From 41eec4740f9fc68190690e5fac071f8ea089b850 Mon Sep 17 00:00:00 2001
From: Stephen Sawchuk <sawchuk@gmail.com>
Date: Mon, 13 Jun 2016 10:26:59 -0400
Subject: [PATCH] datastore: automatically rollback a transaction

---
 lib/datastore/transaction.js | 43 ++++++++++++++++++++++++++----------
 1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/lib/datastore/transaction.js b/lib/datastore/transaction.js
index d78da8b043d0..8b56393839ac 100644
--- a/lib/datastore/transaction.js
+++ b/lib/datastore/transaction.js
@@ -102,8 +102,12 @@ nodeutil.inherits(Transaction, DatastoreRequest);
 /**
  * Commit the remote transaction and finalize the current transaction instance.
  *
+ * If the commit request fails, we will automatically rollback the transaction.
+ *
  * @param {function} callback - The callback function.
  * @param {?error} callback.err - An error returned while making this request.
+ *   If the commit fails, we automatically try to rollback the transaction (see
+ *   {module:datastore/transaction#rollback}).
  * @param {object} callback.apiResponse - The full API response.
  *
  * @example
@@ -114,7 +118,7 @@ nodeutil.inherits(Transaction, DatastoreRequest);
  * });
  */
 Transaction.prototype.commit = function(callback) {
-  var that = this;
+  var self = this;
 
   callback = callback || util.noop;
 
@@ -179,7 +183,7 @@ Transaction.prototype.commit = function(callback) {
       var method = modifiedEntity.method;
       var args = modifiedEntity.args.reverse();
 
-      DatastoreRequest.prototype[method].call(that, args, util.noop);
+      DatastoreRequest.prototype[method].call(self, args, util.noop);
     });
 
   var protoOpts = {
@@ -195,14 +199,29 @@ Transaction.prototype.commit = function(callback) {
 
   this.request_(protoOpts, reqOpts, function(err, resp) {
     if (err) {
-      callback(err, resp);
+      // Rollback automatically for the user.
+      self.rollback(function(err) {
+        if (err) {
+          err.message = [
+            'The commit was not successful and the transaction could not be',
+            'rolled back. You may want to try calling transaction.rollback().',
+            '\n',
+            '\n',
+            err.message
+          ].join('');
+        }
+
+        // Provide the response items from the failed commit to the user. A
+        // successful rollback should be transparent.
+        callback(err, resp);
+      });
       return;
     }
 
     // The `callbacks` array was built previously. These are the callbacks that
     // handle the API response normally when using the DatastoreRequest.save and
     // .delete methods.
-    that.requestCallbacks_.forEach(function(cb) {
+    self.requestCallbacks_.forEach(function(cb) {
       cb(null, resp);
     });
 
@@ -277,10 +296,10 @@ Transaction.prototype.createQuery = function() {
  * });
  */
 Transaction.prototype.delete = function(entities) {
-  var that = this;
+  var self = this;
 
   arrify(entities).forEach(function(ent) {
-    that.modifiedEntities_.push({
+    self.modifiedEntities_.push({
       entity: {
         key: ent
       },
@@ -311,7 +330,7 @@ Transaction.prototype.delete = function(entities) {
  * });
  */
 Transaction.prototype.rollback = function(callback) {
-  var that = this;
+  var self = this;
 
   callback = callback || util.noop;
 
@@ -321,7 +340,7 @@ Transaction.prototype.rollback = function(callback) {
   };
 
   this.request_(protoOpts, {}, function(err, resp) {
-    that.skipCommit = true;
+    self.skipCommit = true;
 
     callback(err || null, resp);
   });
@@ -358,7 +377,7 @@ Transaction.prototype.rollback = function(callback) {
  * });
  */
 Transaction.prototype.run = function(callback) {
-  var that = this;
+  var self = this;
 
   callback = callback || util.noop;
 
@@ -373,7 +392,7 @@ Transaction.prototype.run = function(callback) {
       return;
     }
 
-    that.id = resp.transaction;
+    self.id = resp.transaction;
 
     callback(null, resp);
   });
@@ -491,10 +510,10 @@ Transaction.prototype.run = function(callback) {
  * });
  */
 Transaction.prototype.save = function(entities) {
-  var that = this;
+  var self = this;
 
   arrify(entities).forEach(function(ent) {
-    that.modifiedEntities_.push({
+    self.modifiedEntities_.push({
       entity: {
         key: ent.key
       },