SERVER-44477 Use correct collection count in cluster MR when determining whether to drop and reshard target

2 files changed, 64 insertions, 3 deletions

diff --git a/jstests/sharding/mapReduce_outSharded_checkUUID.js b/jstests/sharding/mapReduce_outSharded_checkUUID.js
index 0de45c127c7..4a0044c563c 100644
--- a/jstests/sharding/mapReduce_outSharded_checkUUID.js
+++ b/jstests/sharding/mapReduce_outSharded_checkUUID.js
@@ -35,6 +35,7 @@
             bulk.insert({j: j, i: i});
         }
     }
+
     assert.writeOK(bulk.execute());
 
     function map() {
@@ -47,14 +48,14 @@
     // sharded src sharded dst
     var suffix = "InShardedOutSharded";
 
-    // Check that merge to an existing empty sharded collection works and creates a new UUID after
+    // Check that merge to an existing empty sharded collection works and preserves the UUID after
     // M/R
     st.adminCommand({shardcollection: "mrShard.outSharded", key: {"_id": 1}});
     var origUUID = getUUIDFromConfigCollections(st.s, "mrShard.outSharded");
     var out = db.srcSharded.mapReduce(map, reduce, {out: {merge: "outSharded", sharded: true}});
     verifyOutput(out, 512);
     var newUUID = getUUIDFromConfigCollections(st.s, "mrShard.outSharded");
-    assert.neq(origUUID, newUUID);
+    assert.eq(origUUID, newUUID);
 
     // Shard1 is the primary shard and only one chunk should have been written, so the chunk with
     // the new UUID should have been written to it.
@@ -166,5 +167,4 @@
     assert.eq(newUUID, getUUIDFromListCollections(st.shard1.getDB("mrShard"), "replaceUnsharded"));
 
     st.stop();
-
 })();
diff --git a/jstests/sharding/mr_merge_to_existing.js b/jstests/sharding/mr_merge_to_existing.js
new file mode 100644
index 00000000000..53f2263c74b
--- /dev/null
+++ b/jstests/sharding/mr_merge_to_existing.js
@@ -0,0 +1,61 @@
+// Test that running mapReduce to a target collection which is empty on the primary shard does *not*
+// drop and reshard it.
+(function() {
+
+    load("jstests/libs/fixture_helpers.js");
+
+    var st = new ShardingTest({
+        shards: 2,
+        rs: {nodes: 2},
+    });
+
+    const dbName = "test";
+    const collName = jsTestName();
+
+    let mongosConn = st.s;
+    assert.commandWorked(mongosConn.getDB(dbName).runCommand({create: collName}));
+    st.ensurePrimaryShard(dbName, st.shard0.shardName);
+
+    // Shard the test collection and split it into two chunks.
+    st.shardColl(collName,
+                 {_id: 1} /* Shard key */,
+                 {_id: 2} /* Split at */,
+                 {_id: 2} /* Move the chunk to its own shard */,
+                 dbName,
+                 true /* Wait until documents orphaned by the move get deleted */);
+
+    // Seed the source collection.
+    let sourceColl = mongosConn.getDB(dbName)[collName];
+    assert.commandWorked(sourceColl.insert({key: 1}));
+    assert.commandWorked(sourceColl.insert({key: 2}));
+    assert.commandWorked(sourceColl.insert({key: 3}));
+    assert.commandWorked(sourceColl.insert({key: 4}));
+
+    // Shard the target collection.
+    let mergeColl = mongosConn.getDB(dbName).mr_merge_out;
+    st.shardColl("mr_merge_out",
+                 {_id: 1} /* Shard key */,
+                 {_id: 2} /* Split at */,
+                 {_id: 2} /* Move the chunk containing {_id: 2} to its own shard */,
+                 dbName);
+
+    // Insert a single document to the target collection and ensure that it lives on the non-primary
+    // shard.
+    assert.commandWorked(mergeColl.insert({_id: 5, value: 1}));
+
+    function map() {
+        emit(this.key, 1);
+    }
+    function reduce(key, values) {
+        return Array.sum(values);
+    }
+
+    // Run the mapReduce to merge to the existing sharded collection.
+    assert.commandWorked(
+        sourceColl.mapReduce(map, reduce, {out: {merge: mergeColl.getName(), sharded: true}}));
+
+    // Verify that the previous document still exists in the target collection.
+    assert.eq(mergeColl.find({_id: 5}).itcount(), 1);
+
+    st.stop();
+})();