Attempting to call Write() multiple times from separate thread(s) causes crash [gRPC] [C++]

Question

I'm attempting to write an async streaming gRPC server (following this example) in C++ where multiple calls to Write are performed on a separate thread. Unfortunately, this causes a SIGSEGV on my system. The server is able to perform one write before it crashes. The below code provides a simple example of what I'm attempting to do. The overloaded call operator receives a message from a separate thread and executes the Write() call, writing MyMessage to the stream.

void MyServer::HandleRpcs() {
    new CallData(&m_service, m_queue.get());
    void* tag;
    bool ok;
    while (true) {
        GPR_ASSERT(m_queue->Next(&tag, &ok));
        GPR_ASSERT(ok);
        static_cast<CallData*>(tag)->Proceed();
    }
}

void MyServer::CallData::Proceed() {
    if (m_state == CREATE) {
        m_state = PROCESS;
        m_service->RequestRpc(&m_context, &m_request, &m_responder, m_queue, m_queue, this);
    }
    else if (m_state == PROCESS) {
        new CallData(m_service, m_queue);
        // Request the RPC here, which begins the message calls to the overloaded () operator
    }
    else {
        GPR_ASSERT(m_state == FINISH);
        delete this;
    }
}

void MyServer::CallData::operator()(Message message) {
    std::lock_guard<std::recursive_mutex> lock{m_serverMutex};
    MyStream stream;
    stream.set_message(message.payload);
    m_responder.Write(stream, this);
    PushTaskToQueue();
}

void MyServer::CallData::PushTaskToQueue() {
    // m_alarm is a member of CallData
    m_alarm.Set(m_queue, gpr_now(gpr_clock_type::GPR_CLOCK_REALTIME), this);
}

Answer 1

Turns out I had a misunderstanding of gRPC and the completion queue. I was calling Write() before the completion queue returns the tag, which caused the crash. To resolve this, I created a static void* member variable in MyServer called m_tag and passed it into the Next function's tag parameter, like so:

GPR_ASSERT(m_queue->Next(&m_tag, &ok));

Then, I checked if the tag matches up with the handler's this pointer in the overloaded call operator:

if (m_tag != this) return;

And I then saw my message stream come through.