Question

Please consider the following code and the explanation from this Mozilla tutorial “Using web workers”:

var myWorker = new Worker('my_worker.js');
myWorker.onmessage = function(event) {
  print("Called back by the worker!\n");
};

Line 1 in this example creates and
starts running the worker thread.
Line 2 sets the onmessage handler for
the worker to a function that is
called when the worker calls its own
postMessage() function.

The thread is started in the moment the Worker constructor is called. I wonder if there might be a race-condition on setting the onmessage handler. For example if the web worker posts a message before onmessage is set.

Does someone know more about this?

Update:

Andrey pointed out that the web worker should start its work, when it receives a message, like in the Fibonacci example in the Mozilla tutorial. But doesn’t that create a new race-condition on setting the onmessage handler in the web worker?

For example:

The main script:

var myWorker = new Worker('worker.js');
myWorker.onmessage = function(evt) {..};
myWorker.postMessage('start');

The web worker script (‘worker.js’)

var result = [];
onmessage = function(evt) {..};

And then consider the following execution path:

main thread                                  web worker
var worker = new Worker("worker.js");
                                             var result = [];
myWorker.onmessage = ..
myWorker.postMessage('start');
                                             onmessage = ..

The “var result = []” line can be left out, it will still be the same effect.

And this is a valid execution path, I tried it out by setting a timeout in the web worker! At the moment I can not see, how to use web workers without running into race-conditions?!

Answer 1

The answer is that both the main script and the web worker have a MessagePort queue which collects the messages before the initial worker script returns.

For details, see this thread on the WHATWG help mailing list:
https://web.archive.org/web/20140401202427/http://lists.whatwg.org/pipermail/help-whatwg.org/2010-August/000606.html