Fix server shutdown ordering issue

src/cpp/server/server_cc.cc

@@ -1009,9 +1009,6 @@ Server::Server(
 Server::~Server() {
   {
     grpc::internal::ReleasableMutexLock lock(&mu_);
-    if (callback_cq_ != nullptr) {
-      callback_cq_->Shutdown();
-    }
     if (started_ && !shutdown_) {
       lock.Unlock();
       Shutdown();
@@ -1020,6 +1017,10 @@ Server::~Server() {
       for (const auto& value : sync_req_mgrs_) {
         value->Shutdown();
       }
+      if (callback_cq_ != nullptr) {
+        callback_cq_->Shutdown();
+        callback_cq_ = nullptr;
+      }
     }
   }
 
@@ -1315,6 +1316,13 @@ void Server::ShutdownInternal(gpr_timespec deadline) {
         &callback_reqs_mu_, [this] { return callback_reqs_outstanding_ == 0; });
   }
 
+  // Shutdown the callback CQ. The CQ is owned by its own shutdown tag, so it
+  // will delete itself at true shutdown.
+  if (callback_cq_ != nullptr) {
+    callback_cq_->Shutdown();
+    callback_cq_ = nullptr;
+  }
+
   // Drain the shutdown queue (if the previous call to AsyncNext() timed out
   // and we didn't remove the tag from the queue yet)
   while (shutdown_cq.Next(&tag, &ok)) {