ModelBaseClass: support promise-based observers

Allow the observer functions passed to `ModelBaseClass.observe`
to return a promise instead of calling the callback.

lib/model.js

@@ -567,7 +567,15 @@ ModelBaseClass.notifyObserversOf = function(operation, context, callback) {
 
     async.eachSeries(
       observers,
-      function(fn, next) { fn(context, next); },
+      function notifySingleObserver(fn, next) {
+        var retval = fn(context, next);
+        if (retval && typeof retval.then === 'function') {
+          retval.then(
+            function() { next(); },
+            next // error handler
+          );
+        }
+      },
       function(err) { callback(err, context) }
     );
   });

package.json

@@ -23,6 +23,7 @@
     "node >= 0.6"
   ],
   "devDependencies": {
+    "bluebird": "^2.9.9",
     "mocha": "^2.1.0",
     "should": "^5.0.0"
   },

test/async-observer.test.js

@@ -1,5 +1,6 @@
 var ModelBuilder = require('../').ModelBuilder;
 var should = require('./init');
+var Promise = global.Promise || require('bluebird');
 
 describe('async observer', function() {
   var TestModel;
@@ -93,6 +94,27 @@ describe('async observer', function() {
       done();
     });
   });
+
+  it('resolves promises returned by observers', function(done) {
+    TestModel.observe('event', function(ctx) {
+      return Promise.resolve('value-to-ignore');
+    });
+    TestModel.notifyObserversOf('event', {}, function(err, ctx) {
+      // the test times out when the promises are not supported
+      done();
+    });
+  });
+
+  it('handles rejected promise returned by an observer', function(done) {
+    var testError = new Error('expected test error');
+    TestModel.observe('event', function(ctx) {
+      return Promise.reject(testError);
+    });
+    TestModel.notifyObserversOf('event', {}, function(err, ctx) {
+      err.should.eql(testError);
+      done();
+    });
+  });
 });
 
 function pushAndNext(array, value) {