DynamoDB: transact_write_items does not return Item in CancellationReasons

[magicmark]

moto does not appear to return Item under err.response["CancellationReasons"][n] when a transaction fails, and we've set "ReturnValuesOnConditionCheckFailure": "ALL_OLD".

I've created a minimal repro with a failing test case here: https://github.com/magicmark/moto-issue-demo.git

╔═ markl@C02DG2X9MD6Q: T/tmp.TOXHwuqG/moto-issue-demo git:(main)via 🐍 v3.8.9 
╚═ ♪ poetry show moto 
name         : moto
version      : 3.1.1

Repro instructions

$ git clone https://github.com/magicmark/moto-issue-demo.git
$ cd moto-issue-demo
$ poetry install
$ poetry run pytest -vvv

Output

        try:
            order_breakfast()
        except BaseException as err:
>           assert 'Item' in err.response["CancellationReasons"][0]
E           AssertionError: assert 'Item' in {'Code': 'ConditionalCheckFailed', 'Message': 'The conditional request failed'}

tests/test_order_breakfast.py:40: AssertionError
============================================================= short test summary info =============================================================
FAILED tests/test_order_breakfast.py::test_breakfast_order - AssertionError: assert 'Item' in {'Code': 'ConditionalCheckFailed', 'Message': 'The...
================================================================ 1 failed in 0.67s ================================================================

What's the problem?

With DynamoDB+boto3, when a transaction fails due to a ConditionExpression failing,
AND we've set "ReturnValuesOnConditionCheckFailure": "ALL_OLD", we should see the
current item in the error response.

For example, if we ran order-breakfast when hitting DynamoDB for real, here's the output
of catching the exception and pprinting err.reason:

{'CancellationReasons': [{'Code': 'ConditionalCheckFailed',
                          'Item': {'customer': {'S': 'mark'},
                                   'lock': {'M': {'acquired_at': {'N': '123'}}},
                                   'mealtime': {'S': 'breakfast'}},
                          'Message': 'The conditional request failed'}],
 'Error': {'Code': 'TransactionCanceledException',
           'Message': 'Transaction cancelled, please refer cancellation '
                      'reasons for specific reasons [ConditionalCheckFailed]'},
 'Message': 'Transaction cancelled, please refer cancellation reasons for '
            'specific reasons [ConditionalCheckFailed]',
 'ResponseMetadata': {'HTTPHeaders': {'connection': 'keep-alive',
                                      'content-length': '393',
                                      'content-type': 'application/x-amz-json-1.0',
                                      'date': 'Wed, 08 Jun 2022 18:55:51 GMT',
                                      'server': 'Server',
                                      'x-amz-crc32': '947576560',
                                      'x-amzn-requestid': 'BDBL292TOK52OSPP0GTJTTS2CVVV4KQNSO5AEMVJF66Q9ASUAAJG'},
                      'HTTPStatusCode': 400,
                      'RequestId': 'BDBL292TOK52OSPP0GTJTTS2CVVV4KQNSO5AEMVJF66Q9ASUAAJG',
                      'RetryAttempts': 0}}

Note that we see,Code, Message, and Item.

Under moto, we do not see Item.

[bblommers]

Thanks for raising this and adding a repro-case @magicmark!