Wait for background tasks finished before releasing remoteCache.

So that the last task won't try to shutdown the remoteCache, otherwise, might cause deadlocks.

Fixes #21568.

PiperOrigin-RevId: 635746896
Change-Id: Ic321216dd0bda05ef1ad18367c8ebbcd1a1d272c

src/main/java/com/google/devtools/build/lib/remote/RemoteExecutionService.java

@@ -1596,15 +1596,19 @@ public void shutdown() {
     }
 
     if (remoteCache != null) {
-      remoteCache.release();
-
       try {
         backgroundTaskPhaser.awaitAdvanceInterruptibly(backgroundTaskPhaser.arrive());
       } catch (InterruptedException e) {
         buildInterrupted.set(true);
         remoteCache.shutdownNow();
         Thread.currentThread().interrupt();
       }
+
+      // Only release the remoteCache once all background tasks have been finished. Otherwise, the
+      // last task might try to close the remoteCache inside the callback of network response which
+      // might cause deadlocks.
+      // See https://github.com/bazelbuild/bazel/issues/21568.
+      remoteCache.release();
     }
 
     if (remoteExecutor != null) {