SERVER-31948: Do not lock the `_indexObserverMutex` on the `informIndexObserver` code path.r3.6.0-rc4

IndexObservers were part of the background collection/index consistency validation, but
the feature did not make it in for 3.6. IndexObservers currently cannot be installed, so
we'll gain some perf by not locking the mutex for what is otherwise a no-op.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/mongo/db/catalog/collection_impl.cpp b/src/mongo/db/catalog/collection_impl.cpp
index 689b340923a..5d5922c690f 100644
--- a/src/mongo/db/catalog/collection_impl.cpp
+++ b/src/mongo/db/catalog/collection_impl.cpp
@@ -1005,6 +1005,15 @@ void CollectionImpl::informIndexObserver(OperationContext* opCtx,
                                          const IndexDescriptor* descriptor,
                                          const IndexKeyEntry& indexEntry,
                                          const ValidationOperation operation) const {
+    // The index observer was used for a project that would allow concurrent validation of
+    // collection/index consistency while updates were happening to the collection. That project did
+    // not make it in for 3.6. This mutex is in a hot code path, so we're going to avoid locking it
+    // for the time being. See SERVER-31948.
+    const bool unusedReturnEarlyForPerf = true;
+    if (unusedReturnEarlyForPerf) {
+        return;
+    }
+
     stdx::lock_guard<stdx::mutex> lock(_indexObserverMutex);
     if (_indexObserver) {
         _indexObserver->inform(opCtx, descriptor, std::move(indexEntry), operation);