FPM pool manager

[bukka]

Description

FPM allows creating multiple pools which except other things provides some sort of separation between processes. There is still however lots of shared resources because MINIT happens in master process which is common for all pools. That's problematic for extension like opcache that creates shared memory between all pools. This gets even more problematic if pools have different users / groups configured due to potential permission errors when accessing shared resources.

The proposed solution for this problem is to create extra layer between master and children processes. It would be a pool manager process that would handle creation of children, logs, MINIT and other things currently handled by master process. Master process would then become very thin and just supervise pool manager(s) as well as some other common things (e.g. loading configuration). This would be also beneficial for improving graceful reload and extending of status and access logging for resource information related to the whole pool like for example request for adding utime and stime.

[bukka]

Here is a list of related issues that would be resolved by this:

• https://bugs.php.net/bug.php?id=75440 - Fpm reload should be graceful, not killing running processes (possibly master could just re-read config and let proc mangers deal with it
• https://bugs.php.net/bug.php?id=60961 - Graceful Restart (USR2) isn't very graceful (similar to above bug listing problems with graceful reload)
• php-fpm: graceful restart without blocking/losing requests #3758 - php-fpm: graceful restart without blocking/losing requests (Mike's old PR with some useful discussion about reload)
• https://bugs.php.net/bug.php?id=74778 - opcache_clear() puts php-fpm master in a cpu loop
• https://bugs.php.net/bug.php?id=74709 - PHP-FPM process eating 100% CPU attempting to use kill_all_lockers (separating opache MINIT)
• php-fpm trying to kill another user's pool results in an infinite loop and 99% cpu usage #8072 - php-fpm trying to kill another user's pool results in an infinite loop and 99% cpu usage (separating opache MINIT)