allow null tag

src/cpp/server/server_context.cc

diff --git a/src/cpp/server/server_context.cc b/src/cpp/server/server_context.cc
index 0d09519..0437339 100644
--- a/src/cpp/server/server_context.cc
+++ b/src/cpp/server/server_context.cc
@@ -50,18 +50,22 @@
 class ServerContext::CompletionOp GRPC_FINAL : public CallOpSetInterface {
  public:
   // initial refs: one in the server context, one in the cq
-  CompletionOp() : refs_(2), finalized_(false), cancelled_(0) {}
+  CompletionOp() : has_tag_(false), tag_(nullptr), refs_(2), finalized_(false), cancelled_(0) {}
 
   void FillOps(grpc_op* ops, size_t* nops) GRPC_OVERRIDE;
   bool FinalizeResult(void** tag, bool* status) GRPC_OVERRIDE;
 
   bool CheckCancelled(CompletionQueue* cq);
 
-  void set_tag(void* tag) { tag_ = tag; }
+  void set_tag(void* tag) {
+    has_tag_ = true;
+    tag_ = tag;
+  }
 
   void Unref();
 
  private:
+  bool has_tag_;
   void* tag_;
   grpc::mutex mu_;
   int refs_;
@@ -94,7 +98,7 @@
   grpc::unique_lock<grpc::mutex> lock(mu_);
   finalized_ = true;
   bool ret = false;
-  if (tag_) {
+  if (has_tag_) {
     *tag = tag_;
     ret = true;
   }
@@ -110,6 +114,7 @@
 
 ServerContext::ServerContext()
     : completion_op_(nullptr),
+      has_notify_when_done_tag_(false),
       async_notify_when_done_tag_(nullptr),
       call_(nullptr),
       cq_(nullptr),
@@ -118,6 +123,7 @@
 ServerContext::ServerContext(gpr_timespec deadline, grpc_metadata* metadata,
                              size_t metadata_count)
     : completion_op_(nullptr),
+      has_notify_when_done_tag_(false),
       async_notify_when_done_tag_(nullptr),
       deadline_(deadline),
       call_(nullptr),
@@ -143,7 +149,9 @@
 void ServerContext::BeginCompletionOp(Call* call) {
   GPR_ASSERT(!completion_op_);
   completion_op_ = new CompletionOp();
-  completion_op_->set_tag(async_notify_when_done_tag_);
+  if (has_notify_when_done_tag_) {
+    completion_op_->set_tag(async_notify_when_done_tag_);
+  }
   call->PerformOps(completion_op_);
 }