLock.getStatus());

            warning() << errmsg;
            return false;
        }

        BSONObj chunkInfo =
            BSON("min" << min << "max" << max << "from" << fromShardName << "to" << toShardName);

        grid.catalogManager()->logChange(
            txn->getClient()->clientAddress(true), "moveChunk.start", ns, chunkInfo);

        // Always refresh our metadata remotely
        ChunkVersion origShardVersion;
        Status refreshStatus = shardingState.refreshMetadataNow(txn, ns, &origShardVersion);

        if (!refreshStatus.isOK()) {
            errmsg = str::stream() << "moveChunk cannot start migrate of chunk "
                                   << "[" << minKey << "," << maxKey << ")"
                                   << causedBy(refreshStatus.reason());

            warning() << errmsg;
            return false;
        }

        if (origShardVersion.majorVersion() == 0) {
            // It makes no sense to migrate if our version is zero and we have no chunks
            errmsg = str::stream() << "moveChunk cannot start migrate of chunk "
                                   << "[" << minKey << "," << maxKey << ")"
                                   << " with zero shard version";

            warning() << errmsg;
            return false;
        }

        // From mongos >= v3.0.
        BSONElement epochElem(cmdObj["epoch"]);
        if (epochElem.type() == jstOID) {
            OID cmdEpoch = epochElem.OID();

            if (cmdEpoch != origShardVersion.epoch()) {
                errmsg = str::stream() << "moveChunk cannot move chunk "
                                       << "[" << minKey << "," << maxKey << "), "
                                       << "collection may have been dropped. "
                                       << "current epoch: " << origShardVersion.epoch()
                                       << ", cmd epoch: " << cmdEpoch;
                warning() << errmsg;
                return false;
            }
        }

        // Get collection metadata
        const CollectionMetadataPtr origCollMetadata(shardingState.getCollectionMetadata(ns));

        // With nonzero shard version, we must have metadata
        invariant(NULL != origCollMetadata);

        ChunkVersion origCollVersion = origCollMetadata->getCollVersion();
        BSONObj shardKeyPattern = origCollMetadata->getKeyPattern();

        // With nonzero shard version, we must have a coll version >= our shard version
        invariant(origCollVersion >= origShardVersion);

        // With nonzero shard version, we must have a shard key
        invariant(!shardKeyPattern.isEmpty());

        ChunkType origChunk;
        if (!origCollMetadata->getNextChunk(min, &origChunk) || origChunk.getMin().woCompare(min) ||
            origChunk.getMax().woCompare(max)) {
            // Our boundaries are different from those passed in
            errmsg = str::stream() << "moveChunk cannot find chunk "
                                   << "[" << minKey << "," << maxKey << ")"
                                   << " to migrate, the chunk boundaries may be stale";

            warning() << errmsg;
            return false;
        }

        log() << "moveChunk request accepted at version " << origShardVersion;

        timing.done(2);
        MONGO_FP_PAUSE_WHILE(moveChunkHangAtStep2);

        // 3.
        MigrateStatusHolder statusHolder(txn, ns, min, max, shardKeyPattern);

        if (statusHolder.isAnotherMigrationActive()) {
            errmsg = "moveChunk is already in progress from this shard";
            warning() << errmsg;
            return false;
        }

        ConnectionString fromShardCS;
        ConnectionString toShardCS;

        // Resolve the shard connection strings.
        {
            std::shared_ptr<Shard> fromShard = grid.shardRegistry()->getShard(fromShardName);
            uassert(28674,
                    str::stream() << "Source shard " << fromShardName
                                  << " is missing. This indicates metadata corruption.",
                    fromShard);

            fromShardCS = fromShard->getConnString();

            std::shared_ptr<Shard> toShard = grid.shardRegistry()->getShard(toShardName);
            uassert(28675,
                    str::stream() << "Destination shard " << toShardName
                                  << " is missing. This indicates metadata corruption.",
                    toShard);

            toShardCS = toShard->getConnString();
        }

        {
            // See comment at the top of the function for more information on what
            // synchronization is used here.
            if (!migrateFromStatus.storeCurrentLocs(txn, maxChunkSize, errmsg, result)) {
                warning() << errmsg;
                return false;
            }

            ScopedDbConnection connTo(toShardCS);
            BSONObj res;
            bool ok;

            const bool isSecondaryThrottle(writeConcern.shouldWaitForOtherNodes());

            BSONObjBuilder recvChunkStartBuilder;
            recvChunkStartBuilder.append("_recvChunkStart", ns);
            recvChunkStartBuilder.append("from", fromShardCS.toString());
            recvChunkStartBuilder.append("fromShardName", fromShardName);
            recvChunkStartBuilder.append("toShardName", toShardName);
            recvChunkStartBuilder.append("min", min);
            recvChunkStartBuilder.append("max", max);
            recvChunkStartBuilder.append("shardKeyPattern", shardKeyPattern);
            recvChunkStartBuilder.append("configServer", shardingState.getConfigServer());
            recvChunkStartBuilder.append("secondaryThrottle", isSecondaryThrottle);

            // Follow the same convention in moveChunk.
            if (isSecondaryThrottle && !secThrottleObj.isEmpty()) {
                recvChunkStartBuilder.append("writeConcern", secThrottleObj);
            }

            try {
                ok = connTo->runCommand("admin", recvChunkStartBuilder.done(), res);
            } catch (DBException& e) {
                errmsg = str::stream() << "moveChunk could not contact to: shard " << toShardName
                                       << " to start transfer" << causedBy(e);
                warning() << errmsg;
                return false;
            }

            connTo.done();

            if (!ok) {
                errmsg = "moveChunk failed to engage TO-shard in the data transfer: ";
                verify(res["errmsg"].type());
                errmsg += res["errmsg"].String();
                result.append("cause", res);
                warning() << errmsg;
                return false;
            }
        }
        timing.done(3);
        MONGO_FP_PAUSE_WHILE(moveChunkHangAtStep3);

        // 4.

        // Track last result from TO shard for sanity check
        BSONObj res;
        for (int i = 0; i < 86400; i++) {  // don't want a single chunk move to take more than a day
            invariant(!txn->lockState()->isLocked());

            // Exponential sleep backoff, up to 1024ms. Don't sleep much on the first few
            // iterations, since we want empty chunk migrations to be fast.
            sleepmillis(1 << std::min(i, 10));

            ScopedDbConnection conn(toShardCS);
            bool ok;
            res = BSONObj();
            try {
                ok = conn->runCommand("admin", BSON("_recvChunkStatus" << 1), res);
                res = res.getOwned();
            } catch (DBException& e) {
                errmsg = str::stream() << "moveChunk could not contact to: shard " << toShardName
                                       << " to monitor transfer" << causedBy(e);
                warning() << errmsg;
                return false;
            }

            conn.done();

            if (res["ns"].str() != ns || res["from"].str() != fromShardCS.toString() ||
                !res["min"].isABSONObj() || res["min"].Obj().woCompare(min) != 0 ||
                !res["max"].isABSONObj() || res["max"].Obj().woCompare(max) != 0) {
                // This can happen when the destination aborted the migration and
                // received another recvChunk before this thread sees the transition
                // to the abort state. This is currently possible only if multiple migrations
                // are happening at once. This is an unfortunate consequence of the shards not
                // being able to keep track of multiple incoming and outgoing migrations.
                errmsg = str::stream() << "Destination shard aborted migration, "
                                          "now running a new one: " << res;
                warning() << errmsg;
                return false;
            }

            LOG(0) << "moveChunk data transfer progress: " << res
                   << " my mem used: " << migrateFromStatus.mbUsed() << migrateLog;

            if (!ok || res["state"].String() == "fail") {
                warning() << "moveChunk error transferring data caused migration abort: " << res
                          << migrateLog;
                errmsg = "data transfer error";
                result.append("cause", res);
                return false;
            }

            if (res["state"].String() == "steady")
                break;

            if (migrateFromStatus.mbUsed() > (500 * 1024 * 1024)) {
                // This is too much memory for us to use for this so we're going to abort
                // the migrate
                ScopedDbConnection conn(toShardCS);

                BSONObj res;
                if (!conn->runCommand("admin", BSON("_recvChunkAbort" << 1), res)) {
                    warning() << "Error encountered while trying to abort migration on "
                              << "destination shard" << toShardCS;
                }

                res = res.getOwned();
                conn.done();
                error() << "aborting migrate because too much memory used res: " << res
                        << migrateLog;
                errmsg = "aborting migrate because too much memory used";
                result.appendBool("split", true);
                return false;
            }

            txn->checkForInterrupt();
        }

        timing.done(4);
        MONGO_FP_PAUSE_WHILE(moveChunkHangAtStep4);

        // 5.

        // Before we get into the critical section of the migration, let's double check
        // that the docs have been cloned, the config servers are reachable,
        // and the lock is in place.
        log() << "About to check if it is safe to enter critical section";

        // Ensure all cloned docs have actually been transferred
        std::size_t locsRemaining = migrateFromStatus.cloneLocsRemaining();
        if (locsRemaining != 0) {
            errmsg = str::stream() << "moveChunk cannot enter critical section before all data is"
                                   << " cloned, " << locsRemaining << " locs were not transferred"
                                   << " but to-shard reported " << res;

            // Should never happen, but safe to abort before critical section
            error() << errmsg << migrateLog;
            dassert(false);
            return false;
        }

        // Ensure distributed lock still held
        Status lockStatus = scopedDistLock.getValue().checkStatus();
        if (!lockStatus.isOK()) {
            errmsg = str::stream() << "not entering migrate critical section because "
                                   << lockStatus.toString();
            warning() << errmsg;
            return false;
        }

        log() << "About to enter migrate critical section";

        {
            // 5.a
            // we're under the collection lock here, so no other migrate can change maxVersion
            // or CollectionMetadata state
            migrateFromStatus.setInCriticalSection(true);
            ChunkVersion myVersion = origCollVersion;
            myVersion.incMajor();

            {
                ScopedTransaction transaction(txn, MODE_IX);
                Lock::DBLock lk(txn->lockState(), nsToDatabaseSubstring(ns), MODE_IX);
                Lock::CollectionLock collLock(txn->lockState(), ns, MODE_X);
                verify(myVersion > shardingState.getVersion(ns));

                // bump the metadata's version up and "forget" about the chunk being moved
                // this is not the commit point but in practice the state in this shard won't
                // until the commit it done
                shardingState.donateChunk(txn, ns, min, max, myVersion);
            }

            log() << "moveChunk setting version to: " << myVersion << migrateLog;

            // 5.b
            // we're under the collection lock here, too, so we can undo the chunk donation because
            // no other state change could be ongoing

            BSONObj res;
            bool ok;

            try {
                ScopedDbConnection connTo(toShardCS, 35.0);
                ok = connTo->runCommand("admin", BSON("_recvChunkCommit" << 1), res);
                connTo.done();
            } catch (DBException& e) {
                errmsg = str::stream() << "moveChunk could not contact to: shard "
                                       << toShardCS.toString() << " to commit transfer"
                                       << causedBy(e);
                warning() << errmsg;
                ok = false;
            }

            if (!ok || MONGO_FAIL_POINT(failMigrationCommit)) {
                log() << "moveChunk migrate commit not accepted by TO-shard: " << res
                      << " resetting shard version to: " << origShardVersion << migrateLog;
                {
                    ScopedTransaction transaction(txn, MODE_IX);
                    Lock::DBLock dbLock(txn->lockState(), nsToDatabaseSubstring(ns), MODE_IX);
                    Lock::CollectionLock collLock(txn->lockState(), ns, MODE_X);

                    log() << "moveChunk collection lock acquired to reset shard version from "
                             "failed migration";

                    // revert the chunk manager back to the state before "forgetting" about the
                    // chunk
                    shardingState.undoDonateChunk(txn, ns, origCollMetadata);
                }
                log() << "Shard version successfully reset to clean up failed migration";

                errmsg = "_recvChunkCommit failed!";
                result.append("cause", res);
                return false;
            }

            log() << "moveChunk migrate commit accepted by TO-shard: " << res << migrateLog;

            // 5.c

            // version at which the next highest lastmod will be set
            // if the chunk being moved is the last in the shard, nextVersion is that chunk's
            // lastmod otherwise the highest version is from the chunk being bumped on the
            // FROM-shard
            ChunkVersion nextVersion;

            // we want to go only once to the configDB but perhaps change two chunks, the one being
            // migrated and another local one (so to bump version for the entire shard)
            // we use the 'applyOps' mechanism to group the two updates and make them safer
            // TODO pull config update code to a module

            BSONArrayBuilder updates;
            {
                // update for the chunk being moved
                BSONObjBuilder op;
                op.append("op", "u");
                op.appendBool("b", false /* no upserting */);
                op.append("ns", ChunkType::ConfigNS);

                BSONObjBuilder n(op.subobjStart("o"));
                n.append(ChunkType::name(), Chunk::genID(ns, min));
                myVersion.addToBSON(n, ChunkType::DEPRECATED_lastmod());
                n.append(ChunkType::ns(), ns);
                n.append(ChunkType::min(), min);
                n.append(ChunkType::max(), max);
                n.append(ChunkType::shard(), toShardName);
                n.done();

                BSONObjBuilder q(op.subobjStart("o2"));
                q.append(ChunkType::name(), Chunk::genID(ns, min));
                q.done();

                updates.append(op.obj());
            }

            nextVersion = myVersion;

            // if we have chunks left on the FROM shard, update the version of one of them as
            // well.  we can figure that out by grabbing the metadata installed on 5.a

            const CollectionMetadataPtr bumpedCollMetadata(shardingState.getCollectionMetadata(ns));
            if (bumpedCollMetadata->getNumChunks() > 0) {
                // get another chunk on that shard
                ChunkType bumpChunk;
                bool chunkRes =
                    bumpedCollMetadata->getNextChunk(bumpedCollMetadata->getMinKey(), &bumpChunk);
                BSONObj bumpMin = bumpChunk.getMin();
                BSONObj bumpMax = bumpChunk.getMax();

                (void)chunkRes;  // for compile warning on non-debug
                dassert(chunkRes);
                dassert(bumpMin.woCompare(min) != 0);

                BSONObjBuilder op;
                op.append("op", "u");
                op.appendBool("b", false);
                op.append("ns", ChunkType::ConfigNS);

                nextVersion.incMinor();  // same as used on donateChunk
                BSONObjBuilder n(op.subobjStart("o"));
                n.append(ChunkType::name(), Chunk::genID(ns, bumpMin));
                nextVersion.addToBSON(n, ChunkType::DEPRECATED_lastmod());
                n.append(ChunkType::ns(), ns);
                n.append(ChunkType::min(), bumpMin);
                n.append(ChunkType::max(), bumpMax);
                n.append(ChunkType::shard(), fromShardName);
                n.done();

                BSONObjBuilder q(op.subobjStart("o2"));
                q.append(ChunkType::name(), Chunk::genID(ns, bumpMin));
                q.done();

                updates.append(op.obj());

                log() << "moveChunk updating self version to: " << nextVersion << " through "
                      << bumpMin << " -> " << bumpMax << " for collection '" << ns << "'"
                      << migrateLog;

            } else {
                log() << "moveChunk moved last chunk out for collection '" << ns << "'"
                      << migrateLog;
            }

            BSONArrayBuilder preCond;
            {
                BSONObjBuilder b;
                b.append("ns", ChunkType::ConfigNS);
                b.append("q",
                         BSON("query" << BSON(ChunkType::ns(ns)) << "orderby"
                                      << BSON(ChunkType::DEPRECATED_lastmod() << -1)));
                {
                    BSONObjBuilder bb(b.subobjStart("res"));
                    // TODO: For backwards compatibility, we can't yet require an epoch here
                    bb.appendTimestamp(ChunkType::DEPRECATED_lastmod(), origCollVersion.toLong());
                    bb.done();
                }
                preCond.append(b.obj());
            }

            Status applyOpsStatus{Status::OK()};
            try {
                // For testing migration failures
                if (MONGO_FAIL_POINT(failMigrationConfigWritePrepare)) {
                    throw DBException("mock migration failure before config write",
                                      PrepareConfigsFailedCode);
                }

                applyOpsStatus =
                    grid.catalogManager()->applyChunkOpsDeprecated(updates.arr(), preCond.arr());

                if (MONGO_FAIL_POINT(failMigrationApplyOps)) {
                    throw SocketException(SocketException::RECV_ERROR,
                                          shardingState.getConfigServer());
                }
            } catch (const DBException& ex) {
                warning() << ex << migrateLog;
                applyOpsStatus = ex.toStatus();
            }

            if (applyOpsStatus == ErrorCodes::PrepareConfigsFailedCode) {
                // In the process of issuing the migrate commit, the SyncClusterConnection
                // checks that the config servers are reachable. If they are not, we are
                // sure that the applyOps command was not sent to any of the configs, so we
                // can safely back out of the migration here, by resetting the shard
                // version that we bumped up to in the donateChunk() call above.

                log() << "About to acquire moveChunk coll lock to reset shard version from "
                      << "failed migration";

                {
                    ScopedTransaction transaction(txn, MODE_IX);
                    Lock::DBLock dbLock(txn->lockState(), nsToDatabaseSubstring(ns), MODE_IX);
                    Lock::CollectionLock collLock(txn->lockState(), ns, MODE_X);

                    // Revert the metadata back to the state before "forgetting"
                    // about the chunk.
                    shardingState.undoDonateChunk(txn, ns, origCollMetadata);
                }

                log() << "Shard version successfully reset to clean up failed migration";

                errmsg = "Failed to send migrate commit to configs because " + errmsg;
                return false;

            } else if (!applyOpsStatus.isOK()) {
                // this could be a blip in the connectivity
                // wait out a few seconds and check if the commit request made it
                //
                // if the commit made it to the config, we'll see the chunk in the new shard and
                // there's no action
                // if the commit did not make it, currently the only way to fix this state is to
                // bounce the mongod so that the old state (before migrating) be brought in

                warning() << "moveChunk commit outcome ongoing" << migrateLog;
                sleepsecs(10);

                // look for the chunk in this shard whose version got bumped
                // we assume that if that mod made it to the config, the applyOps was successful
                try {
                    std::vector<ChunkType> newestChunk;
                    Status status = grid.catalogManager()->getChunks(
                        BSON(ChunkType::ns(ns)),
                        BSON(ChunkType::DEPRECATED_lastmod() << -1),
                        1,
                        &newestChunk);
                    uassertStatusOK(status);

                    ChunkVersion checkVersion;
                    if (!newestChunk.empty()) {
                        invariant(newestChunk.size() == 1);
                        checkVersion = newestChunk[0].getVersion();
                    }

                    if (checkVersion.equals(nextVersion)) {
                        log() << "moveChunk commit confirmed" << migrateLog;
                        errmsg.clear();

                    } else {
                        error() << "moveChunk commit failed: version is at " << checkVersion
                                << " instead of " << nextVersion << migrateLog;
                        error() << "TERMINATING" << migrateLog;
                        dbexit(EXIT_SHARDING_ERROR);
                    }
                } catch (...) {
                    error() << "moveChunk failed to get confirmation of commit" << migrateLog;
                    error() << "TERMINATING" << migrateLog;
                    dbexit(EXIT_SHARDING_ERROR);
                }
            }

            migrateFromStatus.setInCriticalSection(false);

            // 5.d
            BSONObjBuilder commitInfo;
            commitInfo.appendElements(chunkInfo);
            if (res["counts"].type() == Object) {
                commitInfo.appendElements(res["counts"].Obj());
            }

            grid.catalogManager()->logChange(
                txn->getClient()->clientAddress(true), "moveChunk.commit", ns, commitInfo.obj());
        }

        migrateFromStatus.done(txn);
        timing.done(5);
        MONGO_FP_PAUSE_WHILE(moveChunkHangAtStep5);

        // 6.
        // NOTE: It is important that the distributed collection lock be held for this step.
        RangeDeleter* deleter = getDeleter();
        RangeDeleterOptions deleterOptions(
            KeyRange(ns, min.getOwned(), max.getOwned(), shardKeyPattern));
        deleterOptions.writeConcern = writeConcern;
        deleterOptions.waitForOpenCursors = true;
        deleterOptions.fromMigrate = true;
        deleterOptions.onlyRemoveOrphanedDocs = true;
        deleterOptions.removeSaverReason = "post-cleanup";

        if (waitForDelete) {
            log() << "doing delete inline for cleanup of chunk data" << migrateLog;

            string errMsg;
            // This is an immediate delete, and as a consequence, there could be more
            // deletes happening simultaneously than there are deleter worker threads.
            if (!deleter->deleteNow(txn, deleterOptions, &errMsg)) {
                log() << "Error occured while performing cleanup: " << errMsg;
            }
        } else {
            log() << "forking for cleanup of chunk data" << migrateLog;

            string errMsg;
            if (!deleter->queueDelete(txn,
                                      deleterOptions,
                                      NULL,  // Don't want to be notified.
                                      &errMsg)) {
                log() << "could not queue migration cleanup: " << errMsg;
            }
        }
        timing.done(6);
        MONGO_FP_PAUSE_WHILE(moveChunkHangAtStep6);

        return true;
    }

} moveChunkCmd;

bool ShardingState::inCriticalMigrateSection() {
    return migrateFromStatus.getInCriticalSection();
}

bool ShardingState::waitTillNotInCriticalSection(int maxSecondsToWait) {
    return migrateFromStatus.waitTillNotInCriticalSection(maxSecondsToWait);
}

/* -----
   below this are the "to" side commands

   command to initiate
   worker thread
     does initial clone
     pulls initial change set
     keeps pulling
     keeps state
   command to get state
   commend to "commit"
*/

// Enabling / disabling these fail points pauses / resumes MigrateStatus::_go(), the thread
// that receives a chunk migration from the donor.
MONGO_FP_DECLARE(migrateThreadHangAtStep1);
MONGO_FP_DECLARE(migrateThreadHangAtStep2);
MONGO_FP_DECLARE(migrateThreadHangAtStep3);
MONGO_FP_DECLARE(migrateThreadHangAtStep4);
MONGO_FP_DECLARE(migrateThreadHangAtStep5);

class MigrateStatus {
public:
    enum State { READY, CLONE, CATCHUP, STEADY, COMMIT_START, DONE, FAIL, ABORT };

    MigrateStatus()
        : _active(false),
          _numCloned(0),
          _clonedBytes(0),
          _numCatchup(0),
          _numSteady(0),
          _state(READY) {}

    void setState(State newState) {
        std::lock_guard<std::mutex> sl(_mutex);
        _state = newState;
    }

    State getState() const {
        std::lock_guard<std::mutex> sl(_mutex);
        return _state;
    }

    /**
     * Returns OK if preparation was successful.
     */
    Status prepare(const std::string& ns,
                   const std::string& fromShard,
                   const BSONObj& min,
                   const BSONObj& max,
                   const BSONObj& shardKeyPattern) {
        std::lock_guard<std::mutex> lk(_mutex);

        if (_active) {
            return Status(ErrorCodes::ConflictingOperationInProgress,
                          str::stream() << "Active migration already in progress "
                                        << "ns: " << _ns << ", from: " << _from << ", min: " << _min
                                        << ", max: " << _max);
        }

        _state = READY;
        _errmsg = "";

        _ns = ns;
        _from = fromShard;
        _min = min;
        _max = max;
        _shardKeyPattern = shardKeyPattern;

        _numCloned = 0;
        _clonedBytes = 0;
        _numCatchup = 0;
        _numSteady = 0;

        _active = true;

        return Status::OK();
    }

    void go(OperationContext* txn,
            const std::string& ns,
            BSONObj min,
            BSONObj max,
            BSONObj shardKeyPattern,
            const std::string& fromShard,
            const OID& epoch,
            const WriteConcernOptions& writeConcern) {
        try {
            _go(txn, ns, min, max, shardKeyPattern, fromShard, epoch, writeConcern);
        } catch (std::exception& e) {
            {
                std::lock_guard<std::mutex> sl(_mutex);
                _state = FAIL;
                _errmsg = e.what();
            }

            error() << "migrate failed: " << e.what() << migrateLog;
        } catch (...) {
            {
                std::lock_guard<std::mutex> sl(_mutex);
                _state = FAIL;
                _errmsg = "UNKNOWN ERROR";
            }

            error() << "migrate failed with unknown exception" << migrateLog;
        }

        if (getState() != DONE) {
            // Unprotect the range if needed/possible on unsuccessful TO migration
            ScopedTransaction transaction(txn, MODE_IX);
            Lock::DBLock dbLock(txn->lockState(), nsToDatabaseSubstring(ns), MODE_IX);
            Lock::CollectionLock collLock(txn->lockState(), ns, MODE_X);

            string errMsg;
            if (!shardingState.forgetPending(txn, ns, min, max, epoch, &errMsg)) {
                warning() << errMsg;
            }
        }

        setActive(false);
    }

    void _go(OperationContext* txn,
             const std::string& ns,
             BSONObj min,
             BSONObj max,
             BSONObj shardKeyPattern,
             const std::string& fromShard,
             const OID& epoch,
             const WriteConcernOptions& writeConcern) {
        verify(getActive());
        verify(getState() == READY);
        verify(!min.isEmpty());
        verify(!max.isEmpty());

        DisableDocumentValidation validationDisabler(txn);

        log() << "starting receiving-end of migration of chunk " << min << " -> " << max
              << " for collection " << ns << " from " << fromShard << " at epoch "
              << epoch.toString();

        string errmsg;
        MoveTimingHelper timing(txn, "to", ns, min, max, 5 /* steps */, &errmsg, "", "");

        ScopedDbConnection conn(fromShard);
        conn->getLastError();  // just test connection

        NamespaceString nss(ns);
        {
            // 0. copy system.namespaces entry if collection doesn't already exist
            OldClientWriteContext ctx(txn, ns);
            if (!repl::getGlobalReplicationCoordinator()->canAcceptWritesFor(nss)) {
                errmsg = str::stream() << "Not primary during migration: " << ns
                                       << ": checking if collection exists";
                warning() << errmsg;
                setState(FAIL);
                return;
            }

            // Only copy if ns doesn't already exist
            Database* db = ctx.db();
            Collection* collection = db->getCollection(ns);

            if (!collection) {
                list<BSONObj> infos = conn->getCollectionInfos(
                    nsToDatabase(ns), BSON("name" << nsToCollectionSubstring(ns)));

                BSONObj options;
                if (infos.size() > 0) {
                    BSONObj entry = infos.front();
                    if (entry["options"].isABSONObj()) {
                        options = entry["options"].Obj();
                    }
                }

                WriteUnitOfWork wuow(txn);
                Status status = userCreateNS(txn, db, ns, options, false);
                if (!status.isOK()) {
                    warning() << "failed to create collection [" << ns << "] "
                              << " with options " << options << ": " << status;
                }
                wuow.commit();
            }
        }

        {
            // 1. copy indexes

            vector<BSONObj> indexSpecs;
            {
                const std::list<BSONObj> indexes = conn->getIndexSpecs(ns);
                indexSpecs.insert(indexSpecs.begin(), indexes.begin(), indexes.end());
            }

            ScopedTransaction transaction(txn, MODE_IX);
            Lock::DBLock lk(txn->lockState(), nsToDatabaseSubstring(ns), MODE_X);
            OldClientContext ctx(txn, ns);
            if (!repl::getGlobalReplicationCoordinator()->canAcceptWritesFor(nss)) {
                errmsg = str::stream() << "Not primary during migration: " << ns;
                warning() << errmsg;
                setState(FAIL);
                return;
            }

            Database* db = ctx.db();
            Collection* collection = db->getCollection(ns);
            if (!collection) {
                errmsg = str::stream() << "collection dropped during migration: " << ns;
                warning() << errmsg;
                setState(FAIL);
                return;
            }

            MultiIndexBlock indexer(txn, collection);

            indexer.removeExistingIndexes(&indexSpecs);

            if (!indexSpecs.empty()) {
                // Only copy indexes if the collection does not have any documents.
                if (collection->numRecords(txn) > 0) {
                    errmsg = str::stream() << "aborting migration, shard is missing "
                                           << indexSpecs.size() << " indexes and "
                                           << "collection is not empty. Non-trivial "
                                           << "index creation should be scheduled manually";
                    warning() << errmsg;
                    setState(FAIL);
                    return;
                }

                Status status = indexer.init(indexSpecs);
                if (!status.isOK()) {
                    errmsg = str::stream() << "failed to create index before migrating data. "
                                           << " error: " << status.toString();
                    warning() << errmsg;
                    setState(FAIL);
                    return;
                }

                status = indexer.insertAllDocumentsInCollection();
                if (!status.isOK()) {
                    errmsg = str::stream() << "failed to create index before migrating data. "
                                           << " error: " << status.toString();
                    warning() << errmsg;
                    setState(FAIL);
                    return;
                }

                WriteUnitOfWork wunit(txn);
                indexer.commit();

                for (size_t i = 0; i < indexSpecs.size(); i++) {
                    // make sure to create index on secondaries as well
                    getGlobalServiceContext()->getOpObserver()->onCreateIndex(
                        txn, db->getSystemIndexesName(), indexSpecs[i], true /* fromMigrate */);
                }

                wunit.commit();
            }

            timing.done(1);
            MONGO_FP_PAUSE_WHILE(migrateThreadHangAtStep1);
        }

        {
            // 2. delete any data already in range
            RangeDeleterOptions deleterOptions(
                KeyRange(ns, min.getOwned(), max.getOwned(), shardKeyPattern));
            deleterOptions.writeConcern = writeConcern;
            // No need to wait since all existing cursors will filter out this range when
            // returning the results.
            deleterOptions.waitForOpenCursors = false;
            deleterOptions.fromMigrate = true;
            deleterOptions.onlyRemoveOrphanedDocs = true;
            deleterOptions.removeSaverReason = "preCleanup";

            string errMsg;

            if (!getDeleter()->deleteNow(txn, deleterOptions, &errMsg)) {
                warning() << "Failed to queue delete for migrate abort: " << errMsg;
                setState(FAIL);
                return;
            }

            {
                // Protect the range by noting that we're now starting a migration to it
                ScopedTransaction transaction(txn, MODE_IX);
                Lock::DBLock dbLock(txn->lockState(), nsToDatabaseSubstring(ns), MODE_IX);
                Lock::CollectionLock collLock(txn->lockState(), ns, MODE_X);

                if (!shardingState.notePending(txn, ns, min, max, epoch, &errmsg)) {
                    warning() << errmsg;
                    setState(FAIL);
                    return;
                }
            }

            timing.done(2);
            MONGO_FP_PAUSE_WHILE(migrateThreadHangAtStep2);
        }

        State currentState = getState();
        if (currentState == FAIL || currentState == ABORT) {
            string errMsg;
            RangeDeleterOptions deleterOptions(
                KeyRange(ns, min.getOwned(), max.getOwned(), shardKeyPattern));
            deleterOptions.writeConcern = writeConcern;
            // No need to wait since all existing cursors will filter out this range when
            // returning the results.
            deleterOptions.waitForOpenCursors = false;
            deleterOptions.fromMigrate = true;
            deleterOptions.onlyRemoveOrphanedDocs = true;

            if (!getDeleter()->queueDelete(txn, deleterOptions, NULL /* notifier */, &errMsg)) {
                warning() << "Failed to queue delete for migrate abort: " << errMsg;
            }
        }

        {
            // 3. initial bulk clone
            setState(CLONE);

            while (true) {
                BSONObj res;
                if (!conn->runCommand("admin",
                                      BSON("_migrateClone" << 1),
                                      res)) {  // gets array of objects to copy, in disk order
                    setState(FAIL);
                    errmsg = "_migrateClone failed: ";
                    errmsg += res.toString();
                    error() << errmsg << migrateLog;
                    conn.done();
                    return;
                }

                BSONObj arr = res["objects"].Obj();
                int thisTime = 0;

                BSONObjIterator i(arr);
                while (i.more()) {
                    txn->checkForInterrupt();

                    if (getState() == ABORT) {
                        errmsg = str::stream() << "Migration abort requested while "
                                               << "copying documents";
                        error() << errmsg << migrateLog;
                        return;
                    }

                    BSONObj docToClone = i.next().Obj();
                    {
                        OldClientWriteContext cx(txn, ns);

                        BSONObj localDoc;
                        if (willOverrideLocalId(txn,
                                                ns,
                                                min,
                                                max,
                                                shardKeyPattern,
                                                cx.db(),
                                                docToClone,
                                                &localDoc)) {
                            string errMsg = str::stream() << "cannot migrate chunk, local document "
                                                          << localDoc << " has same _id as cloned "
                                                          << "remote document " << docToClone;

                            warning() << errMsg;

                            // Exception will abort migration cleanly
                            uasserted(16976, errMsg);
                        }

                        Helpers::upsert(txn, ns, docToClone, true);
                    }
                    thisTime++;

                    {
                        std::lock_guard<std::mutex> statsLock(_mutex);
                        _numCloned++;
                        _clonedBytes += docToClone.objsize();
                    }

                    if (writeConcern.shouldWaitForOtherNodes()) {
                        repl::ReplicationCoordinator::StatusAndDuration replStatus =
                            repl::getGlobalReplicationCoordinator()->awaitReplication(
                                txn,
                                repl::ReplClientInfo::forClient(txn->getClient()).getLastOp(),
                                writeConcern);
                        if (replStatus.status.code() == ErrorCodes::WriteConcernFailed) {
                            warning() << "secondaryThrottle on, but doc insert timed out; "
                                         "continuing";
                        } else {
                            massertStatusOK(replStatus.status);
                        }
                    }
                }

                if (thisTime == 0)
                    break;
            }

            timing.done(3);
            MONGO_FP_PAUSE_WHILE(migrateThreadHangAtStep3);
        }

        // If running on a replicated system, we'll need to flush the docs we cloned to the
        // secondaries
        repl::OpTime lastOpApplied = repl::ReplClientInfo::forClient(txn->getClient()).getLastOp();

        {
            // 4. do bulk of mods
            setState(CATCHUP);
            while (true) {
                BSONObj res;
                if (!conn->runCommand("admin", BSON("_transferMods" << 1), res)) {
                    setState(FAIL);
                    errmsg = "_transferMods failed: ";
                    errmsg += res.toString();
                    error() << "_transferMods failed: " << res << migrateLog;
                    conn.done();
                    return;
                }
                if (res["size"].number() == 0)
                    break;

                apply(txn, ns, min, max, shardKeyPattern, res, &lastOpApplied);

                const int maxIterations = 3600 * 50;
                int i;
                for (i = 0; i < maxIterations; i++) {
                    txn->checkForInterrupt();

                    if (getState() == ABORT) {
                        errmsg = str::stream() << "Migration abort requested while waiting "
                                               << "for replication at catch up stage";
                        error() << errmsg << migrateLog;

                        return;
                    }

                    if (opReplicatedEnough(txn, lastOpApplied, writeConcern))
                        break;

                    if (i > 100) {
                        warning() << "secondaries having hard time keeping up with migrate"
                                  << migrateLog;
                    }

                    sleepmillis(20);
                }

                if (i == maxIterations) {
                    errmsg = "secondary can't keep up with migrate";
                    error() << errmsg << migrateLog;
                    conn.done();
                    setState(FAIL);
                    return;
                }
            }

            timing.done(4);
            MONGO_FP_PAUSE_WHILE(migrateThreadHangAtStep4);
        }

        {
            // pause to wait for replication
            // this will prevent us from going into critical section until we're ready
            Timer t;
            while (t.minutes() < 600) {
                txn->checkForInterrupt();

                if (getState() == ABORT) {
                    errmsg = "Migration abort requested while waiting for replication";
                    error() << errmsg << migrateLog;
                    return;
                }

                log() << "Waiting for replication to catch up before entering critical section";
                if (flushPendingWrites(txn, ns, min, max, lastOpApplied, writeConcern))
                    break;
                sleepsecs(1);
            }

            if (t.minutes() >= 600) {
                setState(FAIL);
                errmsg = "Cannot go to critical section because secondaries cannot keep up";
                error() << errmsg << migrateLog;
                return;
            }
        }

        {
            // 5. wait for commit

            setState(STEADY);
            bool transferAfterCommit = false;
            while (getState() == STEADY || getState() == COMMIT_START) {
                txn->checkForInterrupt();

                // Make sure we do at least one transfer after recv'ing the commit message
                // If we aren't sure that at least one transfer happens *after* our state
                // changes to COMMIT_START, there could be mods still on the FROM shard that
                // got logged *after* our _transferMods but *before* the critical section.
                if (getState() == COMMIT_START)
                    transferAfterCommit = true;

                BSONObj res;
                if (!conn->runCommand("admin", BSON("_transferMods" << 1), res)) {
                    log() << "_transferMods failed in STEADY state: " << res << migrateLog;
                    errmsg = res.toString();
                    setState(FAIL);
                    conn.done();
                    return;
                }

                if (res["size"].number() > 0 &&
                    apply(txn, ns, min, max, shardKeyPattern, res, &lastOpApplied)) {
                    continue;
                }

                if (getState() == ABORT) {
                    return;
                }

                // We know we're finished when:
                // 1) The from side has told us that it has locked writes (COMMIT_START)
                // 2) We've checked at least one more time for un-transmitted mods
                if (getState() == COMMIT_START && transferAfterCommit == true) {
                    if (flushPendingWrites(txn, ns, min, max, lastOpApplied, writeConcern))
                        break;
                }

                // Only sleep if we aren't committing
                if (getState() == STEADY)
                    sleepmillis(10);
            }

            if (getState() == FAIL) {
                errmsg = "timed out waiting for commit";
                return;
            }

            timing.done(5);
            MONGO_FP_PAUSE_WHILE(migrateThreadHangAtStep5);
        }

        setState(DONE);
        conn.done();
    }

    void status(BSONObjBuilder& b) {
        std::lock_guard<std::mutex> sl(_mutex);

        b.appendBool("active", _active);

        b.append("ns", _ns);
        b.append("from", _from);
        b.append("min", _min);
        b.append("max", _max);
        b.append("shardKeyPattern", _shardKeyPattern);

        b.append("state", stateToString(_state));
        if (_state == FAIL) {
            b.append("errmsg", _errmsg);
        }

        BSONObjBuilder bb(b.subobjStart("counts"));
        bb.append("cloned", _numCloned);
        bb.append("clonedBytes", _clonedBytes);
        bb.append("catchup", _numCatchup);
        bb.append("steady", _numSteady);
        bb.done();
    }

    bool apply(OperationContext* txn,
               const string& ns,
               BSONObj min,
               BSONObj max,
               BSONObj shardKeyPattern,
               const BSONObj& xfer,
               repl::OpTime* lastOpApplied) {
        repl::OpTime dummy;
        if (lastOpApplied == NULL) {
            lastOpApplied = &dummy;
        }

        bool didAnything = false;

        if (xfer["deleted"].isABSONObj()) {
            ScopedTransaction transaction(txn, MODE_IX);
            Lock::DBLock dlk(txn->lockState(), nsToDatabaseSubstring(ns), MODE_IX);
            Helpers::RemoveSaver rs("moveChunk", ns, "removedDuring");

            BSONObjIterator i(xfer["deleted"].Obj());
            while (i.more()) {
                Lock::CollectionLock clk(txn->lockState(), ns, MODE_X);
                OldClientContext ctx(txn, ns);

                BSONObj id = i.next().Obj();

                // do not apply deletes if they do not belong to the chunk being migrated
                BSONObj fullObj;
                if (Helpers::findById(txn, ctx.db(), ns.c_str(), id, fullObj)) {
                    if (!isInRange(fullObj, min, max, shardKeyPattern)) {
                        log() << "not applying out of range deletion: " << fullObj << migrateLog;

                        continue;
                    }
                }

                if (serverGlobalParams.moveParanoia) {
                    rs.goingToDelete(fullObj);
                }

                deleteObjects(txn,
                              ctx.db(),
                              ns,
                              id,
                              PlanExecutor::YIELD_MANUAL,
                              true /* justOne */,
                              false /* god */,
                              true /* fromMigrate */);

                *lastOpApplied = repl::ReplClientInfo::forClient(txn->getClient()).getLastOp();
                didAnything = true;
            }
        }

        if (xfer["reload"].isABSONObj()) {
            BSONObjIterator i(xfer["reload"].Obj());
            while (i.more()) {
                OldClientWriteContext cx(txn, ns);

                BSONObj updatedDoc = i.next().Obj();

                BSONObj localDoc;
                if (willOverrideLocalId(
                        txn, ns, min, max, shardKeyPattern, cx.db(), updatedDoc, &localDoc)) {
                    string errMsg = str::stream()
                        << "cannot migrate chunk, local document " << localDoc
                        << " has same _id as reloaded remote document " << updatedDoc;

                    warning() << errMsg;

                    // Exception will abort migration cleanly
                    uasserted(16977, errMsg);
                }

                // We are in write lock here, so sure we aren't killing
                Helpers::upsert(txn, ns, updatedDoc, true);

                *lastOpApplied = repl::ReplClientInfo::forClient(txn->getClient()).getLastOp();
                didAnything = true;
            }
        }

        return didAnything;
    }

    /**
     * Checks if an upsert of a remote document will override a local document with the same _id
     * but in a different range on this shard.
     * Must be in WriteContext to avoid races and DBHelper errors.
     * TODO: Could optimize this check out if sharding on _id.
     */
    bool willOverrideLocalId(OperationContext* txn,
                             const string& ns,
                             BSONObj min,
                             BSONObj max,
                             BSONObj shardKeyPattern,
                             Database* db,
                             BSONObj remoteDoc,
                             BSONObj* localDoc) {
        *localDoc = BSONObj();
        if (Helpers::findById(txn, db, ns.c_str(), remoteDoc, *localDoc)) {
            return !isInRange(*localDoc, min, max, shardKeyPattern);
        }

        return false;
    }

    /**
     * Returns true if the majority of the nodes and the nodes corresponding to the given
     * writeConcern (if not empty) have applied till the specified lastOp.
     */
    bool opReplicatedEnough(OperationContext* txn,
                            const repl::OpTime& lastOpApplied,
                            const WriteConcernOptions& writeConcern) {
        WriteConcernOptions majorityWriteConcern;
        majorityWriteConcern.wTimeout = -1;
        majorityWriteConcern.wMode = WriteConcernOptions::kMajority;
        Status majorityStatus = repl::getGlobalReplicationCoordinator()
                                    ->awaitReplication(txn, lastOpApplied, majorityWriteConcern)
                                    .status;

        if (!writeConcern.shouldWaitForOtherNodes()) {
            return majorityStatus.isOK();
        }

        // Also enforce the user specified write concern after "majority" so it covers
        // the union of the 2 write concerns.

        WriteConcernOptions userWriteConcern(writeConcern);
        userWriteConcern.wTimeout = -1;
        Status userStatus = repl::getGlobalReplicationCoordinator()
                                ->awaitReplication(txn, lastOpApplied, userWriteConcern)
                                .status;

        return majorityStatus.isOK() && userStatus.isOK();
    }

    bool flushPendingWrites(OperationContext* txn,
                            const std::string& ns,
                            BSONObj min,
                            BSONObj max,
                            const repl::OpTime& lastOpApplied,
                            const WriteConcernOptions& writeConcern) {
        if (!opReplicatedEnough(txn, lastOpApplied, writeConcern)) {
            repl::OpTime op(lastOpApplied);
            OCCASIONALLY warning() << "migrate commit waiting for a majority of slaves for '" << ns
                                   << "' " << min << " -> " << max << " waiting for: " << op
                                   << migrateLog;
            return false;
        }

        log() << "migrate commit succeeded flushing to secondaries for '" << ns << "' " << min
              << " -> " << max << migrateLog;

        {
            // Get global lock to wait for write to be commited to journal.
            ScopedTransaction transaction(txn, MODE_S);
            Lock::GlobalRead lk(txn->lockState());

            // if durability is on, force a write to journal
            if (getDur().commitNow(txn)) {
                log() << "migrate commit flushed to journal for '" << ns << "' " << min << " -> "
                      << max << migrateLog;
            }
        }

        return true;
    }

    static string stateToString(State state) {
        switch (state) {
            case READY:
                return "ready";
            case CLONE:
                return "clone";
            case CATCHUP:
                return "catchup";
            case STEADY:
                return "steady";
            case COMMIT_START:
                return "commitStart";
            case DONE:
                return "done";
            case FAIL:
                return "fail";
            case ABORT:
                return "abort";
        }
        verify(0);
        return "";
    }

    bool startCommit() {
        std::unique_lock<std::mutex> lock(_mutex);

        if (_state != STEADY) {
            return false;
        }

        const auto deadline = system_clock::now() + seconds(30);
        _state = COMMIT_START;
        while (_active) {
            if (std::cv_status::timeout == isActiveCV.wait_until(lock, deadline)) {
                _state = FAIL;
                log() << "startCommit never finished!" << migrateLog;
                return false;
            }
        }

        if (_state == DONE) {
            return true;
        }

        log() << "startCommit failed, final data failed to transfer" << migrateLog;
        return false;
    }

    void abort() {
        std::lock_guard<std::mutex> sl(_mutex);
        _state = ABORT;
        _errmsg = "aborted";
    }

    bool getActive() const {
        std::lock_guard<std::mutex> lk(_mutex);
        return _active;
    }
    void setActive(bool b) {
        std::lock_guard<std::mutex> lk(_mutex);
        _active = b;
        isActiveCV.notify_all();
    }

    // Guards all fields.
    mutable std::mutex _mutex;
    bool _active;
    std::condition_variable isActiveCV;

    std::string _ns;
    std::string _from;
    BSONObj _min;
    BSONObj _max;
    BSONObj _shardKeyPattern;

    long long _numCloned;
    long long _clonedBytes;
    long long _numCatchup;
    long long _numSteady;

    State _state;
    std::string _errmsg;

} migrateStatus;

void migrateThread(std::string ns,
                   BSONObj min,
                   BSONObj max,
                   BSONObj shardKeyPattern,
                   std::string fromShard,
                   OID epoch,
                   WriteConcernOptions writeConcern) {
    Client::initThread("migrateThread");
    OperationContextImpl txn;
    if (getGlobalAuthorizationManager()->isAuthEnabled()) {
        ShardedConnectionInfo::addHook();
        AuthorizationSession::get(txn.getClient())->grantInternalAuthorization();
    }

    migrateStatus.go(&txn, ns, min, max, shardKeyPattern, fromShard, epoch, writeConcern);
}

/**
 * Command for initiating the recipient side of the migration to start copying data
 * from the donor shard.
 *
 * {
 *   _recvChunkStart: "namespace",
 *   congfigServer: "hostAndPort",
 *   from: "hostAndPort",
 *   fromShardName: "shardName",
 *   toShardName: "shardName",
 *   min: {},
 *   max: {},
 *   shardKeyPattern: {},
 *
 *   // optional
 *   secondaryThrottle: bool, // defaults to true
 *   writeConcern: {} // applies to individual writes.
 * }
 */
class RecvChunkStartCommand : public ChunkCommandHelper {
public:
    void help(std::stringstream& h) const {
        h << "internal";
    }
    RecvChunkStartCommand() : ChunkCommandHelper("_recvChunkStart") {}

    virtual bool isWriteCommandForConfigServer() const {
        return false;
    }
    virtual void addRequiredPrivileges(const std::string& dbname,
                                       const BSONObj& cmdObj,
                                       std::vector<Privilege>* out) {
        ActionSet actions;
        actions.addAction(ActionType::internal);
        out->push_back(Privilege(ResourcePattern::forClusterResource(), actions));
    }
    bool run(OperationContext* txn,
             const string&,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        // Active state of TO-side migrations (MigrateStatus) is serialized by distributed
        // collection lock.
        if (migrateStatus.getActive()) {
            errmsg = "migrate already in progress";
            return false;
        }

        // Pending deletes (for migrations) are serialized by the distributed collection lock,
        // we are sure we registered a delete for a range *before* we can migrate-in a
        // subrange.
        const size_t numDeletes = getDeleter()->getTotalDeletes();
        if (numDeletes > 0) {
            errmsg = str::stream() << "can't accept new chunks because "
                                   << " there are still " << numDeletes
                                   << " deletes from previous migration";

            warning() << errmsg;
            return false;
        }

        if (!shardingState.enabled()) {
            if (!cmdObj["configServer"].eoo()) {
                dassert(cmdObj["configServer"].type() == String);
                ShardingState::initialize(cmdObj["configServer"].String());
            } else {
                errmsg = str::stream()
                    << "cannot start recv'ing chunk, "
                    << "sharding is not enabled and no config server was provided";

                warning() << errmsg;
                return false;
            }
        }

        if (!cmdObj["toShardName"].eoo()) {
            dassert(cmdObj["toShardName"].type() == String);
            shardingState.gotShardName(cmdObj["toShardName"].String());
        }

        string ns = cmdObj.firstElement().String();
        BSONObj min = cmdObj["min"].Obj().getOwned();
        BSONObj max = cmdObj["max"].Obj().getOwned();

        // Refresh our collection manager from the config server, we need a collection manager
        // to start registering pending chunks.
        // We force the remote refresh here to make the behavior consistent and predictable,
        // generally we'd refresh anyway, and to be paranoid.
        ChunkVersion currentVersion;
        Status status = shardingState.refreshMetadataNow(txn, ns, &currentVersion);

        if (!status.isOK()) {
            errmsg = str::stream() << "cannot start recv'ing chunk "
                                   << "[" << min << "," << max << ")" << causedBy(status.reason());

            warning() << errmsg;
            return false;
        }

        // Process secondary throttle settings and assign defaults if necessary.
        WriteConcernOptions writeConcern;
        status = writeConcern.parseSecondaryThrottle(cmdObj, NULL);

        if (!status.isOK()) {
            if (status.code() != ErrorCodes::WriteConcernNotDefined) {
                warning() << status.toString();
                return appendCommandStatus(result, status);
            }

            writeConcern = getDefaultWriteConcern();
        } else {
            repl::ReplicationCoordinator* replCoordinator = repl::getGlobalReplicationCoordinator();

            if (replCoordinator->getReplicationMode() ==
                    repl::ReplicationCoordinator::modeMasterSlave &&
                writeConcern.shouldWaitForOtherNodes()) {
                warning() << "recvChunk cannot check if secondary throttle setting "
                          << writeConcern.toBSON()
                          << " can be enforced in a master slave configuration";
            }

            Status status = replCoordinator->checkIfWriteConcernCanBeSatisfied(writeConcern);
            if (!status.isOK() && status != ErrorCodes::NoReplicationEnabled) {
                warning() << status.toString();
                return appendCommandStatus(result, status);
            }
        }

        if (writeConcern.shouldWaitForOtherNodes() &&
            writeConcern.wTimeout == WriteConcernOptions::kNoTimeout) {
            // Don't allow no timeout.
            writeConcern.wTimeout = kDefaultWTimeoutMs;
        }

        BSONObj shardKeyPattern;
        if (cmdObj.hasField("shardKeyPattern")) {
            shardKeyPattern = cmdObj["shardKeyPattern"].Obj().getOwned();
        } else {
            // shardKeyPattern may not be provided if another shard is from pre 2.2
            // In that case, assume the shard key pattern is the same as the range
            // specifiers provided.
            BSONObj keya = Helpers::inferKeyPattern(min);
            BSONObj keyb = Helpers::inferKeyPattern(max);
            verify(keya == keyb);

            warning()
                << "No shard key pattern provided by source shard for migration."
                   " This is likely because the source shard is running a version prior to 2.2."
                   " Falling back to assuming the shard key matches the pattern of the min and max"
                   " chunk range specifiers.  Inferred shard key: " << keya;

            shardKeyPattern = keya.getOwned();
        }

        const string fromShard(cmdObj["from"].String());

        // Set the TO-side migration to active
        Status prepareStatus = migrateStatus.prepare(ns, fromShard, min, max, shardKeyPattern);

        if (!prepareStatus.isOK()) {
            return appendCommandStatus(result, prepareStatus);
        }

        std::thread m(migrateThread,
                      ns,
                      min,
                      max,
                      shardKeyPattern,
                      fromShard,
                      currentVersion.epoch(),
                      writeConcern);

        m.detach();
        result.appendBool("started", true);
        return true;
    }

} recvChunkStartCmd;

class RecvChunkStatusCommand : public ChunkCommandHelper {
public:
    void help(std::stringstream& h) const {
        h << "internal";
    }
    RecvChunkStatusCommand() : ChunkCommandHelper("_recvChunkStatus") {}
    virtual void addRequiredPrivileges(const std::string& dbname,
                                       const BSONObj& cmdObj,
                                       std::vector<Privilege>* out) {
        ActionSet actions;
        actions.addAction(ActionType::internal);
        out->push_back(Privilege(ResourcePattern::forClusterResource(), actions));
    }
    bool run(OperationContext* txn,
             const string&,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        migrateStatus.status(result);
        return 1;
    }

} recvChunkStatusCommand;

class RecvChunkCommitCommand : public ChunkCommandHelper {
public:
    void help(std::stringstream& h) const {
        h << "internal";
    }
    RecvChunkCommitCommand() : ChunkCommandHelper("_recvChunkCommit") {}
    virtual void addRequiredPrivileges(const std::string& dbname,
                                       const BSONObj& cmdObj,
                                       std::vector<Privilege>* out) {
        ActionSet actions;
        actions.addAction(ActionType::internal);
        out->push_back(Privilege(ResourcePattern::forClusterResource(), actions));
    }
    bool run(OperationContext* txn,
             const string&,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        bool ok = migrateStatus.startCommit();
        migrateStatus.status(result);
        return ok;
    }

} recvChunkCommitCommand;

class RecvChunkAbortCommand : public ChunkCommandHelper {
public:
    void help(std::stringstream& h) const {
        h << "internal";
    }
    RecvChunkAbortCommand() : ChunkCommandHelper("_recvChunkAbort") {}
    virtual void addRequiredPrivileges(const std::string& dbname,
                                       const BSONObj& cmdObj,
                                       std::vector<Privilege>* out) {
        ActionSet actions;
        actions.addAction(ActionType::internal);
        out->push_back(Privilege(ResourcePattern::forClusterResource(), actions));
    }
    bool run(OperationContext* txn,
             const string&,
             BSONObj& cmdObj,
             int,
             string& errmsg,
             BSONObjBuilder& result) {
        migrateStatus.abort();
        migrateStatus.status(result);
        return true;
    }

} recvChunkAboortCommand;


class IsInRangeTest : public StartupTest {
public:
    void run() {
        BSONObj min = BSON("x" << 1);
        BSONObj max = BSON("x" << 5);
        BSONObj skey = BSON("x" << 1);

        verify(!isInRange(BSON("x" << 0), min, max, skey));
        verify(isInRange(BSON("x" << 1), min, max, skey));
        verify(isInRange(BSON("x" << 3), min, max, skey));
        verify(isInRange(BSON("x" << 4), min, max, skey));
        verify(!isInRange(BSON("x" << 5), min, max, skey));
        verify(!isInRange(BSON("x" << 6), min, max, skey));

        BSONObj obj = BSON("n" << 3);
        BSONObj min2 = BSON("x" << BSONElementHasher::hash64(obj.firstElement(), 0) - 2);
        BSONObj max2 = BSON("x" << BSONElementHasher::hash64(obj.firstElement(), 0) + 2);
        BSONObj hashedKey = BSON("x"
                                 << "hashed");

        verify(isInRange(BSON("x" << 3), min2, max2, hashedKey));
        verify(!isInRange(BSON("x" << 3), min, max, hashedKey));
        verify(!isInRange(BSON("x" << 4), min2, max2, hashedKey));

        LOG(1) << "isInRangeTest passed" << migrateLog;
    }
} isInRangeTest;
}