c++ - grpc c++ 异步完成队列事件

Question

我试图理解 grpc c++ 异步模型流程。这篇文章(link)已经解释了我的很多疑惑。这是 grpc_asycn_server 的代码。为了了解 CompletionQueue 何时获取请求，我添加了一些打印语句，如下所示:

首先在 HandleRpcs() 函数中。

void HandleRpcs() {
    // Spawn a new CallData instance to serve new clients.
    new CallData(&service_, cq_.get());
    void* tag;  // uniquely identifies a request.
    bool ok;
    int i = 0;
    while (true) {
      std::cout << i << std::endl; ///////////////////////////////
      // Block waiting to read the next event from the completion queue. The
      // event is uniquely identified by its tag, which in this case is the
      // memory address of a CallData instance.
      // The return value of Next should always be checked. This return value
      // tells us whether there is any kind of event or cq_ is shutting down.
      GPR_ASSERT(cq_->Next(&tag, &ok));
      GPR_ASSERT(ok);
      static_cast<CallData*>(tag)->Proceed();
      i++;
    }
  }

以及在proceed()函数内部:

void Proceed() {
  if (status_ == CREATE) {
    // Make this instance progress to the PROCESS state.
    status_ = PROCESS;

    // As part of the initial CREATE state, we *request* that the system
    // start processing SayHello requests. In this request, "this" acts are
    // the tag uniquely identifying the request (so that different CallData
    // instances can serve different requests concurrently), in this case
    // the memory address of this CallData instance.
    std::cout<<"RequestSayHello called"<<std::endl; ////////////////////////////
    service_->RequestSayHello(&ctx_, &request_, &responder_, cq_, cq_,
                              this);
  } else if (status_ == PROCESS) {
    // Spawn a new CallData instance to serve new clients while we process
    // the one for this CallData. The instance will deallocate itself as
    // part of its FINISH state.
    new CallData(service_, cq_);

    // The actual processing.
    std::string prefix("Hello ");
    reply_.set_message(prefix + request_.name());

    // And we are done! Let the gRPC runtime know we've finished, using the
    // memory address of this instance as the uniquely identifying tag for
    // the event.
    status_ = FINISH;
    responder_.Finish(reply_, Status::OK, this);
  } else {
    std::cout<<"deallocated"<<std::endl; ////////////////////////////
    GPR_ASSERT(status_ == FINISH);
    // Once in the FINISH state, deallocate ourselves (CallData).
    delete this;
  }
}

一旦我运行服务器和一个客户端( client )，服务器就会打印以下内容:

RequestSayHello called
i = 0
RequestSayHello called
i = 1
deallocated
i = 2

第二个 RequestSayHello 调用 是有意义的，因为创建了新的 CallData 实例。我的问题是为什么proceed()函数第二次执行并打印dealloced？

Answer 1

完成队列 (cq_) 结构处理多种类型的事件，包括请求事件和响应事件。第一次调用 proceed() 进入 CallData 对象状态机的 PROCESS 阶段。

在此阶段:
1. 创建一个新的CallData对象；正如您提到的，这会将请求事件插入到 cq_ 中
2. 使用回复对象调用responder_；这会将响应事件插入到 cq_

中

收到来自 cq_ 的响应事件后，会在第一个 CallData 对象上再次调用 proceed()，该对象现在位于 FINISH 状态，因此执行清理并打印 dealulated。