From 089509d380e0be952d2de0437a45dfd9bf664ffb Mon Sep 17 00:00:00 2001
From: Chris Eineke <chris@ei.ne.ke>
Date: Wed, 7 Jan 2015 02:15:04 -0500
Subject: [PATCH] Introduce join guard.

---
 guard.js | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/guard.js b/guard.js
index 38924cae..5b9abe3f 100644
--- a/guard.js
+++ b/guard.js
@@ -15,6 +15,7 @@ define(function(require) {
 	var slice = Array.prototype.slice;
 
 	guard.n = n;
+	guard.join = join;
 
 	return guard;
 
@@ -31,7 +32,9 @@ define(function(require) {
 			var args = slice.call(arguments);
 
 			return when(condition()).withThis(this).then(function(exit) {
-				return when(f.apply(this, args))['finally'](exit);
+				return when(f.apply(this, args)).tap(exit);
+			}, function (exit) {
+				return exit;
 			});
 		};
 	}
@@ -68,5 +71,38 @@ define(function(require) {
 		}
 	}
 
+	/**
+	 * Creates a condition that allows only one concurrent execution of a guarded function
+	 */
+	function join() {
+		var first = false;
+		var value;
+		var waiting = [];
+
+		return function enter() {
+			return when.promise(function(resolve, reject) {
+				if (!first) {
+					resolve(exit);
+					first = true;
+				} else {
+					waiting.push([ resolve, reject ]);
+				}
+			});
+		};
+
+		function exit(result) {
+			waiting.forEach(function (waiting) {
+				waiting[1](result);
+			});
+			reset();
+		}
+
+		function reset() {
+			first = false;
+			value = undefined;
+			waiting = [];
+		}
+	}
+
 });
 }(typeof define === 'function' && define.amd ? define : function(factory) { module.exports = factory(require); }));