Fix #136: roll back transactions on exceptions.

Cherry-pick:  3221122

gcloud/datastore/test_transaction.py

@@ -112,7 +112,6 @@ def test_context_manager_no_raise(self):
  self.assertEqual(xact.id(), None)
 
  def test_context_manager_w_raise(self):
- # See https://github.com/GoogleCloudPlatform/gcloud-python/issues/136
  class Foo(Exception):
  pass
  _DATASET = 'DATASET'
@@ -127,13 +126,10 @@ class Foo(Exception):
  self.assertTrue(connection._xact is xact)
  raise Foo()
  except Foo:
- pass # XXX
- # self.assertEqual(xact.id(), None)
- # self.assertEqual(connection._rolled_back, _DATASET))
- # self.assertEqual(connection._xact, None)
- # XXX should *not* have committed
- self.assertEqual(connection._committed, (_DATASET, mutation))
- # self.assertEqual(connection._committed, None)
+ self.assertEqual(xact.id(), None)
+ self.assertEqual(connection._rolled_back, _DATASET)
+ self.assertEqual(connection._xact, None)
+ self.assertEqual(connection._committed, None)
  self.assertTrue(connection._xact is None)
  self.assertEqual(xact.id(), None)
 

gcloud/datastore/transaction.py

@@ -21,18 +21,15 @@ class Transaction(object):
  ... entity1.save()
  ... entity2.save()
 
- To rollback a transaction if there is an error::
+ By derault, the transaction is rolled back is an error::
 
  >>> from gcloud import datastore
  >>> dataset = datastore.get_dataset('dataset-id', email, key_path)
  >>> with dataset.transaction() as t:
- ... try:
- ... do_some_work()
- ... entity1.save()
- ... except:
- ... t.rollback()
+ ... do_some_work()
+ ... raise Exception() # rolls back
 
- If the transaction isn't rolled back,
+ If the transaction block exists without an exception,
  it will commit by default.
 
  .. warning::
@@ -249,4 +246,7 @@ def __enter__(self):
  return self
 
  def __exit__(self, exc_type, exc_val, exc_tb):
- self.commit()
+ if exc_type is None:
+ self.commit()
+ else:
+ self.rollback()