DISCARD without MULTI

Hey folks,

I came across a weird issue today where one of our processes started raising ResponseError: DISCARD without MULTI when calling execute() on a pipeline object. As far as I can tell, this error should be impossible to receive since redis-py is supposed to take care of any discard/unwatching when using pipeline objects. This suggests a bug in redis-py not properly cleaning up after itself, although I’ve been reading the code and I don’t see yet how this is possible. I also don’t know how to reproduce the bug.

ERROR 2016-10-29 04:34:32.807 error expiring
Traceback (most recent call last):
  File "dmserver.py", line 1477, in push_in_worker
    redis_ops.cq_expire_items(channel_key)
  File "redis_ops.py", line 129, in cq_expire_items
    pipe.execute()
  File "/usr/lib/python2.7/dist-packages/redis/client.py", line 2626, in execute
    return execute(conn, stack, raise_on_error)
  File "/usr/lib/python2.7/dist-packages/redis/client.py", line 2521, in _execute_transaction
    self.immediate_execute_command('DISCARD')
  File "/usr/lib/python2.7/dist-packages/redis/client.py", line 2461, in immediate_execute_command
    return self.parse_response(conn, command_name, **options)
  File "/usr/lib/python2.7/dist-packages/redis/client.py", line 2584, in parse_response
    self, connection, command_name, **options)
  File "/usr/lib/python2.7/dist-packages/redis/client.py", line 585, in parse_response
    response = connection.read_response()
  File "/usr/lib/python2.7/dist-packages/redis/connection.py", line 582, in read_response
    raise response
ResponseError: DISCARD without MULTI

I’m using Redis 3.0.6 and redis-py 2.10.5.

Anyone seen anything like this before and have any suggestions?

[dlcron]

I’ve also had this issue recently.

Let my summarise it just if anyone is also struggling.

You clearly cannot do the following:

> redis_db.sadd('123', *[])
*** redis.exceptions.ResponseError: wrong number of arguments for 'sadd' command

But following is allowed:

> pipe = redis_db.pipeline()
> pipe.multi()
> pipe.sadd('123', *[])
> pipe.execute()
*** redis.exceptions.ResponseError: DISCARD without MULTI

Long story short, if you are using pipeline, each of the command is queued. If one of the queued command results with an error, you might get DISCARD without MULTI. In my case arguments to sadd were computed dynamically and sometimes ended up as an empty list. This was clearly an issue in my application logic.

Since we keep the commands in the stack we could handle this specific case by introspecting the stack arguments and throw an error as soon as we add something there. In my case it is possible to know this sooner, without calling execute on pipeline.

> [..]/redis/client.py(2879)execute()
  return execute(conn, stack, raise_on_error)  # <---- we have queued commands in stack

What do you guys think about this?

[andymccurdy]

This was fixing in redis-py 3.5.0. Forgot to close this issue when the error was resolved.