From a PR comment.

In ReshardingRecipientService, _changeStreamsMonitorStarted SharedPromise is fulfilled 4 times, three of those times under exclusive lock. But in _createAndStartChangeStreamsMonitor, the promise is fulfilled without holding an exclusive lock.

Code snippet:

    if (!_metadata.getPerformVerification() || _skipCloningAndApplying ||
        inPotentialAbortScenario(_recipientCtx.getState()) ||
        _changeStreamsMonitorStarted.getFuture().isReady() ||
        _changeStreamsMonitorCompleted.getFuture().isReady()) {
        return;
    }

    _createChangeStreamsMonitor(executor, factory);
    _changeStreamsMonitorStarted.emplaceValue();

The check of isReady will catch most cases of double fulfillment, but there is a window here where isReady returns false, but then the SharedPromise is fulfilled before emplaceValue is called. This would cause a exception, as a SharedPromise can only be fulfilled once.

The fix is to put the isReady/emplaceValue sequence under an exclusive lock.