Loading or saving a post with enough (large) revisions exhausts available memory

[mlbrgl]

This is a follow-up to #34560.

What steps should be taken to consistently reproduce the problem?

• create a post with enough (large) revisions (see "Steps to reproduce" from #34560).
• Reduce WP_MAX_MEMORY_LIMIT so that it is enough to load one revision but not enough to load tens / hundreds of revisions (e.g. 128M)
• save post -> Updating failed. The response is not a valid JSON response.
• edit post -> **Fatal error**: Allowed memory size of 134217728 bytes exhausted

Does the problem occur even when you deactivate all plugins and use the default theme?

• yes

Likely cause
It seems full post revisions (including post content) are still being loaded when editing a post. This is likely the cause of high memory consumption. To look into this:

• on a standard WordPress installation, activate ​https://wordpress.org/plugins/debug-bar/ with

  <?php
  define( 'SAVEQUERIES', true );
  ?>
  

• edit a post with revisions enabled
• check the "Queries" tab of the debug bar (might need to turn off Gutenberg full screen mode to see it) and look for the query that loads the exhaustive list of full revisions (wp_posts.*):

SELECT wp_posts.* FROM wp_posts 
WHERE 1=1 AND wp_posts.post_parent = [[THE CURRENT POST ID]] 
AND wp_posts.post_type = 'revision' 
AND ((wp_posts.post_status = 'inherit')) 
ORDER BY wp_posts.post_date DESC, wp_posts.ID DESC

[mlbrgl]

Saving a post with enough large revisions

[mlbrgl]

Editing a post with enough large revisions

[mlbrgl]

The likely offending query exhausting available memory on posts with enough large revisions

[flixos90]

Thank you for the report @mlbrgl, and welcome to Trac. Apologies for this only being reviewed now after all this time.

Is this still a problem you're facing? Would it be an option to limit the number of revisions that are saved for a post?

Overall, this seems like a valid issue, but I'm unsure how it could be addressed other than having adequate memory limit. Do you have any suggestions?

cc @spacedmonkey @adamsilverstein for visibility

[mlbrgl]

Hi @flixos90, thanks for getting back to me, and no worries about timing :)

IIRC, we did end up limiting the number of saved revisions at the time, but moved off of WP since.

I'm fine with closing if this doesn't need to be tracked anymore.