Memcached flags loses during load from snapshot

[sbkoto]

not fixed on 1.19.0

set item 4 3600 4
test
STORED
get item
VALUE item 4 4
test
END

after reload dragonfly and load from snapshot:

get item
VALUE item 0 4
test
END

Originally posted by @sbkoto in #1758 (comment)

[romange]

definition of done - rdb_test reloads the snapshot file with mcflags.

[chakaz]

Thanks for filing this issue, @sbkoto. We plan to work on this soon.

Some more context:

• memcached allows adding an integer flag to entries. In this case, the flag's value is 4
• This type of flag metadata does not exist in Redis, so accessing it is only possible via the memcached API
• We currently do not have support for this in rdb_save.cc and rdb_load.cc. We should add it, using a custom opcode (like we have for the STICKY bit today), because it's not supported nativly in RDB format
• We do have support for this for stable sync. So a replica will accept changes for the flag. But it will not get the original value in full sync, for the same reason it is not saved to files.
• IMHO, in addition to rdb_test, we should also add replication test to make sure this information is sent to replicas