Fix interned strings buffer check if 0 free bytes

[MichaIng]

• Resolves: [Bug]: The OPcache interned strings buffer is nearly full #31223 (comment)
  At least this case made me review the code and see the loophole.

Summary

With #32902 it was meant to be avoided to recommend raising the interned strings buffer size above a quarter of the total OPcache size. This works as long as there is at least 1 byte free, but does not apply if the buffer is filled completely.

This commit switches the conditions so that the interned strings buffer size must be smaller than a quarter of the total OPcache size for the warning to be shown. That the buffer must be either filled completely or by more than 90% remains untouched.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[szaimen]

LGTM but didnt test

[szaimen]

/backport to stable25

[szaimen]

/backport to stable24

[MichaIng]

Tested up and down with a bunch of combinations of OPcache size and interned strings buffer size and it works all as expected. And indeed, sometimes the interned strings buffer free memory was exactly 0 bytes, in which case the recommendation previously was still shown, even if interned strings buffer was already a quarter of overall OPcache size or larger. This in theory could have lead to a vicious cycle, at least if the interned strings buffer keeps being exhausted until last byte (which was not always the case in my tests, sometimes some bytes stayed free, which makes sense if there is no small fitting string to buffer):

• Admin reacts to interned strings buffer recommendation and increases it.
• Since the interned strings buffer is allocated directly as used part of the overall OPcache, now the OPcache could be used above the limit, triggering a related recommendation to raise its size. Admin reacts to it.
• Due to the still unknown circumstances in some cases, especially also with Wordpress on same PHP instance, the interned strings buffer keeps being filled, triggering another warning.
• Admin reacts, then OPcache again gets too small, etc etc.

So to finally silence all noise on that topic, this should be merged 😄.

[szaimen]

/rebase

[Badb0yBadb0y]

Seems like it is not available in 27. I have the warning here too:

The OPcache interned strings buffer is nearly full. To assure that repeating strings can be effectively cached, it is recommended to apply opcache.interned_strings_buffer to your PHP configuration with a value higher than 8.

[MichaIng]

That this warning is shown in your case does not mean that you are effected by the bug fixed with this PR. Double check your OPcache settings, that opcache.interned_strings_buffer is set to at least 25% of opcache.memory_consumption, in case use opcache_get_status or opcache-gui to verify that those settings are applied correctly/effectively for requests to your Nextcloud instance.