SERVER-42484 Ensure we are inside a WriteUnitOfWork when writing index keys during initial sync collection cloning

(cherry picked from commit c41af3e315a115b04a53706837d36a729d174fc3)

2 files changed, 50 insertions, 3 deletions

diff --git a/jstests/replsets/initial_sync_clone_multikey.js b/jstests/replsets/initial_sync_clone_multikey.js
new file mode 100644
index 00000000000..0cf39825261
--- /dev/null
+++ b/jstests/replsets/initial_sync_clone_multikey.js
@@ -0,0 +1,36 @@
+/**
+ * Test initial sync cloning of a collection that contains a multikey index when hybrid index builds
+ * are disabled.
+ */
+(function() {
+"use strict";
+
+const replTest = new ReplSetTest({nodes: 1});
+replTest.startSet();
+replTest.initiate();
+
+const dbName = jsTest.name();
+const collName = "test";
+
+const primary = replTest.getPrimary();
+const primaryDB = primary.getDB(dbName);
+
+jsTestLog("Creating the collection and an index.");
+assert.commandWorked(primaryDB.createCollection(collName));
+assert.commandWorked(primaryDB[collName].createIndex({"x": 1}, {background: true}));
+
+// Make the index multikey.
+primaryDB[collName].insert({x: [1, 2]});
+
+jsTestLog("Adding a secondary node to do the initial sync.");
+replTest.add({setParameter: "enableHybridIndexBuilds=false"});
+
+jsTestLog("Re-initiating replica set with the new secondary.");
+replTest.reInitiate();
+
+// Wait until initial sync completes.
+jsTestLog("Waiting until initial sync completes.");
+replTest.awaitSecondaryNodes();
+replTest.awaitReplication();
+replTest.stopSet();
+})();
\ No newline at end of file
diff --git a/src/mongo/db/repl/collection_bulk_loader_impl.cpp b/src/mongo/db/repl/collection_bulk_loader_impl.cpp
index 68f760d284a..56322162fe1 100644
--- a/src/mongo/db/repl/collection_bulk_loader_impl.cpp
+++ b/src/mongo/db/repl/collection_bulk_loader_impl.cpp
@@ -154,9 +154,20 @@ Status CollectionBulkLoaderImpl::insertDocuments(const std::vector<BSONObj>::con
             }
 
             if (loc) {
-                // Inserts index entries into the external sorter. This will not update
-                // pre-existing indexes.
-                status = _addDocumentToIndexBlocks(doc, loc.get());
+                // Inserts index entries into the external sorter. This will not update pre-existing
+                // indexes. Wrap this in a WUOW since the index entry insertion may modify the
+                // durable record store which can throw a write conflict exception.
+                status =
+                    writeConflictRetry(_opCtx.get(), "_addDocumentToIndexBlocks", _nss.ns(), [&] {
+                        WriteUnitOfWork wunit(_opCtx.get());
+                        status = _addDocumentToIndexBlocks(doc, loc.get());
+                        // We only need to commit the unit of work if the index insertion was
+                        // successful.
+                        if (status.isOK()) {
+                            wunit.commit();
+                        }
+                        return status;
+                    });
             }
 
             if (!status.isOK()) {