XREADGROUP returns None when stream length is less than COUNT with BLOCK

[jjmerchante]

Describe the bug

Calling XREADGROUP with COUNT and BLOCK returns None when the number of items in the stream is less than COUNT. On the first call from the _blocking method, it claims the available items, and because the total items are less than COUNT it returns None. On the second call, after BLOCK milliseconds, no items remain in the stream, resulting in 0 items being returned.

fakeredis-py/fakeredis/commands_mixins/streams_mixin.py

Line 344 in 697fc90

if first_pass and (count is None or len(stream_results) < count):

Expected behavior

According to Redis documentation, "When the BLOCK command is passed, but there is data to return at least in one of the streams passed, the command is executed synchronously exactly like if the BLOCK option would be missing."

To Reproduce

For example:

from fakeredis import FakeStrictRedis

connection = FakeStrictRedis()

connection.xadd('test-events', {"message": "hello"})
connection.xadd('test-events', {"message": "bye"})

connection.xgroup_create('test-events', 'group1', id='0', mkstream=True)
messages = connection.xreadgroup(groupname='group1',
    consumername='consumer1',
    streams={'test-events': '>'},
    count=10,
    block=5000
)
# messages is an empty array

Running in a redis-cli doesn't block when the number of items is lower than COUNT and returns the results:

XADD stream 1-1 message "hello"
XGROUP CREATE stream group1 0
XREADGROUP GROUP group1 consumer1 COUNT 10 BLOCK 5000 STREAMS stream '>'

I think len(stream_results) < count should be removed from the condition, but let me know if there’s a specific reason for keeping it.

Thank you