1 files changed, 31 insertions, 10 deletions

diff --git a/include/grpc++/impl/codegen/async_unary_call.h b/include/grpc++/impl/codegen/async_unary_call.h
index f0f909686b..e472f04f56 100644
--- a/include/grpc++/impl/codegen/async_unary_call.h
+++ b/include/grpc++/impl/codegen/async_unary_call.h
@@ -32,13 +32,18 @@ namespace grpc {
 class CompletionQueue;
 extern CoreCodegenInterface* g_core_codegen_interface;
 
-/// An interface relevant for async client side unary RPCS (which send
+/// An interface relevant for async client side unary RPCs (which send
 /// one request message to a server and receive one response message).
 template <class R>
 class ClientAsyncResponseReaderInterface {
  public:
   virtual ~ClientAsyncResponseReaderInterface() {}
 
+  /// Start the call that was set up by the constructor, but only if the
+  /// constructor was invoked through the "Prepare" API which doesn't actually
+  /// start the call
+  virtual void StartCall() = 0;
+
   /// Request notification of the reading of initial metadata. Completion
   /// will be notified by \a tag on the associated completion queue.
   /// This call is optional, but if it is used, it cannot be used concurrently
@@ -70,9 +75,10 @@ template <class R>
 class ClientAsyncResponseReader final
     : public ClientAsyncResponseReaderInterface<R> {
  public:
-  /// Start a call and write the request out.
+  /// Start a call and write the request out if \a start is set.
   /// \a tag will be notified on \a cq when the call has been started (i.e.
   /// intitial metadata sent) and \a request has been written out.
+  /// If \a start is not set, the actual call must be initiated by StartCall
   /// Note that \a context will be used to fill in custom initial metadata
   /// used to send to the server when starting the call.
   template <class W>
@@ -80,11 +86,11 @@ class ClientAsyncResponseReader final
                                            CompletionQueue* cq,
                                            const RpcMethod& method,
                                            ClientContext* context,
-                                           const W& request) {
+                                           const W& request, bool start) {
     Call call = channel->CreateCall(method, context, cq);
     return new (g_core_codegen_interface->grpc_call_arena_alloc(
         call.call(), sizeof(ClientAsyncResponseReader)))
-        ClientAsyncResponseReader(call, context, request);
+        ClientAsyncResponseReader(call, context, request, start);
   }
 
   // always allocated against a call arena, no memory free required
@@ -92,13 +98,20 @@ class ClientAsyncResponseReader final
     assert(size == sizeof(ClientAsyncResponseReader));
   }
 
+  void StartCall() override {
+    assert(!started_);
+    started_ = true;
+    StartCallInternal();
+  }
+
   /// See \a ClientAsyncResponseReaderInterface::ReadInitialMetadata for
   /// semantics.
   ///
   /// Side effect:
   ///   - the \a ClientContext associated with this call is updated with
   ///     possible initial and trailing metadata sent from the server.
-  void ReadInitialMetadata(void* tag) {
+  void ReadInitialMetadata(void* tag) override {
+    assert(started_);
     GPR_CODEGEN_ASSERT(!context_->initial_metadata_received_);
 
     meta_buf.set_output_tag(tag);
@@ -111,7 +124,8 @@ class ClientAsyncResponseReader final
   /// Side effect:
   ///   - the \a ClientContext associated with this call is updated with
   ///     possible initial and trailing metadata sent from the server.
-  void Finish(R* msg, Status* status, void* tag) {
+  void Finish(R* msg, Status* status, void* tag) override {
+    assert(started_);
     finish_buf.set_output_tag(tag);
     if (!context_->initial_metadata_received_) {
       finish_buf.RecvInitialMetadata(context_);
@@ -125,15 +139,22 @@ class ClientAsyncResponseReader final
  private:
   ClientContext* const context_;
   Call call_;
+  bool started_;
 
   template <class W>
-  ClientAsyncResponseReader(Call call, ClientContext* context, const W& request)
-      : context_(context), call_(call) {
-    init_buf.SendInitialMetadata(context->send_initial_metadata_,
-                                 context->initial_metadata_flags());
+  ClientAsyncResponseReader(Call call, ClientContext* context, const W& request,
+                            bool start)
+      : context_(context), call_(call), started_(start) {
+    // Bind the metadata at time of StartCallInternal but set up the rest here
     // TODO(ctiller): don't assert
     GPR_CODEGEN_ASSERT(init_buf.SendMessage(request).ok());
     init_buf.ClientSendClose();
+    if (start) StartCallInternal();
+  }
+
+  void StartCallInternal() {
+    init_buf.SendInitialMetadata(context_->send_initial_metadata_,
+                                 context_->initial_metadata_flags());
     call_.PerformOps(&init_buf);
   }