Transaction is left open when error happens in Datastore API

This one is hard to reproduce, but I noticed that when using a transaction with a with statement and an error happens inside the transaction itself (a 500 error for example), it appears that the transaction is left open.

with self.client.transaction():
  # do stuff

# create entities, put, put_multi, etc

Exception:

Traceback (most recent call last):
  File "my_client.py", line 190, in do_stuff
    with self.client.transaction():
  File "site-packages/gcloud/datastore/batch.py", line 275, in __enter__
    self.begin()
  File "site-packages/gcloud/datastore/transaction.py", line 131, in begin
    self._id = self.connection.begin_transaction(self.project)
  File "site-packages/gcloud/datastore/connection.py", line 307, in begin_transaction
    _datastore_pb2.BeginTransactionResponse)
  File "site-packages/gcloud/datastore/connection.py", line 124, in _rpc
    data=request_pb.SerializeToString())
  File "site-packages/gcloud/datastore/connection.py", line 98, in _request
    raise make_exception(headers, error_status.message, use_json=False)
InternalServerError: 500 internal error.

After the exception, any operations to the Datastore using the same client have no effect, and no error is reported.
So my guess is that for some reason the transaction was left open and the changes are never committed.