From a14d55980c2cdc565d4704a7e3ad37e4e535c1b2 Mon Sep 17 00:00:00 2001
From: Scott Hernandez <scotthernandez@gmail.com>
Date: Tue, 5 Jan 2016 13:27:39 -0500
Subject: SERVER-21867: Throw WCE when next isn't next for WTRecordStore or
 WTIndex during forward iteration

(cherry picked from commit 490baef98fde3131a5a328c1f809ab02cecd5c62)
---
 .../db/storage/wiredtiger/wiredtiger_index.cpp     | 34 ++++++++++++++++++++--
 .../storage/wiredtiger/wiredtiger_record_store.cpp | 19 +++++++++++-
 2 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
index 9c51f2cc2a8..bd522ebf4bc 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_index.cpp
@@ -51,6 +51,7 @@
 #include "mongo/db/storage/storage_options.h"
 #include "mongo/stdx/memory.h"
 #include "mongo/util/assert_util.h"
+#include "mongo/util/hex.h"
 #include "mongo/util/fail_point.h"
 #include "mongo/util/log.h"
 #include "mongo/util/mongoutils/str.h"
@@ -72,6 +73,8 @@
 namespace mongo {
 namespace {
 
+MONGO_FP_DECLARE(WTEmulateOutOfOrderNextIndexKey);
+
 using std::string;
 using std::vector;
 
@@ -617,7 +620,7 @@ public:
 
         if (!_lastMoveWasRestore)
             advanceWTCursor();
-        updatePosition();
+        updatePosition(true);
         return curr(parts);
     }
 
@@ -807,7 +810,7 @@ protected:
      * be called after a restore that did not restore to original state since that does not
      * logically move the cursor until the following call to next().
      */
-    void updatePosition() {
+    void updatePosition(bool inNext = false) {
         _lastMoveWasRestore = false;
         if (_cursorAtEof) {
             _eof = true;
@@ -820,6 +823,33 @@ protected:
         WT_CURSOR* c = _cursor->get();
         WT_ITEM item;
         invariantWTOK(c->get_key(c, &item));
+
+        const auto isForwardNextCall = _forward && inNext && !_key.isEmpty();
+        if (isForwardNextCall) {
+            // Due to a bug in wired tiger (SERVER-21867) sometimes calling next
+            // returns something prev.
+            const int cmp =
+                std::memcmp(_key.getBuffer(), item.data, std::min(_key.getSize(), item.size));
+            bool nextNotIncreasing = cmp > 0 || (cmp == 0 && _key.getSize() > item.size);
+
+            if (MONGO_FAIL_POINT(WTEmulateOutOfOrderNextIndexKey)) {
+                log() << "WTIndex::updatePosition simulating next key not increasing.";
+                nextNotIncreasing = true;
+            }
+
+            if (nextNotIncreasing) {
+                // Our new key is less than the old key which means the next call moved to !next.
+                log() << "WTIndex::updatePosition -- the new key ( " << toHex(item.data, item.size)
+                      << ") is less than the previous key (" << _key.toString()
+                      << "), which is a bug.";
+
+                // Force a retry of the operation from our last known position by acting as-if
+                // we received a WT_ROLLBACK error.
+                throw WriteConflictException();
+            }
+        }
+
+        // Store (a copy of) the new item data as the current key for this cursor.
         _key.resetFromBuffer(item.data, item.size);
 
         if (atOrPastEndPointAfterSeeking()) {
diff --git a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
index 030b2dd7647..a38a6fd517b 100644
--- a/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
+++ b/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp
@@ -91,6 +91,7 @@ bool shouldUseOplogHack(OperationContext* opCtx, const std::string& uri) {
 }  // namespace
 
 MONGO_FP_DECLARE(WTWriteConflictException);
+MONGO_FP_DECLARE(WTEmulateOutOfOrderNextRecordId);
 
 const std::string kWiredTigerEngineName = "wiredTiger";
 
@@ -483,7 +484,23 @@ public:
         _skipNextAdvance = false;
         int64_t key;
         invariantWTOK(c->get_key(c, &key));
-        const RecordId id = _fromKey(key);
+        RecordId id = _fromKey(key);
+
+        if (_forward && MONGO_FAIL_POINT(WTEmulateOutOfOrderNextRecordId)) {
+            log() << "WTEmulateOutOfOrderNextRecordId fail point has triggerd so RecordId is now "
+                     "RecordId(1) instead of " << id;
+            // Replace the found RecordId with a (small) fake one.
+            id = RecordId{1};
+        }
+
+        if (_forward && _lastReturnedId >= id) {
+            log() << "WTCursor::next -- c->next_key ( " << id
+                  << ") was not greater than _lastReturnedId (" << _lastReturnedId
+                  << ") which is a bug.";
+            // Force a retry of the operation from our last known position by acting as-if
+            // we received a WT_ROLLBACK error.
+            throw WriteConflictException();
+        }
 
         if (!isVisible(id)) {
             _eof = true;
-- 
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