With custom page templates set per page, surprising behavior after switching themes

[ericlewis]

If you have custom page templates set for a page, and you switch to a new theme that has no custom templates, bizarre behavior ensues.

When editing one of these pages, upon saving, everything looks fine. But actually, there was is a silent failure around post.php:2903:

if ( !empty($page_template) && 'page' == $data['post_type'] ) {
		$post->page_template = $page_template;
		$page_templates = wp_get_theme()->get_page_templates();
		if ( 'default' != $page_template && ! isset( $page_templates[ $page_template ] ) ) {
			if ( $wp_error )
				return new WP_Error('invalid_page_template', __('The page template is invalid.'));
			else
				return 0;
		}
		update_post_meta($post_ID, '_wp_page_template',  $page_template);
	}

Since the page template is still set for this page, but does not exist in the theme, the function returns early, which means we don't hit a few crucial hooks - edit_post, save_post, and wp_insert_post.

[nofearinc]

I was able to reproduce that as well. Creating a page and assigning a page template from a theme works OK, when switching to a new theme and updating the page it returns earlier in the code above. The problem exists "forever", i.e. if you save the page with the old template a couple times, it would still fall in that same block (and hooks such as save_post are not fired at all).

Any way we could always ignore that issue and just set the default template (or at least whenever $wp_error is false?)

[wonderboymusic]

In 31020:

If you set '_wp_page_template' for a post, and then switch to a theme that doesn't support the template, subsequent calls to wp_insert_post() will bail early without running a plethora of hooks. This shouldn't happen. We should set the template to 'default' and continue through the rest of the function. The function will still bail if $wp_error is true via wp_insert_post( $arr, true ).

Props nofearinc.
Fixes #25334.

[wonderboymusic]

#29269 was marked as a duplicate.

[ocean90]

#21268 was marked as a duplicate.