Is it possible to use workers in a Greasemonkey script?

Question

I would like to use the Web Worker facility introduced in Firefox 3.5 to enhance a Greasemonkey script I'm working on.

Is this even possible?

I've done some experimentation, but I can't get past the issue of loading a worker script from an arbitrary domain.

For example, this does not work:

var myWorker = new Worker("http://dl.getdropbox.com/u/93604/js/worker.js");

This code generates an error message in my Firebug console:

Failed to load script: http://dl.getdropbox.com/u/93604/js/worker.js (nsresult = 0x805303f4)

Apparently there's a limitation that does not allow you to start a worker from a URL that's not relative to the base URL of the calling script. You can load a worker script at a relative URL like this just fine:

var myWorker = new Worker("worker.js");

But there's no way for me to get the worker script on the user's filesystem so that it could be at a path relative to the calling script.

Am I screwed here? Should I give up on trying to use workers within my Greasemonkey script?

Answer 1

For years I thought it wasn't possible to use web workers in GM. Of course the first idea was to use data-urls. But the Worker constructor didn't seem to accept them.

Today I tried it again and it worked without any problems at first. Only when I started to use functions of the GM API the Worker constructor stopped working.

Seemingly Firefox has a bug that prevents you from accessing Worker from a sandbox with X-ray vision. Even evaluating typeof Worker throws an exception. So the only way to use workers is to get the unwrapped version from the unwrapped window:

var echoWorker = new unsafeWindow.Worker("data:text/javascript," +
    "self.onmessage = function(e) {\n" +
    "    self.postMessage(e.data);\n" +
    "};"
);

Of course you have to be careful about special characters. It's better to encode the script with base64:

var dataURL = 'data:text/javascript;base64,' + btoa(script);
var worker = unsafeWindow.Worker(dataURL);

Alternatively you can also use blob-urls:

var blob = new Blob([script], {type: 'text/javascript'});
var blobURL = URL.createObjectURL(blob);
var worker = new unsafeWindow.Worker(blobURL);
URL.revokeObjectURL(blobURL);

If you really want to use a script hosted on a different domain that's not a problem because same origin policy doesn't apply for GM_xmlhttpRequest:

function createWorkerFromExternalURL(url, callback) {
    GM_xmlhttpRequest({
        method: 'GET',
        url: url,
        onload: function(response) {
            var script, dataURL, worker = null;
            if (response.status === 200) {
                script = response.responseText;
                dataURL = 'data:text/javascript;base64,' + btoa(script);
                worker = new unsafeWindow.Worker(dataURL);
            }
            callback(worker);
        },
        onerror: function() {
            callback(null);
        }
    });
}

Answer 2

By now (10 years later), it's possible to use Web Workers with Firefox 77 and Tampermonkey. I've tested sucessfully using inline workers:

var blob = new Blob(["onmessage = function(e){postMessage('whats up?');console.log(e.data)}"], {type: 'text/javascript'})

var url = URL.createObjectURL(blob)

var worker = new Worker(url)

worker.onmessage = function(e){
  console.log(e.data) 
}

worker.postMessage('hey there!')

With Chrome or other extension like Greasemonkey ou Violentmonkey, i'ts not working because of CSP worker-src (see violation cases at https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/worker-src). This is why is not possible to use HTTP URLs or string as argument of Worker constructor, only works with blob URLs in this very specific case.

Still, there is a catch about the context of Workers. They can't access DOM, window, document or parent objects (see features available to workers at https://www.html5rocks.com/en/tutorials/workers/basics/).

Answer 3

See:

Can I load a web worker script from an absolute URL?