Javascript semaphore / test-and-set / lock?

user_13055 0
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Is there such a thing as an atomic test-and-set, semaphore, or lock in Javascript?

I have javascript invoking async background processes via a custom protocol (the background process literally runs in a separate process, unrelated to the browser). I believe I'm running into a race condition; the background process returns between my test and my set, screwing things up on the javascript side. I need a test-and-set operation to make it a real semaphore.

Here's the javascript code that attempts to detect background processes and queue them up:

Call = function () {

var isRunning = true,
    queue = [];

return  {
    // myPublicProperty: "something",

    call: function (method) {
            if (isRunning) {
                console.log("Busy, pushing " + method);
                queue.push(method);
            } else {
                isRunning = true;
                objccall(method);
            }
        },

        done: function() {
            isRunning = false;
            if (queue.length > 0) {
                Call.call(queue.shift());
            }
        }
    };
}();

Call is a singleton that implements the queuing; anybody that wants to invoke an external process does Call.call("something") .

Any ideas?

concurrency semaphore javascript
7 Answers

JavaScript has no locking semantics because JS is not a multi threaded language. Multiple threads can only operate concurrently in completely distinct contexts -- eg. HTML5 Worker threads, or in things like multiple instances of JavaScriptCore API's context object (I assume SpiderMonkey has a similar concept). They can't have shared state, so in essence all execution is atomic.

Okay, as you have now provided some of your code i assume you have something akin to:

External Process:
<JSObject>.isRunning = true;
doSomething()
<JSObject>.done()

Or some such (using appropriate APIs). In which case I would expect the JS engine to block if JS is executing in the context of your js object (which is what JavaScriptCore would do), failing that you will probably need to put a manual lock in place around js execution.

What engine are you using to do all of this? I ask because based on your description it sounds like you're setting a flag from a secondary thread from a non-JS language using the C/C++ API provided by that language, and most JS engines assume that any state manipulation made via the API will be occurring on a single thread, typically the same thread that all execution occurs on.

olliej_7840

Maybe you could implement a basic integer semaphore, just add the variable into the DOM and lock/unlock it and make sure your functions keep checking it, else timeout them =)

If you are using a framework such as Mootools you could try to handle the flow of the app with events such as onComplete and so on.

user_539910

I had the same issue, here is how I solved it. It works fine for two concurrent processes. If you have three processes or more, it is possible that two processes start together.

var isRunning = false;
...
var id = setInterval(function(){ //loop until isRunning becomes false
            if (!isRunning){
                isRunning = true;
                //do something synchronous or use a promise
                isRunning = false;
                clearInterval(id); // stop the loop here
            }
        , 10);

It is better than the while loop because it solves concurrency/async issues for reading/setting isRunning.

user_115980390

I used the async-mutex package to solve an issue with Expo SQLite not blocking BEGIN TRANSACTION in iOS. You create the mutex and wrap your critical sections in the runExclusive method

    return this.mutex.runExclusive(async () => {
      try {
        await this.executeSqlAsync('begin immediate transaction');
        const tx = new AsyncSQLTransaction(this);
        const rs = await callback(tx);
        await this.executeSqlAsync('commit');
        return rs;
      } catch (error) {
        await this.executeSqlAsync('rollback');
        throw error;
      }
    });
user_2420420
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