stub: avoid race in onHalfClose server StreamObserver

stub/src/main/java/io/grpc/stub/ServerCalls.java

@@ -129,6 +129,7 @@ private final class UnaryServerCallListener extends ServerCall.Listener<ReqT> {
       private final ServerCall<ReqT, RespT> call;
       private final ServerCallStreamObserverImpl<ReqT, RespT> responseObserver;
       private boolean canInvoke = true;
+      private boolean wasReady;
       private ReqT request;
 
       // Non private to avoid synthetic class
@@ -171,7 +172,7 @@ public void onHalfClose() {
         method.invoke(request, responseObserver);
         request = null;
         responseObserver.freeze();
-        if (call.isReady()) {
+        if (wasReady) {
           // Since we are calling invoke in halfClose we have missed the onReady
           // event from the transport so recover it here.
           onReady();
@@ -188,6 +189,7 @@ public void onCancel() {
 
       @Override
       public void onReady() {
+        wasReady = true;
         if (responseObserver.onReadyHandler != null) {
           responseObserver.onReadyHandler.run();
         }