Merge branch 'grpc:master' into transport_call_creds

Author: AshZhang

xds/src/main/java/io/grpc/xds/XdsNameResolver.java

@@ -537,17 +537,21 @@ private boolean retainCluster(String cluster) {
 
     private void releaseCluster(final String cluster) {
       int count = clusterRefs.get(cluster).refCount.decrementAndGet();
+      if (count < 0) {
+        throw new AssertionError();
+      }
       if (count == 0) {
         syncContext.execute(new Runnable() {
           @Override
           public void run() {
-            if (clusterRefs.get(cluster).refCount.get() == 0) {
-              clusterRefs.remove(cluster);
-              if (resolveState.lastConfigOrStatus.hasValue()) {
-                updateResolutionResult(resolveState.lastConfigOrStatus.getValue());
-              } else {
-                resolveState.cleanUpRoutes(resolveState.lastConfigOrStatus.getStatus());
-              }
+            if (clusterRefs.get(cluster).refCount.get() != 0) {
+              throw new AssertionError();
+            }
+            clusterRefs.remove(cluster);
+            if (resolveState.lastConfigOrStatus.hasValue()) {
+              updateResolutionResult(resolveState.lastConfigOrStatus.getValue());
+            } else {
+              resolveState.cleanUpRoutes(resolveState.lastConfigOrStatus.getStatus());
             }
           }
         });