I have a Worker that shares a SharedArrayBuffer with the "main thread". To work correctly, I have to make sure that the worker has access to the SAB before the main thread accesses to it. (EDIT: The code creating the worker has to be in a seperate function (EDIT2: which returns an array pointing to the SAB).) (Maybe, already this is not possible, you'll tell me).
The initial code looks like this:
function init() {
var code = `onmessage = function(event) {
console.log('starting');
var buffer=event.data;
var arr = new Uint32Array(buffer);// I need to have this done before accessing the buffer again from the main
//some other code, manipulating the array
}`
var buffer = new SharedArrayBuffer(BUFFER_ELEMENT_SIZE);
var blob = new Blob([code], { "type": 'application/javascript' });
var url = window.URL || window.webkitURL;
var blobUrl = url.createObjectURL(blob);
var counter = new Worker(blobUrl);
counter.postMessage(buffer);
let res = new Uint32Array(buffer);
return res;
}
function test (){
let array = init();
console.log('main');
//accessing the SAB again
};
The worker code is always executed after test(), the console shows always main, then starting.
Using timeouts does not help. Consider the following code for test:
function test (){
let array = [];
console.log('main');
setTimeout(function(){
array = initSAB();
},0);
setTimeout(function(){
console.log('main');
//accessing the SAB again
},0);
console.log('end');
};
The console shows end first, followed by main, followed by starting.
However, assigning the buffer to a global array outside the test() function does the job, even without timeouts.
My questions are the following:
- why does the worker does not start directly after the message was send (= received?). AFAIK, workers have their own event queue, so they should not rely on the main stack becoming empty?
- Is there a specification detailing when a worker starts working after sending a message?
- Is there a way to make sure the worker has started before accessing the SAB again without using global variables? (One could use busy waiting, but I beware...) There is probably no way, but I want to be sure.
Edit
To be more precise:
- In a completly parallel running scenario, the Worker would be able to handle the message immediately after it was posted. This is obviously not the case.
- Most Browser API (and Worker is such an API) use a callback queue to handle calls to the API. But if this applied, the message would be posted/handled before the timeout calbacks were executed.
- To go even further: If I try busy waiting after postMessage by reading from the SAB until it changes one value will block the program infinitely. For me, it means that the Browser does not posts the message until the call stack is empty As far as I know, this behaviour is not documentated and I cannot explain it.
To summerize: I want to know how the browser determines when to post the message and to handle it by the worker, if the call of postMessage is inside a function. I already found a workaround (global variables), so I'm more interested in how it works behind the scenes. But if someone can show me a working example, I'll take it.
EDIT 2:
The code using the global variable (the code that works fine) looks like this
function init() {
//Unchanged
}
var array = init(); //global
function test (){
console.log('main');
//accessing the SAB again
};
It prints starting, then main to the console.
What is also worth noticing : If I debug the code with the Firefox Browser (Chrome not tested) I get the result I want without the global variable (starting before main) Can someone explain?
