xreadgroup throws an exception if message was trimmed when reading pending messages

Version: 3.0.1

Platform: Python 3.6.6 on Debian 9 and Redis 5.0.2

Description: If messages get deleted from a STREAM via XADD with MAXLEN or XTRIM in a stream where there is an existing consumer group after it’s consumed but before being acked, when you consume the pending messages using XREADGROUP with id 0, redis returns a (nil) as the payload of the messages instead of field key/value pairs. This causes xreadgroup to throw an exception:

[2019-01-10 12:00:10,206] [16] [ERROR] Error reading messages
Traceback (most recent call last):
  File "/code/app/daemons/__init__.py", line 38, in run
    stream_messages = self.redis_client.xreadgroup("consumer-group", "consumer", {"stream": "0"}, count=1000)
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 2185, in xreadgroup
    return self.execute_command('XREADGROUP', *pieces)
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 755, in execute_command
    return self.parse_response(connection, command_name, **options)
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 774, in parse_response
    return self.response_callbacks[command_name](response, **options)
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 275, in parse_xread
    return [[nativestr(r[0]), parse_stream_list(r[1])] for r in response]
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 275, in <listcomp>
    return [[nativestr(r[0]), parse_stream_list(r[1])] for r in response]
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 246, in parse_stream_list
    return [(r[0], pairs_to_dict(r[1])) for r in response]
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 246, in <listcomp>
    return [(r[0], pairs_to_dict(r[1])) for r in response]
  File "/usr/local/lib/python3.6/site-packages/redis/client.py", line 195, in pairs_to_dict
    it = iter(response)
TypeError: 'NoneType' object is not iterable

You can see this is the expected behaviour in Redis’s source code: https://github.com/antirez/redis/blob/5.0.2/src/t_stream.c#L1031

[soloestoy]

relate to https://github.com/antirez/redis/issues/5718

[andymccurdy]

Merged. Thanks!