The slave sends \n keepalive messages to the master while parsing the rdb,
and later sends REPLCONF ACK once a second. rarely, the master recives both
a linefeed char and a REPLCONF in the same read, \n*3\r\n$8\r\nREPLCONF\r\n...
and it tries to trim two chars (\r\n) from the query buffer,
trimming the '*' from *3\r\n$8\r\nREPLCONF\r\n...
then the master tries to process a command starting with '3' and replies to
the slave a bunch of -ERR and one +OK.
although the slave silently ignores these (prints a log message), this corrupts
the replication offset at the slave since the slave increases the replication
offset, and the master did not.
other than the fix in processInlineBuffer, i did several other improvments
while hunting this very rare bug.
- when redis replies with "unknown command" it includes a portion of the
arguments, not just the command name. so it would be easier to understand
what was recived, in my case, on the slave side, it was -ERR, but
the "arguments" were the interesting part (containing info on the error).
- about a year ago i added code in addReplyErrorLength to print the error to
the log in case of a reply to master (since this string isn't actually
trasmitted to the master), now changed that block to print a similar log
message to indicate an error being sent from the master to the slave.
note that the slave is marked as CLIENT_SLAVE only after PSYNC was received,
so this will not cause any harm for REPLCONF, and will only indicate problems
that are gonna corrupt the replication stream anyway.
- two places were c->reply was emptied, and i wanted to reset sentlen
this is a precaution (i did not actually see such a problem), since a
non-zero sentlen will cause corruption to be transmitted on the socket.
Reading the PR gave me the opportunity to better specify what the code
was doing in places where I was not immediately sure about what was
going on. Moreover I documented the structure in server.h so that people
reading the header file will immediately understand what the structure
is useful for.
A) slave buffers didn't count internal fragmentation and sds unused space,
this caused them to induce eviction although we didn't mean for it.
B) slave buffers were consuming about twice the memory of what they actually needed.
- this was mainly due to sdsMakeRoomFor growing to twice as much as needed each time
but networking.c not storing more than 16k (partially fixed recently in 237a38737).
- besides it wasn't able to store half of the new string into one buffer and the
other half into the next (so the above mentioned fix helped mainly for small items).
- lastly, the sds buffers had up to 30% internal fragmentation that was wasted,
consumed but not used.
C) inefficient performance due to starting from a small string and reallocing many times.
what i changed:
- creating dedicated buffers for reply list, counting their size with zmalloc_size
- when creating a new reply node from, preallocate it to at least 16k.
- when appending a new reply to the buffer, first fill all the unused space of the
previous node before starting a new one.
other changes:
- expose mem_not_counted_for_evict info field for the benefit of the test suite
- add a test to make sure slave buffers are counted correctly and that they don't cause eviction
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.
To detect when the group (or the whole key) is destroyed to send an
error to the consumers blocked in such group is a problem, so we leave
the consumers listening, the sysadmin is free to create or destroy
groups assuming she/he knows what to do. However a client may be blocked
in a given consumer group, that is later destroyed. Then the stream
receives new elements. In that case there is no sane behavior to serve
the consumer... but to report an error about the group no longer
existing.
More about detecting this synchronously and why it is not done:
1. Normally we don't do that, we leave clients blocked for other data
types such as lists.
2. When we free a stream object there is no longer information about
what was the key it was associated with, so while destroying the
consumer groups we miss the info to unblock the clients in that moment.
3. Objects can be reclaimed in other threads where it is no longer safe
to do client operations.
When a client blocks for a consumer group, we don't know the actual ID
we want to be served: other clients blocked in the same consumer group
may be served first, so the consumer group latest delivered ID changes.
This was not handled correctly, all the clients in the consumer group
were unblocked without data but the first.
