[ DDS ] Fix race condition in Mutex

The lock was being released but not reacquired when the next request
resumed, potentially leading to a race.

Change-Id: Ib00ae40ca8c1dfff6b98bbf642032c57d498fa40
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/394760
Auto-Submit: Ben Konyi <[email protected]>
Reviewed-by: Jessy Yameogo <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>

Author: bkonyi

pkg/dds/lib/src/utils/mutex.dart

@@ -72,11 +72,15 @@ class Mutex {
   }
 
   void _releaseLock() {
-    _locked = false;
     if (_outstandingRequests.isNotEmpty) {
       final request = _outstandingRequests.removeFirst();
       request.complete();
+      return;
     }
+    // Only release the lock if no other requests are pending to prevent races
+    // between the next request from the queue to be handled and incoming
+    // requests.
+    _locked = false;
   }
 
   int _weakGuards = 0;