Change behavior to properly account for possibility of NULL

tag. This can happen if the tag is actually an integer being
typecast to void*

To avoid breaking the API of existing Next calls, I've made
a new AsyncNext method with a tri-state return that indicates
whether there is a shutdown, an actual event, or a timeout.

Still needs proper testing for the AsyncNext method specifically.

include/grpc++/completion_queue.h

@@ -76,10 +76,19 @@ class CompletionQueue {
   explicit CompletionQueue(grpc_completion_queue *take);
   ~CompletionQueue();
 
+  // Tri-state return for Next: SHUTDOWN, GOT_EVENT, TIMEOUT
+  enum NextStatus {SHUTDOWN, GOT_EVENT, TIMEOUT};
+
   // Blocking (until deadline) read from queue.
-  // Returns false if the queue is ready for destruction, true otherwise
+  // Returns false if the queue is ready for destruction, true if event
-  // If the deadline passed, *tag will be null
+
-  bool Next(void **tag, bool *ok, gpr_timespec deadline=gpr_inf_future);
+  bool Next(void **tag, bool *ok) {
+    return (AsyncNext(tag,ok,gpr_inf_future) != SHUTDOWN);
+  }
+
+  // Nonblocking (until deadline) read from queue.
+  // Cannot rely on result of tag or ok if return is TIMEOUT
+  NextStatus AsyncNext(void **tag, bool *ok, gpr_timespec deadline);
 
   // Shutdown has to be called, and the CompletionQueue can only be
   // destructed when false is returned from Next().

src/cpp/common/completion_queue.cc

@@ -57,24 +57,23 @@ class EventDeleter {
   }
 };
 
-bool CompletionQueue::Next(void** tag, bool* ok, gpr_timespec deadline) {
+CompletionQueue::NextStatus CompletionQueue::AsyncNext(void** tag, bool* ok,
+                                                       gpr_timespec deadline) {
   std::unique_ptr<grpc_event, EventDeleter> ev;
 
   for (;;) {
     ev.reset(grpc_completion_queue_next(cq_, deadline));
     if (!ev) { /* got a NULL back because deadline passed */
-      *ok = true;
+      return TIMEOUT;
-      *tag = nullptr;
-      return true;
     }
     if (ev->type == GRPC_QUEUE_SHUTDOWN) {
-      return false;
+      return SHUTDOWN;
     }
     auto cq_tag = static_cast<CompletionQueueTag*>(ev->tag);
     *ok = ev->data.op_complete == GRPC_OP_OK;
     *tag = cq_tag;
     if (cq_tag->FinalizeResult(tag, ok)) {
-      return true;
+      return GOT_EVENT;
     }
   }
 }