websockets - calling dataAvailableForRead can cause errors

[schveiguy]

If you have a simple web page which just sends hello every 1 msec, and then you have the server read in a loop like this:

void handleConn(scope WebSocket sock)
{
    while (sock.connected) {
        if(sock.dataAvailableForRead) {
            auto msg = sock.receiveText();
            writeln(msg, " msg.length=", msg.length);
        }
        sleep(1.msecs);
    }
}

You will run into a crash.

The issue is that when a WebSocket is created, it starts a read task, which reads messages as they come in.

But the call to dataAvailableForRead boils down to also trying a read if there is no data available.

Having both these tasks reading at the same time causes this assert to fail:

https://github.com/vibe-d/vibe-core/blob/master/source/vibe/core/net.d#L663

The sequence of events is:

1. Task A gets the next frame off the web socket via

   vibe-http/source/vibe/http/websockets.d

   Line 991 in 7e4c53e

   skipFrame(); // reads the first frame

   . This starts a read call on the socket, which asynchronously waits for data
2. Task B calls dataAvailableForRead on the socket, which also sets up a read call.
3. The web messages are coming in with the correct timing such that both reads succeed, but not in sync. Therefore, the assert is hit because there is still data on the buffer.

At least, I believe this is the case, it is hard to tell.

I think the correct fix is to have dataAvailableForRead to avoid attempting a read on websockets, and just only check for buffer data available.