SERVER-39608 Prevent the OplogFetcher from enqueing a document twice when using StartingPoint::kEnqueueFirstDoc

1 files changed, 14 insertions, 2 deletions

diff --git a/src/mongo/db/repl/oplog_fetcher.cpp b/src/mongo/db/repl/oplog_fetcher.cpp
index 8828d7f5e4b..84ebd79b4da 100644
--- a/src/mongo/db/repl/oplog_fetcher.cpp
+++ b/src/mongo/db/repl/oplog_fetcher.cpp
@@ -439,8 +439,16 @@ StatusWith<BSONObj> OplogFetcher::_onSuccessfulBatch(const Fetcher::QueryRespons
 
         LOG(1) << "oplog fetcher successfully fetched from " << _getSource();
 
-        // If this is the first batch, no rollback is needed and we don't want to enqueue the first
-        // document, skip it.
+        // We do not always enqueue the first document. We elect to skip it for the following
+        // reasons:
+        //    1. This is the first batch and no rollback is needed. Callers specify
+        //       StartingPoint::kSkipFirstDoc when they want this behavior.
+        //    2. We have already enqueued that document in a previous attempt. We can get into
+        //       this situation if we had a batch with StartingPoint::kEnqueueFirstDoc that failed
+        //       right after that first document was enqueued. In such a scenario, we would not
+        //       have advanced the lastFetched opTime, so we skip past that document to avoid
+        //       duplicating it.
+
         if (_startingPoint == StartingPoint::kSkipFirstDoc) {
             firstDocToApply++;
         }
@@ -488,6 +496,10 @@ StatusWith<BSONObj> OplogFetcher::_onSuccessfulBatch(const Fetcher::QueryRespons
         return status;
     }
 
+    // Start skipping the first doc after at least one doc has been enqueued in the lifetime
+    // of this fetcher.
+    _startingPoint = StartingPoint::kSkipFirstDoc;
+
     if (_dataReplicatorExternalState->shouldStopFetching(
             _getSource(), replSetMetadata, oqMetadata)) {
         str::stream errMsg;