spanner: consider exporting Transaction._rolled_back as Transaction.rolled_back

[odeke-em]

I am currently dealing with a situation where a Transaction might have been rolled back but the exception wasn't directly passed back to me as per

======================================================================
ERROR: test_concurrent_delete_with_save (basic.tests.ConcurrentSaveTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/google/api_core/grpc_helpers.py", line 57, in error_remapped_callable
    return callable_(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/grpc/_channel.py", line 565, in __call__
    return _end_unary_response_blocking(state, call, False, None)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/grpc/_channel.py", line 467, in _end_unary_response_blocking
    raise _Rendezvous(state, None, None, deadline)
grpc._channel._Rendezvous: <_Rendezvous of RPC that terminated with:
	status = StatusCode.FAILED_PRECONDITION
	details = "Cannot start a read or query within a transaction after Commit() or Rollback() has been called."
	debug_error_string = "{"created":"@1580864794.999511000","description":"Error received from peer ipv6:[2607:f8b0:4007:803::200a]:443","file":"src/core/lib/surface/call.cc","file_line":1046,"grpc_message":"Cannot start a read or query within a transaction after Commit() or Rollback() has been called.","grpc_status":9}"
>

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Users/emmanuelodeke/Library/Python/3.7/lib/python/site-packages/spanner/dbapi/cursor.py", line 89, in execute
    self.__handle_insert(self.__get_txn(), sql, args or None)
  File "/Users/emmanuelodeke/Library/Python/3.7/lib/python/site-packages/spanner/dbapi/cursor.py", line 139, in __handle_insert
    param_types=param_types,
  File "/Users/emmanuelodeke/Library/Python/3.7/lib/python/site-packages/spanner/dbapi/cursor.py", line 356, in handle_txn_exec_with_retry
    return txn_method(*args, **kwargs)
  File "/Users/emmanuelodeke/Library/Python/3.7/lib/python/site-packages/google/cloud/spanner_v1/transaction.py", line 202, in execute_update
    metadata=metadata,
  File "/Users/emmanuelodeke/Library/Python/3.7/lib/python/site-packages/google/cloud/spanner_v1/gapic/spanner_client.py", line 810, in execute_sql
    request, retry=retry, timeout=timeout, metadata=metadata
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/google/api_core/gapic_v1/method.py", line 143, in __call__
    return wrapped_func(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/google/api_core/retry.py", line 277, in retry_wrapped_func
    on_error=on_error,
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/google/api_core/retry.py", line 182, in retry_target
    return target()
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/google/api_core/timeout.py", line 214, in func_with_timeout
    return func(*args, **kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/google/api_core/grpc_helpers.py", line 59, in error_remapped_callable
    six.raise_from(exceptions.from_grpc_error(exc), exc)
  File "<string>", line 3, in raise_from
google.api_core.exceptions.FailedPrecondition: 400 Cannot start a read or query within a transaction after Commit() or Rollback() has been called.

and I see that we exported the attribute committed as per

python-spanner/google/cloud/spanner_v1/transaction.py

Line 37 in 386f738

committed = None

but unfortunately not _rolled_back

python-spanner/google/cloud/spanner_v1/transaction.py

Line 39 in 386f738

_rolled_back = False

which I need in order to ensure that I can correctly run commit or rollback on my transactions without a failed precondition error.