Unexpected behavior when creating and reading from new groups

[kairichard]

Consider the following sequences of command:

# Add a few messages and create a consumer group
XADD mystream * message_id 1
XADD mystream * message_id 2
XADD mystream * message_id 3
XGROUP CREATE mystream grp1 0 

# Returns no messages
XREADGROUP GROUP grp1 cns1 STREAMS mystream 0-0

# Shows that grp1 has 0 pending messages
XINFO groups mystream

This comes of rather unexpected as I need to do the following to get the message from the stream into the group, but only once. Which the docs also mention. But still I personally (n=1) find that to be unexpected and maybe just don't understand the design choice behind it because.

XREADGROUP GROUP grp1 cns1 STREAMS mystream >

It also leads to client code that is rather convoluted and harder to reason about because it will inherntly surface the choice made in redis.