[wp-trac] [WordPress Trac] #49532: Clicking "preview" multiple times on a post causes an identical autosave revision to be created and destroyed repeatedly

[WordPress Trac]

#49532: Clicking "preview" multiple times on a post causes an identical autosave
revision to be created and destroyed repeatedly
-----------------------------------+-----------------------------
 Reporter:  inwerpsel              |      Owner:  (none)
     Type:  defect (bug)           |     Status:  new
 Priority:  normal                 |  Milestone:  Awaiting Review
Component:  Revisions              |    Version:  trunk
 Severity:  normal                 |   Keywords:
  Focuses:  rest-api, performance  |
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 How to reproduce:
 Click the preview button multiple times in a row, without making any
 changes to the post in between. This will result with the autosave
 revision being created and destroyed repeatedly (check the response of the
 api request that does the autosave, it alternates between 200 and 400).



 Detailed explanation:

 Suppose the post has no autosave revision yet.

 Clicking preview the first time results in a request being sent to "/wp-
 json/wp/v2/post_type/{id}/autosaves?_locale=user". In that request the
 autosave revision is looked up for the post.
 https://github.com/WordPress/WordPress/blob/master/wp-includes/rest-
 api/endpoints/class-wp-rest-autosaves-controller.php#L360

 Then, because there is no record, it will create a new one "as a special
 post revision" at

 https://github.com/WordPress/WordPress/blob/master/wp-includes/rest-
 api/endpoints/class-wp-rest-autosaves-controller.php#L393

 After creating the request returns 200 and the autosave revision record is
 in the database.

 Pressing the second time it will again look up the autosave revision and
 this time find it. Then an object is created with the $_POST data, but
 which has the old autosave revision's ID. That object is then compared to
 the current post, and if it's not different, wp_delete_post_meta is called
 with the old autosave ID and the record is deleted.


 Notice how now we're back in the same state as at the start (a post with
 no autosave revision). So we can keep on pressing the preview button and
 the record will keep on getting created and deleted, even though it
 contains the exact same data as the post of which it is an autosave.


 Why is this an issue:
 This creates and deletes records in the wp_posts table with no real use,
 causing unwanted load and exhausting the ids of the wp_posts table.


 Proposed solution:
 Either no record should be created in the first place if there is no
 difference, or the existing record shouldn't be deleted, even if it
 doesn't have any changes.

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Ticket URL: <https://core.trac.wordpress.org/ticket/49532>
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