SERVER-53685 Need to handle case of indexes being created on an empty collection when cloning starts.

1 files changed, 45 insertions, 1 deletions

diff --git a/src/mongo/db/repl/tenant_collection_cloner.cpp b/src/mongo/db/repl/tenant_collection_cloner.cpp
index c36dbc7a7cf..9a724df617a 100644
--- a/src/mongo/db/repl/tenant_collection_cloner.cpp
+++ b/src/mongo/db/repl/tenant_collection_cloner.cpp
@@ -82,6 +82,10 @@ TenantCollectionCloner::TenantCollectionCloner(const NamespaceString& sourceNss,
       _sourceDbAndUuid(NamespaceString("UNINITIALIZED")),
       _collectionClonerBatchSize(collectionClonerBatchSize),
       _countStage("count", this, &TenantCollectionCloner::countStage),
+      _checkIfDonorCollectionIsEmptyStage(
+          "checkIfDonorCollectionIsEmpty",
+          this,
+          &TenantCollectionCloner::checkIfDonorCollectionIsEmptyStage),
       _listIndexesStage("listIndexes", this, &TenantCollectionCloner::listIndexesStage),
       _createCollectionStage(
           "createCollection", this, &TenantCollectionCloner::createCollectionStage),
@@ -115,7 +119,11 @@ TenantCollectionCloner::TenantCollectionCloner(const NamespaceString& sourceNss,
 }
 
 BaseCloner::ClonerStages TenantCollectionCloner::getStages() {
-    return {&_countStage, &_listIndexesStage, &_createCollectionStage, &_queryStage};
+    return {&_countStage,
+            &_checkIfDonorCollectionIsEmptyStage,
+            &_listIndexesStage,
+            &_createCollectionStage,
+            &_queryStage};
 }
 
 void TenantCollectionCloner::preStage() {
@@ -175,6 +183,34 @@ BaseCloner::AfterStageBehavior TenantCollectionCloner::countStage() {
     return kContinueNormally;
 }
 
+// This avoids a race where an index may be created and data inserted after we do listIndexes.
+// That would result in doing a createIndexes on a non-empty collection during oplog application.
+// Instead, if the collection is empty before listIndexes, we do not clone the data -- it will be
+// added during oplog application.
+//
+// Note we cannot simply use the count() above, because that checks metadata which may not be 100%
+// accurate.
+BaseCloner::AfterStageBehavior TenantCollectionCloner::checkIfDonorCollectionIsEmptyStage() {
+    auto fieldsToReturn = BSON("_id" << 1);
+    auto cursor =
+        getClient()->query(_sourceDbAndUuid,
+                           {} /* Query */,
+                           1 /* limit */,
+                           0 /* skip */,
+                           &fieldsToReturn,
+                           QueryOption_SecondaryOk,
+                           0 /* batchSize */,
+                           ReadConcernArgs(ReadConcernLevel::kMajorityReadConcern).toBSONInner());
+    _donorCollectionWasEmptyBeforeListIndexes = !cursor->more();
+    LOGV2_DEBUG(5368500,
+                1,
+                "Checked if donor collection was empty",
+                "wasEmpty"_attr = _donorCollectionWasEmptyBeforeListIndexes,
+                "namespace"_attr = _sourceNss.ns(),
+                "tenantId"_attr = _tenantId);
+    return kContinueNormally;
+}
+
 BaseCloner::AfterStageBehavior TenantCollectionCloner::listIndexesStage() {
     // This will be set after a successful listCollections command.
     _operationTime = Timestamp();
@@ -341,6 +377,14 @@ BaseCloner::AfterStageBehavior TenantCollectionCloner::createCollectionStage() {
 }
 
 BaseCloner::AfterStageBehavior TenantCollectionCloner::queryStage() {
+    if (_donorCollectionWasEmptyBeforeListIndexes) {
+        LOGV2_WARNING(5368501,
+                      "Collection was empty at clone time.",
+                      "namespace"_attr = _sourceNss,
+                      "tenantId"_attr = _tenantId);
+        return kContinueNormally;
+    }
+
     // Sets up tracking the lastVisibleOpTime from response metadata.
     auto requestMetadataWriter = [this](OperationContext* opCtx,
                                         BSONObjBuilder* metadataBob) -> Status {