5 ms·

I once had to write a lock for JavaScript, it was beautifully simple.

• vanderZwan 9y ago
  You're making me curious enough to want to see it. Would it fit in a post here or on a short gist?
  • HeyImAlex 9y ago
    It can just be a boolean variable called locked?
    • deleted 9y ago
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    • deleted 9y ago
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    • nitely 9y ago
      Almost. In threaded programming a thread will wait/block until the lock is released and then make use of the protected resource. In async world you would probably just check some flag and if it's "locked" then register a callback to some "unlocked" event to mimic this.
      • amelius 9y ago
        "Promises" are a lot like locks.
      • anonacct37 9y ago
        Lookup twisted deferred locks. https://twistedmatrix.com/documents/9.0.0/api/twisted.internet.defer.DeferredLock.html https://twistedmatrix.com/documents/9.0.0/api/twisted.intern...
        • nitely 9y ago
          Looks nice. How is this implemented?
          • rdtsc 9y ago
            Not gp, but I used Twisted Matrix for a few years. It is usually implemented with a boolean flag and a list of client waiting to be called back when the lock is released. So users of the lock would call acquire and get back a deferred (a promise in other language), but say in the most basic way, a callback is passed to it. When lock can be acquired, that callback is called (or deferred is fired). When done with the resource, must call the release. Release then will trigger the lock to inspect the list of available waiting clients, pick one and fire their callback letting them know they can access the resource, and so on. https://github.com/twisted/twisted/blob/trunk/src/twisted/internet/defer.py#L1581 https://github.com/twisted/twisted/blob/trunk/src/twisted/in...
    • ChristianBundy 9y ago
      Yep. let lock; const log = (msg) => { lock = new Promise(async (resolve) => { await lock; console.log(msg); setTimeout(resolve, 1000); }); }; 'hello world'.split('').forEach(log);
  • foota 9y ago
    I don't have access to the code anymore, unfortunately. But it kept track of a list of people waiting on the lock and then notified the first caller off the list when the lock was released. It was only beautifully simple with respect to what it would have been in a non single-threaded evnironment. I needed it in order to use a library that had some annoying global state in it.
  • johnbender 9y ago
    Not sure if this helps but I hacked this together the other day to avoid adding a real queue to a very simple application. It uses event queue ordering semantics to get FIFO behavior. The read/write combinations to the semaphore variable need not be atomic since we know that if this thread is running there are no other threads that can be reading and then modifying it. MyObject.queueSemaphore = N; MyObject.emitter = new EventEmitter(); MyObject.queue = function( ... ){ // if this run can take place go // otherwise wait for a run-complete to try again if(this.queueSemaphore > 0) { this.queueSemaphore--; } else { // must return a promise that resolves and possibly requeues return new promise((resolve, reject) => { this.emitter.once("run-complete", () => { resolve(true); }); }).then(() => this.queue( ... )); } this.promiseBasedProcess() // ... work .finally(() => { this.queueSemaphore++; this.emitter.emit("run-complete"); }); }
  • johnsonjo 9y ago
    Here's a lock implementation I made just barely as a proof of concept: https://bl.ocks.org/johnsonjo4531/99256568deaf8c0f1685793a4a8205bb https://bl.ocks.org/johnsonjo4531/99256568deaf8c0f1685793a4a... Basically it is used like this: ``` var lock = new AsyncLock(); (async () => { for await(var _ of lock) { console.log("here 1a"); await sleep(1000); console.log("here 1b") } })(); (async () => { for await(var _ of lock) { console.log("here 2a"); await sleep(1000); console.log("here 2b") } })(); /* When Lock is working should print: here 1a here 1b here 2a here 2b Without the lock it would print: here 1a here 2a here 1b here 2b */ ``` If you want to check out the implementation go to the blocks link. (which is basically just a gist that you can run). Here's the actual gist link: https://gist.github.com/johnsonjo4531/99256568deaf8c0f1685793a4a8205bb https://gist.github.com/johnsonjo4531/99256568deaf8c0f168579.... It requires Chrome 63 (at this point Chrome Canary) because it uses async iterators. I used this because I can close the lock automatically in case an error is thrown, the loop is broken. It only iterates one value and only after the lock is unlocked.