xds: Assert XdsNR's cluster ref counting is consistent

It is much harder to debug refcounting problems when we ignore
impossible situations. So make such impossible cases complain loudly so
the bug is obvious.

Author: ejona86

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());
             }
           }
         });