[Webkit-unassigned] [Bug 184285] New: MessagePorts created in WebWorker thread stops working

Tue Apr 3 19:19:04 PDT 2018

https://bugs.webkit.org/show_bug.cgi?id=184285

            Bug ID: 184285
           Summary: MessagePorts created in WebWorker thread stops working
           Product: WebKit
           Version: Safari 11
          Hardware: Macintosh
                OS: macOS 10.13
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: New Bugs
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: ycabon at esri.com

Communication between MessagePort instances created in a WebWorker thread stops after time.
I suspect GC happening.
The following test case is in 2 parts. An html page and a worker script.
The worker creates a message channel and transfer one of the ports back to the main thread.
The ports start communicating then after some 18 exchanges, the communication stops.
I finally figured that if I kept the reference of the MessageChannel that create the ports around the problem disappeared.
I looks like this shouldn't happen.
The test cases creates some arraybuffers and discard them to trigger some GC.

Test case:

//---------------------------------------------------------------------------
// messagechannel_garbagecollected.html
//---------------------------------------------------------------------------

<html>

<body>
  <script>
    var worker = new Worker("./messagechannel_garbagecollected_worker.js");
    var arrays = [];

    worker.addEventListener("message", (event) => {
      var port = event.data.port;

      port.addEventListener("message", (event) => {
        document.body.innerHTML += event.data + "<br />";
        if (arrays.length === 10) {
          arrays = [];
        }
        arrays.push(new ArrayBuffer(500000));
        setTimeout(function() {
          port.postMessage("ping");
        }, 50);
      });

      port.start();
      port.postMessage("ping");
    });

  </script>
</body>

</html>

//---------------------------------------------------------------------------
// messagechannel_garbagecollected_worker.js
//---------------------------------------------------------------------------

var arrays = [];

function startup() {
  var channel = new MessageChannel();
  var port = channel.port1;
  var id = 0;

  port.addEventListener("message", (event) => {
    if (arrays.length === 10) {
      arrays = [];
    }
    arrays.push(new ArrayBuffer(50000));
    setTimeout(function() {
      port.postMessage("pong " + id++);
    }, 50);
  });
  port.start();

  // keeping a dummy reference to the channel keeps it working
  // port["channel"] = channel;

  self.postMessage({
    port: channel.port2
  }, [channel.port2]);
}

startup();

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