Fix racy promises in SpecTestRunner (#2693)

Author: Brian Chen

packages/firestore/src/util/async_queue.ts

@@ -372,9 +372,16 @@ export class AsyncQueue {
    * Waits until all currently queued tasks are finished executing. Delayed
    * operations are not run.
    */
-  drain(): Promise<void> {
-    // It should still be possible to drain the queue after it is shutting down.
-    return this.enqueueEvenAfterShutdown(() => Promise.resolve());
+  async drain(): Promise<void> {
+    // Operations in the queue prior to draining may have enqueued additional
+    // operations. Keep draining the queue until the tail is no longer advanced,
+    // which indicates that no more new operations were enqueued and that all
+    // operations were executed.
+    let currentTail: Promise<unknown>;
+    do {
+      currentTail = this.tail;
+      await currentTail;
+    } while (currentTail !== this.tail);
   }
 
   /**

packages/firestore/test/unit/specs/limbo_spec.test.ts

@@ -476,12 +476,12 @@ describeSpec('Limbo Documents:', [], () => {
         .expectLimboDocs(docB.key, docC.key)
         .client(1)
         .stealPrimaryLease()
+        .expectListen(query, 'resume-token-1000000')
         .client(0)
         .runTimer(TimerId.ClientMetadataRefresh)
         .expectPrimaryState(false)
         .expectLimboDocs()
         .client(1)
-        .expectListen(query, 'resume-token-1000000')
         .watchAcksFull(query, 3 * 1e6)
         .expectLimboDocs(docB.key, docC.key)
         .ackLimbo(3 * 1e6, deletedDocB)