The conclusion, that a xread request can be answered syncronously in
case that the stream's last_id is larger than the passed last-received-id
parameter, assumes, that there must be entries present, which could be
returned immediately.
This assumption fails for empty streams that actually contained some
entries which got removed by xdel, ... .
As result, the client is answered synchronously with an empty result,
instead of blocking for new entries to arrive.
An additional check for a non-empty stream is required.
Slaves and rebooting redis may have different radix tree struct,
by different stream* config options. So propagating approximated
MAXLEN to AOF/slaves may lead to date inconsistency.
If we rewrite the MAXLEN argument as zero when no trimming
was performed, date between master and slave and aof will
be inconsistent, because `xtrim maxlen 0` means delete all
entries in stream.
We don't want to increment the deliveries here, because the sysadmin
reset the consumer group so the desire is likely to restart processing,
and having the PEL polluted with old information is not useful but
probably confusing.
Related to #5111.
To simplify the semantics of blocking for a group, this commit changes
the implementation to better match the description we provide of
conusmer groups: blocking for > will make the consumer waiting for new
elements in the group. However blocking for any other ID will always
serve the local history of the consumer.
However it must be noted that the > ID is actually an alias for the
special ID ms/seq of UINT64_MAX,UINT64_MAX.
Now a MAXLEN of 0 really does what it means: it will create a zero
entries stream. This is useful in order to make sure that the behavior
is identical to XTRIM, that must be able to reduce the stream to zero
elements when MAXLEN is given.
Also now MAXLEN with a count < 0 will return an error.
Currently it does not look it's sensible to generate events for streams
consumer groups modification, being them metadata, however at least for
key-level events, like the creation or removal of a consumer group, I
added a few events here and there. Later we can evaluate if it makes
sense to add more. From the POV instead of WAIT (in Redis transaciton)
and signaling the key as modified, it looks like that the transaction
should not fail when a stream is modified, so no calls are made in
consumer groups related functions to signalModifiedKey().
