Fix block theme featured image preview

[adamsilverstein]

(Improve _set_preview for case when autosave missing)

Problem: Autosaves are deleted when a new autosave is sent with the same data. In the block editor, this causes the preview data to be missing on previews.

Bug exhibited: When using block themes, post previews do not properly display the currently set featured image. Several other data transformations are also missing because _set_preview exits early when it can't locate the autosave.

When investigating an issue with previews in block themes not working correctly, I found an underlying bug that prevents autosaves from being used correctly for previews in some cases. (See ​PR comment).

In src/wp-includes/rest-api/endpoints/class-wp-rest-autosaves-controller.php in the create_post_autosave function - when the autosave sent to the autosave REST endpoint exactly matches the previous autosave, the controller deletes the old autosave revision, then returns an error "There is nothing to save. The autosave and the post content are the same."

Deleting the old autosave revision means that later when src/wp-includes/revision.php _set_preview gets called, and it calls wp_get_post_autosave the autosave is missing, and the preview data swaps are not activated.

The end result of this is that currently if you set a preview image using a block theme on a published post and preview it, the featured image is not displayed correctly. Skipping deleting the previous autosave fixes the issue.

…error.

Trac ticket: https://core.trac.wordpress.org/ticket/54708

[adamsilverstein]

I found several users reporting related bugs on trac: #49532, #52933

I tracked down the source of why we chose to delete this autosave in the endpoint: https://core.trac.wordpress.org/ticket/43316#comment:62 --

"if the post content is identical to the new content that is being auto-saved, the previous autosave revision is deleted to prevent having a redundant revision (i.e. if the update of the previous autosaved revision will make it identical to the current post, there is no need of it)."

This makes sense... except that since we return an error here, a new autosave is never created. So the post now lacks any autosave, and this later causes _set_preview to ​bail early - since it relies on the autosave, breaking preview functionality.

I would propose that since the autosave already exists with the correct data, we should leave it in place so previews continue working as expected. An alternate solution would be to update the login in _set_preview to work even without an autosave present.

[adamsilverstein]

In 54708.2.diff​:

When sending the data to the autosave REST API endpoint:

• If no autosave exists for the user, create one
• If an autosave exists for the user, update it

Also:

• Remove logic that previously would delete existing autosaves that matched the post content which had the unfortunate side effect of breaking previews because preview setup requires and checks for the presence of an autosave.

[adamsilverstein]

After some digging here the underlying bug is actually in _set_preview - when an autosave is missing for a post, the current function bails early, and never applies the tem or thumbnail filters:

add_filter( 'get_the_terms', '_wp_preview_terms_filter', 10, 3 );
add_filter( 'get_post_metadata', '_wp_preview_post_thumbnail_filter', 10, 3 );

The reason an autosave may not exist is: when the autosave content, title and excerpt and content exactly match it's parent post, we delete it. In this case, we can skip the reassignment of these values, but still apply the correct filters to the preview.

[adamsilverstein]

In 54708.3.diff​:

• In _set_preview, when the autosave is missing, skip assignment of content, title and excerpt but still apply filters to get_the_terms and get_post_metadata.

[walbo]

54708.3.diff​ looks good to me.

Can confirm the patch fixes the featured image in block based themes when previewing a post. Also tested in twenty twenty one to confirm there is no regression in non block based themes

[adamsilverstein]

Marking this for 5.9 since it corrects a pretty serious regression where featured images no longer show consistently in previews for block themes. Alternately we could land after release and backport in the first "point" release. I would appreciate a 2nd committer opinion here.

This problem has actually existed for as long as the autosave endpoint, and back in 2018 we added an extra query variable to previews in Gutenberg just to work around the issue (see ​https://github.com/WordPress/gutenberg/issues/9151).

The block theme featured image rendering surfaced this bug again, fixing it in core makes more sense though, this also fixes term filtering for previews which would also break when autosaves are missing.

[audrasjb]

Before 54708.3.diff - I can reproduce the issue

[audrasjb]

After 54708.3.diff - the issue no longer occurs

[audrasjb]

I was able to reproduce the issue, and 54708.3.diff fixes it on my side.

[Mamaduka]

I tested the patch, and the fix works as expected. Thanks, @adamsilverstein.

[audrasjb]

Marking for commit since it was successfully tested.

[hellofromTonya]

Replying to adamsilverstein:

After some digging here the underlying bug is actually in _set_preview - when an autosave is missing for a post, the current function bails early, and never applies the tem or thumbnail filters:

add_filter( 'get_the_terms', '_wp_preview_terms_filter', 10, 3 );
add_filter( 'get_post_metadata', '_wp_preview_post_thumbnail_filter', 10, 3 );

The reason an autosave may not exist is: when the autosave content, title and excerpt and content exactly match it's parent post, we delete it. In this case, we can skip the reassignment of these values, but still apply the correct filters to the preview.

Aha, great find Adam! I too was able to reproduce the issue. Can confirm 54708.3.diff​ resolved it.

[adamsilverstein]

I will be away from my computer for a couple of weeks: anyone is welcome to commit this.

[audrasjb]

Alright, let's get this fixed then.
Self assigning for commit.

Trac ticket: https://core.trac.wordpress.org/ticket/54708

[audrasjb]

Let's switch the ticket to the Revisions component :)

Checks are passing. Committing the patch.

[audrasjb]

In 52433:

Revisions: Improve _set_preview for case when autosave is missing.

This change fixes an issue where autosaves are deleted when a new autosave is sent with the same data. In the block editor, this causes the preview data to be missing on post previews. The end result of this is that if one set a preview image using a block theme on a published post and preview it, the featured image is not displayed correctly. Skipping deleting the previous autosave fixes the issue.

Props adamsilverstein, walbo, audrasjb, Mamaduka, hellofromTonya.
Fixes #54708.

Committed in https://core.trac.wordpress.org/changeset/52433