[wp-trac] [WordPress Trac] #56909: pre_recurse_dirsize filter cannot be used to fill up dirsize_cache and thus breaks performance

[WordPress Trac]

#56909: pre_recurse_dirsize filter cannot be used to fill up dirsize_cache and thus
breaks performance
-------------------------+-------------------------------------------------
 Reporter:  janthiel     |      Owner:  (none)
     Type:  defect       |     Status:  new
  (bug)                  |
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  Networks     |    Version:  5.6
  and Sites              |   Keywords:  2nd-opinion dev-feedback needs-
 Severity:  normal       |  patch
  Focuses:  multisite,   |
  performance            |
-------------------------+-------------------------------------------------
 In 5.6.0 a new filter was introduced to the dirsize calculation
 `pre_recurse_dirsize`. After that filter was introduced the dirsize cache
 was modified to store each folders size separately for a massive
 performance increase ( part of
 https://core.trac.wordpress.org/ticket/19879 ).

 https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-
 includes/functions.php#L8287

 This second change lead to a state where the `pre_recurse_dirsize` filter
 is kind of useless. One cannot access or modify the dirsize cache within
 the filter as the `$dirsize_cache` variable is passed by reference to the
 recursive calls of `recurse_dirsize()`.

 https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-
 includes/functions.php#L8300

 Thus using the `pre_recurse_dirsize` filter renders it impossible to use
 the new, much more efficient dirsize cache based on single folders. I can
 only fill up the total for the top level folder.

 If `pre_recurse_dirsize` is used the code would skip these recursive calls
 to `recurse_dirsize()`. And thus the reference passing of the
 `$dirsize_cache` and filling it with the subfolder sizes.

 One would consider that the filter code could set the `$dirsize_cache` or
 the transient value on its own. This doesn't work as well, as the original
 code works on an in memory version of the `$dirsize_cache` and will
 overwrite any changes done within the filter at the end of its code.

 https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-
 includes/functions.php#L8323-L8328

 This state leads to the bad situation that using the `pre_recurse_dirsize`
 filter will always lead to worse performance. Although the idea behind
 introducing it was to open up for performance improvements.

 I am currently unsure how to fix this in a smart way and am open for any
 thoughts and suggestions.

 (Maybe bad) Ideas I had:
 - Pass the `$dirsize_cache` by the reference to the filter (technically
 impossible as far as I know)
 - Add another filter to disable the dirsize cache saving in
 `recurse_dirsize` to handle everything on our own (would allow full
 backward compat)
 - Move the `pre_recurse_dirsize` to another position (don't really know
 where...)
 - Make `recurse_dirsize` a pluggable function to replace it completely


 Thanks a lot!

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Ticket URL: <https://core.trac.wordpress.org/ticket/56909>
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