SERVER-60420 Release the critical section early in the shard collection slow path

(cherry picked from commit e7c0fc2c3eac5d5fd35b29fcc7d1d5d52946c293)

2 files changed, 22 insertions, 7 deletions

diff --git a/src/mongo/db/s/create_collection_coordinator.cpp b/src/mongo/db/s/create_collection_coordinator.cpp
index 1b0621f2a94..f9892052a46 100644
--- a/src/mongo/db/s/create_collection_coordinator.cpp
+++ b/src/mongo/db/s/create_collection_coordinator.cpp
@@ -512,18 +512,16 @@ ExecutorFuture<void> CreateCollectionCoordinator::_runImpl(
                     }
                 }
 
-                _collectionEmpty = checkIfCollectionIsEmpty(opCtx, nss());
-
+                _createPolicy(opCtx);
                 _createCollectionAndIndexes(opCtx);
 
-                _createPolicyAndChunks(opCtx);
-
                 audit::logShardCollection(opCtx->getClient(),
                                           nss().ns(),
                                           *_doc.getShardKey(),
                                           _doc.getUnique().value_or(false));
 
                 if (_splitPolicy->isOptimized()) {
+                    _createChunks(opCtx);
                     // Block reads/writes from here on if we need to create
                     // the collection on other shards, this way we prevent
                     // reads/writes that should be redirected to another
@@ -548,10 +546,16 @@ ExecutorFuture<void> CreateCollectionCoordinator::_runImpl(
                     _commit(opCtx);
                 }
 
+                // End of the critical section, from now on, read and writes are permitted.
                 RecoverableCriticalSectionService::get(opCtx)->releaseRecoverableCriticalSection(
                     opCtx, nss(), _critSecReason, ShardingCatalogClient::kMajorityWriteConcern);
 
+                // Slow path. Create chunks (which might incur in an index scan) and commit must be
+                // done outside of the critical section to prevent writes from stalling in unsharded
+                // collections.
                 if (!_splitPolicy->isOptimized()) {
+                    _createChunks(opCtx);
+
                     _commit(opCtx);
                 }
 
@@ -723,8 +727,10 @@ void CreateCollectionCoordinator::_createCollectionAndIndexes(OperationContext*
     _collectionUUID = *sharding_ddl_util::getCollectionUUID(opCtx, nss());
 }
 
-void CreateCollectionCoordinator::_createPolicyAndChunks(OperationContext* opCtx) {
-    LOGV2_DEBUG(5277904, 2, "Create collection _createChunks", "namespace"_attr = nss());
+void CreateCollectionCoordinator::_createPolicy(OperationContext* opCtx) {
+    LOGV2_DEBUG(6042001, 2, "Create collection _createPolicy", "namespace"_attr = nss());
+
+    _collectionEmpty = checkIfCollectionIsEmpty(opCtx, nss());
 
     _splitPolicy = InitialSplitPolicy::calculateOptimizationStrategy(
         opCtx,
@@ -735,6 +741,10 @@ void CreateCollectionCoordinator::_createPolicyAndChunks(OperationContext* opCtx
         getTagsAndValidate(opCtx, nss(), _shardKeyPattern->toBSON()),
         getNumShards(opCtx),
         *_collectionEmpty);
+}
+
+void CreateCollectionCoordinator::_createChunks(OperationContext* opCtx) {
+    LOGV2_DEBUG(5277904, 2, "Create collection _createChunks", "namespace"_attr = nss());
 
     _initialChunks = _splitPolicy->createFirstChunks(
         opCtx, *_shardKeyPattern, {*_collectionUUID, ShardingState::get(opCtx)->shardId()});
diff --git a/src/mongo/db/s/create_collection_coordinator.h b/src/mongo/db/s/create_collection_coordinator.h
index 2ef5b0a08a4..3ca202c1778 100644
--- a/src/mongo/db/s/create_collection_coordinator.h
+++ b/src/mongo/db/s/create_collection_coordinator.h
@@ -103,9 +103,14 @@ private:
     void _createCollectionAndIndexes(OperationContext* opCtx);
 
     /**
+     * Creates the appropiate split policy.
+     */
+    void _createPolicy(OperationContext* opCtx);
+
+    /**
      * Given the appropiate split policy, create the initial chunks.
      */
-    void _createPolicyAndChunks(OperationContext* opCtx);
+    void _createChunks(OperationContext* opCtx);
 
     /**
      * If the optimized path can be taken, ensure the collection is already created in all the