Checking OOM by `getMaxMemoryState` inside script might get different result
with `freeMemoryIfNeededAndSafe` at script start, because lua stack and
arguments also consume memory.
This leads to memory `borderline` when memory grows near server.maxmemory:
- `freeMemoryIfNeededAndSafe` at script start detects no OOM, no memory freed
- `getMaxMemoryState` inside script detects OOM, script aborted
We solve this 'borderline' issue by saving OOM state at script start to get
stable lua OOM state.
related to issue #6565 and #5250.
So error message `ERR only (P)SUBSCRIBE / (P)UNSUBSCRIBE / PING / QUIT allowed in this context` will become
`ERR 'get' command submitted, but only (P)SUBSCRIBE / (P)UNSUBSCRIBE / PING / QUIT allowed in this context`
Likely fix#6723.
This is what happens AFAIK: we enter the main loop where we expire stuff
until a given percentage of keys is still found to be logically expired.
There are however other potential exit conditions.
However the "sampled" variable is not always incremented inside the
loop, because we may found no valid slot as we scan the hash table, but
just NULLs ad dict entries. So when the do/while loop condition is
triggered at the end, we do (expired*100/sampled), dividing by zero if
we sampled 0 keys.
Funcion adjustOpenFilesLimit() has an implicit parameter, which is server.maxclients.
This function aims to ajust maximum file descriptor number according to server.maxclients
by best effort, which is "bestlimit" could be lower than "maxfiles" but greater than "oldlimit".
When we try to increase "maxclients" using CONFIG SET command, we could increase maximum
file descriptor number to a bigger value without calling aeResizeSetSize the same time.
When later more and more clients connect to server, the allocated fd could be bigger and bigger,
and eventually exceeds events size of aeEventLoop.events. When new nodes joins the cluster,
new link is created, together with new fd, but when calling aeCreateFileEvent, we did not
check the return value. In this case, we have a non-null "link" but the associated fd is not
registered.
So when we dynamically set "maxclients" we could reach an inconsistency between maximum file
descriptor number of the process and server.maxclients. And later could cause cluster link and link
fd inconsistency.
While setting "maxclients" dynamically, we consider it as failed when resulting "maxclients" is not
the same as expected. We try to restore back the maximum file descriptor number when we failed to set
"maxclients" to the specified value, so that server.maxclients could act as a guard as before.
