GRPC error `Only one write can be pending at a time`

[tom....@gmail.com]

Hi Everyone,

I'm using grpc in combination with rx observables and now I'm running into

an issue where I sometimes get the following exception:

System.InvalidOperationException

Message=Only one write can be pending at a time

at Grpc.Core.Internal.AsyncCallServer`2.CheckSendAllowedOrEarlyResult()

I've been searching for what this means and how to resolve this and came
across a stack overflow post that mentions that there can be only one in-flight
message at any one time. However, I think I did that with my implementation:

public override Task MachineStatusStream(
    StatusStreamRequest request,
    grpc.IServerStreamWriter<StatusStreamResponse> responseStream,
    grpc.ServerCallContext context)

{

    return _alarmPresenter.AlarmStatusStream()
        .Select(status => status.ToGrpc())
        .Select(status => new StatusStreamResponse { Status = status })
        .ForEachAsync(response =>
            responseStream.WriteAsync(response).Wait(context.CancellationToken));

}

Isn't the `.Wait()` in the write async enough to guarantee that the write is finished?

Any pointers or help would be highly appreciated!

Best regards,

Tom

[Robert Engels]

You are using foreachasync so each (multiple in parallel) will perform the write before the wait is hit, which is not valid. Just use foreach instead and drop the wait, or sync on the write. 

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