CapnProto - Request to other server in callback

[Thuy Doan]

Hi all,

I am a newbie to CapnProto. I'd like to have your help.

I want to request a function of server2 in the callback of server1.
But I got an exception like below.
Please help me resolve it.
Many thanks!

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x3000000000020)
  * frame #0: 0x000000010035859c libcapnp-rpc-0.10.2.dylib`capnp::VatNetwork<capnp::rpc::twoparty::VatId, capnp::rpc::twoparty::ProvisionId, capnp::rpc::twoparty::RecipientId, capnp::rpc::twoparty::ThirdPartyCapId, capnp::rpc::twoparty::JoinResult>::baseConnect(capnp::AnyStruct::Reader) + 20
    frame #1: 0x0000000100361a2c libcapnp-rpc-0.10.2.dylib`kj::_::TransformPromiseNode<kj::_::Void, kj::Own<kj::AsyncIoStream>, capnp::EzRpcClient::Impl::Impl(kj::StringPtr, unsigned int, capnp::ReaderOptions)::'lambda'(kj::Own<kj::AsyncIoStream>&&), kj::_::PropagateException>::getImpl(kj::_::ExceptionOrValue&) + 512
    frame #2: 0x00000001004d2220 libkj-async-0.10.2.dylib`kj::_::RunnableImpl<kj::_::TransformPromiseNodeBase::get(kj::_::ExceptionOrValue&)::$_31>::run() + 32
    frame #3: 0x000000010028aaa4 libkj-0.10.2.dylib`kj::_::runCatchingExceptions(kj::_::Runnable&) + 40
    frame #4: 0x00000001004c7e48 libkj-async-0.10.2.dylib`kj::_::TransformPromiseNodeBase::get(kj::_::ExceptionOrValue&) + 64
    frame #5: 0x00000001004c8684 libkj-async-0.10.2.dylib`kj::_::ForkHubBase::fire() + 60
    frame #6: 0x00000001004c654c libkj-async-0.10.2.dylib`kj::_::waitImpl(kj::Own<kj::_::PromiseNode>&&, kj::_::ExceptionOrValue&, kj::WaitScope&, kj::SourceLocation) + 608
    frame #7: 0x0000000100005198 client`kj::Promise<capnp::Response<SampleServer1::CallbackRegisterResults> >::wait(kj::WaitScope&, kj::SourceLocation) + 120
    frame #8: 0x0000000100004b94 client`main + 344
    frame #9: 0x000000010003d08c dyld`start + 520  

Sample source code as below

SampleServer1.capnp:

interface SampleServer1 {

    callbackRegister @0 (callback :Callback) -> ();   //to register a callback

    interface Callback {
        calbackFunc @0 (in :Int32) -> ();
    }
}

SampleServer1::Server Impl:

class SampleServer1Impl : public SampleServer1::Server
{
::kj::Promise<void> callbackRegister(CallbackRegisterContext context){
    auto cb = context.getParams().getCallback());

    auto request = cb.calbackFuncRequest();   //Call callback function
    request.setIn(111);
    auto promise = request.send();

    return kj::READY_NOW;
  }
}

SampleServer1::Callback::Server Impl:

class CallbackImpl : public SampleServer1::Callback::Server
{
::kj::Promise<void> calbackFunc(CalbackFuncContext context){

    capnp::EzRpcClient ezClient2("unix:/tmp/capnp-server-2");
    Server2::Client client2 = ezClient2.getMain<SampleServer2>();

    auto& waitScope = ezClient2.getWaitScope();
    {
      auto request = client2.functionSampleRequest();  //Request to SERVER2
      request.setIn(222);
      auto promise = request.send();
     
      promise.wait(waitScope);
      
    }

    return kj::READY_NOW;
  }
}

SampleServer2.capnp:

interface SampleServer2 {

    functionSample @0 (in :Int32) -> ();
}

SampleServer2::Server impl

class SampleServer2 Impl : public SampleServer2::Server
{
::kj::Promise<void> functionSample(FunctionSampleContext context){
    //Do something
    return kj::READY_NOW;
  }
}

Client implement

  capnp::EzRpcClient ezClient("unix:/tmp/capnp-server-1");
  Subscriber::Client client = ezClient.getMain<SampleServer1>();
  auto& waitScope = ezClient.getWaitScope();

 ::SampleServer1::Callback::Client callback = ::SampleServer1::Callback::Client(kj::heap<CallbackImpl>());

  auto request = client.callbackRegisterRequest();      //Register a callback to Server1
  request.setCallback(callback);
  auto promise = request.send();

[Kenton Varda]

Hi Thuy,

Your code doesn't work because it tries to use nested WaitScopes. When a server-side method is called, the event loop is already running, so you have to use promise.then(lambda) instead of promise.wait(waitScope).

So instead of this:

    auto& waitScope = ezClient2.getWaitScope();
    {
      auto request = client2.functionSampleRequest();  //Request to SERVER2
      request.setIn(222);
      auto promise = request.send();
     
      promise.wait(waitScope);
     
    }

    return kj::READY_NOW;

Try:

    auto request = client2.functionSampleRequest();  //Request to SERVER2
    request.setIn(222);

    return request.send()

        .then([context](capnp::Response<Server2::SampleRequestResult> response) {

      // You can handle the response from server2 here if you want.

      // If there's nothing to do, you can replace `.then()` with `.ignoreResult()`.

    })

-Kenton

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[Thuy Doan]

Thank Kento a lot.

That's work for me!

Vào lúc 20:52:34 UTC+7 ngày Thứ Hai, 12 tháng 9, 2022, ken...@cloudflare.com đã viết: