SERVER-45192 don't allocate during WSM::clear

2 files changed, 13 insertions, 1 deletions

diff --git a/src/mongo/db/exec/document_value/document.h b/src/mongo/db/exec/document_value/document.h
index 5b9d953b287..978d008cae0 100644
--- a/src/mongo/db/exec/document_value/document.h
+++ b/src/mongo/db/exec/document_value/document.h
@@ -334,6 +334,10 @@ public:
         return _storage ? _storage->isModified() : false;
     }
 
+    bool hasExclusivelyOwnedStorage() const {
+        return _storage && !_storage->isShared();
+    }
+
     /// only for testing
     const void* getPtr() const {
         return _storage.get();
diff --git a/src/mongo/db/exec/working_set.cpp b/src/mongo/db/exec/working_set.cpp
index 94e564f8cf2..6529df45b07 100644
--- a/src/mongo/db/exec/working_set.cpp
+++ b/src/mongo/db/exec/working_set.cpp
@@ -113,7 +113,15 @@ WorkingSetID WorkingSet::emplace(WorkingSetMember&& wsm) {
 void WorkingSetMember::clear() {
     _metadata = DocumentMetadataFields{};
     keyData.clear();
-    resetDocument(SnapshotId(), BSONObj());
+    if (doc.value().hasExclusivelyOwnedStorage()) {
+        // Reset the document to point to an empty BSON, which will preserve its underlying
+        // DocumentStorage for future users of this WSM.
+        resetDocument(SnapshotId(), BSONObj());
+    } else {
+        // If the Document doesn't exclusively own its storage, don't do anything. Attempting to
+        // assign it a value (even an empty one) would result in an allocation, which we don't want
+        // here, since this function is very much on the hot path.
+    }
     _state = WorkingSetMember::INVALID;
 }