[feature] Disabling retries is possible now

README.md

@@ -45,6 +45,37 @@ lock("myLock", function(done) {
 });
 ```
 
+### Disabling retries 
+Retries are very useful if you want all operations requesting the locks to
+eventually be executed and completed.
+
+But sometimes you only care about the first lock, and every subsequent attempt
+to acquire the same lock should just be dropped.
+
+You can do this by setting retryDelay timeout to 0:
+
+```javascript
+var client = require("redis").createClient(),
+	lock = require("redis-lock")(client, 0); // <-- disabling retries
+
+// since we disabled retries, a new parameter "error" is passed to the callback
+lock("myLock", function(err, done) {
+	if(!err){
+		// Simulate a 1 second long operation
+		setTimeout(done, 1000);
+	}
+});
+
+// this lock will fail
+lock("myLock", function(err, done) {
+	if(err && err.code == "ALREADY_LOCKED"){
+		// already locked!
+	}else{
+		done()
+	}
+});
+```
+
 ## Installation
 
 	$ npm install redis-lock

index.js

@@ -4,7 +4,7 @@ var util = require('util');
 var defaultTimeout = 5000;
 var promisify = util.promisify || function(x) { return x; };
 
-function acquireLock(client, lockName, timeout, retryDelay, onLockAcquired) {
+function acquireLock(client, lockName, timeout, retryDelay, onLockAcquired, onAlreadyLocked) {
 	function retry() {
 		setTimeout(function() {
 			acquireLock(client, lockName, timeout, retryDelay, onLockAcquired);
@@ -13,7 +13,10 @@ function acquireLock(client, lockName, timeout, retryDelay, onLockAcquired) {
 
 	var lockTimeoutValue = (Date.now() + timeout + 1);
 	client.set(lockName, lockTimeoutValue, 'PX', timeout, 'NX', function(err, result) {
-		if(err || result === null) return retry();
+		if(err || result === null){
+			if(retryDelay) return retry();
+			else return onAlreadyLocked()
+		}
 		onLockAcquired(lockTimeoutValue);
 	});
 }
@@ -23,7 +26,7 @@ module.exports = function(client, retryDelay) {
 		throw new Error("You must specify a client instance of http://github.com/mranney/node_redis");
 	}
 
-	retryDelay = retryDelay || 50;
+	retryDelay = typeof retryDelay === 'undefined' ? 50 : retryDelay;
 
 	function lock(lockName, timeout, taskToPerform) {
 		if(!lockName) {
@@ -38,15 +41,25 @@ module.exports = function(client, retryDelay) {
 		lockName = "lock." + lockName;
 
 		acquireLock(client, lockName, timeout, retryDelay, function(lockTimeoutValue) {
-			taskToPerform(promisify(function(done) {
+			const doneCb = promisify(function(done) {
 				done = done || function() {};
 
 				if(lockTimeoutValue > Date.now()) {
 					client.del(lockName, done);
 				} else {
 					done();
 				}
-			}));
+			})
+
+			if(retryDelay){
+				taskToPerform(doneCb);
+			}else{
+				taskToPerform(null, doneCb);
+			}
+		}, function(){
+			const err = new Error("Locked already acquired")
+			err.code = "ALREADY_LOCKED"
+			taskToPerform(err) // errored acquiring lock
 		});
 	}
 

test/test.js

@@ -1,6 +1,7 @@
 var should = require("should"),
 	redisClient = require("redis").createClient(),
-	lock = require("../index")(redisClient)
+	lock = require("../index")(redisClient),
+	lockWithoutDelay = require("../index")(redisClient, 0),
 	util = require("util");
 
 const delay = ms => new Promise(res => setTimeout(res, ms));
@@ -53,6 +54,35 @@ describe("redis-lock", function() {
 		}
 	});
 
+
+	it("should throw error in second operation if delay is zero and first has lock", function(done) {
+		var errored = 0, success = 0;
+		lockWithoutDelay("testLock", function(err, completed) {
+			setTimeout(function() {
+				if(!err){
+					success++;
+					completed();
+					proceed();
+				}
+			}, 500);	// Longer, started first
+		});
+
+		lockWithoutDelay("testLock", function(err, completed) {
+			if(err && err.code == "ALREADY_LOCKED"){
+				errored++
+			}
+			proceed();
+		});
+
+		function proceed() {
+			if(errored === 1 && success == 1) {
+				errored.should.equal(1);
+				done();
+			}
+		}
+	});
+
+
 	it("shouldn't create a deadlock if the first operation doesn't release the lock within <timeout>", function(done) {
 		var start = new Date();
 		lock("testLock", 300, function(completed) {