The following conditions create a situation where an operation hangs indefinitely:

1. During a replSetReconfig, the variable _currentCommittedSnapshot is set to boost::none in ReplicationCoordinatorImpl::_dropAllSnapshots_inlock.
2. ReplicationCoordinatorImpl::_doneWaitingForReplication_inlock checks !_currentCommittedSnapshot, and returns false if that condition is true.
3. During a replSetReconfig, ReplicationCoordinatorImpl::_wakeReadyWaiters_inlock is called, which signals and removes ThreadWaiters if they are done waiting for replication.

The bug occurs when a ThreadWaiter is removed from the waiter list by the thread performing the replSetReconfig because _doneWaitingForReplication_inlock returns true, but that same thread nulls out _currentCommittedSnapshot, and therefore operation thread's call to _doneWaitingForReplication_inlock returns false. The ThreadWaiter was removed from the list of waiters, and can never be signaled again.

I've detailed this problem in a blog post on my company's engineering blog: https://engineering.vena.io/2021/02/19/what-to-do-when-mongo-3-6-wont-return-your-calls/

I'm opening this against 3.6.20 because the support policy (https://www.mongodb.com/support-policy) shows support extending until April 2021.

I've tested the following patch, and it seems to fix it:

diff --git a/src/mongo/db/repl/replication_coordinator_impl.cpp b/src/mongo/db/repl/replication_coordinator_impl.cpp

index a6a4d0084b..b88b882046 100644

--- a/src/mongo/db/repl/replication_coordinator_impl.cpp

+++ b/src/mongo/db/repl/replication_coordinator_impl.cpp

@@ -1707,6 +1707,17 @@ Status ReplicationCoordinatorImpl::_awaitReplication_inlock(

         if (!stepdownStatus.isOK()) {

             return stepdownStatus;

         }

+

+        // If a replSetReconfig occurred, then all snapshots will be dropped.

+        // `_doneWaitingForReplication_inlock` will fail if there is no current snapshot, and

+        // if this thread's waiter was signaled and removed from the wait list during

+        // replSetReconfig we will enter waitForConditionOrInterruptNoAssertUntil above and

+        // condVar will never be notified.

+        //

+        // If it's null, wait for newly committed snapshot here.

+        while (!_currentCommittedSnapshot) {

+            opCtx->waitForConditionOrInterrupt(_currentCommittedSnapshotCond, *lock);

+        }

     }

     return _checkIfWriteConcernCanBeSatisfied_inlock(writeConcern);

But I also believe that cherry picking https://github.com/mongodb/mongo/commit/fe1b92cee5c133e82845ffbd31b25ab5b66084d3 would fix this issue as well (note I haven't tested this). I was able to reproduce this issue on versions 4.0.0 and 4.0.1, but not 4.0.2, and that commit exists between 4.0.1 and 4.0.2.