SERVER-23807 index build must X-lock collection

Index builds must lock the collection with an X lock, rather than an IX lock, to
prevent a race with concurrent updates.

2 files changed, 13 insertions, 2 deletions

diff --git a/src/mongo/db/commands/create_indexes.cpp b/src/mongo/db/commands/create_indexes.cpp
index 238fdc7c4cb..e906eb9c2ae 100644
--- a/src/mongo/db/commands/create_indexes.cpp
+++ b/src/mongo/db/commands/create_indexes.cpp
@@ -233,7 +233,16 @@ public:
         }
 
         try {
-            Lock::CollectionLock colLock(txn->lockState(), ns.ns(), MODE_IX);
+            // We need an X lock and not an IX lock due to a race with concurrent updates. Consider
+            // the following scenario:
+            //
+            // 1. A document is about to be indexed by a background index builder.
+            // 2. An update concurrently reads the same document.
+            // 3. The update unindexes the old value of the document. However, this is a no-op
+            // because the document wasn't previously indexed, so no write conflict is thrown.
+            // 4. Both the update and the index builder insert their respective index keys into the
+            // index, resulting in more keys in the index than expected.
+            Lock::CollectionLock colLock(txn->lockState(), ns.ns(), MODE_X);
             uassertStatusOK(indexer.insertAllDocumentsInCollection());
         } catch (const DBException& e) {
             invariant(e.getCode() != ErrorCodes::WriteConflict);
diff --git a/src/mongo/db/index_builder.cpp b/src/mongo/db/index_builder.cpp
index ba2cbd6d6ef..0ed860c5dea 100644
--- a/src/mongo/db/index_builder.cpp
+++ b/src/mongo/db/index_builder.cpp
@@ -175,7 +175,9 @@ Status IndexBuilder::_build(OperationContext* txn,
                         dbLock->relockWithMode(MODE_IX);
                     }
 
-                    Lock::CollectionLock colLock(txn->lockState(), ns.ns(), MODE_IX);
+                    // We need an X lock and not an IX lock for background index building. See the
+                    // createIndexes command for a detailed example.
+                    Lock::CollectionLock colLock(txn->lockState(), ns.ns(), MODE_X);
                     status = indexer.insertAllDocumentsInCollection();
                 }