We assume accept handlers may choose to reject a connection and close
it, but connAccept() callers can't distinguish between this state and
other error states requiring connClose().
This makes it safe (and mandatory!) to always call connClose() if
connAccept() fails, and safe for accept handlers to close connections
(which will defer).
Before this commit, when upgrading a replica, expired keys will not
be loaded, thus causing replica having less keys in db. To this point,
master and replica's keys is logically consistent. However, before
the keys in master and replica are physically consistent, that is,
they have the same dbsize, if master got a problem and the replica
got promoted and becomes new master of that partition, and master
updates a key which does not exist on master, but physically exists
on the old master(new replica), the old master would refuse to update
the key, thus causing master and replica data inconsistent.
How could this happen?
That's all because of the wrong judgement of roles while starting up
the server. We can not use server.masterhost to judge if the server
is master or replica, since it fails in cluster mode.
When we start the server, we load rdb and do want to load expired keys,
and do not want to have the ability to active expire keys, if it is
a replica.
"Partial Resynchronization" is a special variant of replication success
that we have to tell systemd about if it is managing redis-server via a
Type=Notify service unit.
The callback approach we took is very efficient, the module can do any
filtering of keys without building any list and cloning strings, it can
also read data from the key's value. but if the user tries to re-open
the key, or any other key, this can cause dict re-hashing (dictFind does
that), and that's very bad to do from inside dictScan.
this commit protects the dict from doing any rehashing during scan, but
also warns the user not to attempt any writes or command calls from
within the callback, for fear of unexpected side effects and crashes.
This makes simpler to give people help when posting such kind of errors
in the mailing list or other help forums, because sometimes the
directive looks well spelled, but the version of Redis they are using is
not able to support it.
