SERVER-37779 Step up enforcement that $changeStream must be first

Adds an additional check which enforces a $changeStream stage must be
the first stage in the pipeline which is robust to the case where a
$changeStream is only ever sent to a mongos.

2 files changed, 5 insertions, 2 deletions

diff --git a/src/mongo/db/pipeline/document_source_change_stream_transform.cpp b/src/mongo/db/pipeline/document_source_change_stream_transform.cpp
index 150b310b30c..405a3bd303c 100644
--- a/src/mongo/db/pipeline/document_source_change_stream_transform.cpp
+++ b/src/mongo/db/pipeline/document_source_change_stream_transform.cpp
@@ -434,6 +434,11 @@ DocumentSource::GetModPathsReturn DocumentSourceChangeStreamTransform::getModifi
 DocumentSource::GetNextResult DocumentSourceChangeStreamTransform::getNext() {
     pExpCtx->checkForInterrupt();
 
+    uassert(50988,
+            "Illegal attempt to execute an internal change stream stage on mongos. A $changeStream "
+            "stage must be the first stage in a pipeline",
+            !pExpCtx->inMongos);
+
     // If we're unwinding an 'applyOps' from a transaction, check if there are any documents we have
     // stored that can be returned.
     if (_txnContext) {
diff --git a/src/mongo/db/pipeline/document_source_check_invalidate.cpp b/src/mongo/db/pipeline/document_source_check_invalidate.cpp
index cedc45598ea..6a86fb8491e 100644
--- a/src/mongo/db/pipeline/document_source_check_invalidate.cpp
+++ b/src/mongo/db/pipeline/document_source_check_invalidate.cpp
@@ -62,8 +62,6 @@ bool isInvalidatingCommand(const boost::intrusive_ptr<ExpressionContext>& pExpCt
 DocumentSource::GetNextResult DocumentSourceCheckInvalidate::getNext() {
     pExpCtx->checkForInterrupt();
 
-    invariant(!pExpCtx->inMongos);
-
     if (_queuedInvalidate) {
         const auto res = DocumentSource::GetNextResult(std::move(_queuedInvalidate.get()));
         _queuedInvalidate.reset();