SERVER-39705 Always observe multikey changes in IndexBuildInterceptor::sideWrite.

This re-applies commit 5bd904dff90a0e6332d6d4630053141e6617c5de with additional
change to the js test hybrid_sparse_multikey_index.js.

1 files changed, 7 insertions, 4 deletions

diff --git a/src/mongo/db/index/index_build_interceptor.cpp b/src/mongo/db/index/index_build_interceptor.cpp
index aaf57c66564..8d8f538691f 100644
--- a/src/mongo/db/index/index_build_interceptor.cpp
+++ b/src/mongo/db/index/index_build_interceptor.cpp
@@ -384,11 +384,10 @@ Status IndexBuildInterceptor::sideWrite(OperationContext* opCtx,
     // `multikeyMetadataKeys` when inserting.
     *numKeysOut = keys.size() + (op == Op::kInsert ? multikeyMetadataKeys.size() : 0);
 
-    if (*numKeysOut == 0) {
-        return Status::OK();
-    }
-
     {
+        // SERVER-39705: It's worth noting that a document may not generate any keys, but be
+        // described as being multikey. This step must be done to maintain parity with `validate`s
+        // expectations.
         stdx::unique_lock<stdx::mutex> lk(_multikeyPathMutex);
         if (_multikeyPaths) {
             MultikeyPathTracker::mergeMultikeyPaths(&_multikeyPaths.get(), multikeyPaths);
@@ -399,6 +398,10 @@ Status IndexBuildInterceptor::sideWrite(OperationContext* opCtx,
         }
     }
 
+    if (*numKeysOut == 0) {
+        return Status::OK();
+    }
+
     std::vector<BSONObj> toInsert;
     for (const auto& key : keys) {
         // Documents inserted into this table must be consumed in insert-order.