Do not call callbacks twice in async boot scripts

See #252 for details

lib/executor.js

@@ -306,11 +306,11 @@ function runScripts(app, list, callback) {
   async.eachSeries(functions, function(f, done) {
     debug('Running script %s', f.path);
     var cb = function(err) {
-      debug('Async function finished %s', f.path);
+      debug('Async function %s %s', err ? 'failed' : 'finished', f.path);
       done(err);
       // Make sure done() isn't called twice, e.g. if a script returns a
       // thenable object and also calls the passed callback.
-      cb = null;
+      cb = function() {};
     };
     try {
       var result = f.func(app, cb);

test/executor.test.js

@@ -373,6 +373,23 @@ describe('executor', function() {
     });
   });
 
+  describe('with boot script returning a promise and calling callback',
+    function() {
+      before(function() {
+        process.promiseAndCallback = true;
+      });
+
+      after(function() {
+        delete process.promiseAndCallback;
+      });
+
+      it('should only call the callback once', function(done) {
+        // Note: Mocha will fail this test if done() is called twice
+        boot.execute(app, simpleAppInstructions(), done);
+      });
+    }
+  );
+
   describe('for mixins', function() {
     var options;
     beforeEach(function() {

test/fixtures/simple-app/boot/promise-callback.js

@@ -0,0 +1,13 @@
+// Copyright IBM Corp. 2017. All Rights Reserved.
+// Node module: loopback-boot
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+var Promise = require('bluebird');
+
+module.exports = function(app, callback) {
+  callback();
+  if (process.promiseAndCallback) {
+    return Promise.reject();
+  }
+};