Allow post data to be filtered if is_customize_preview()

[westonruter]

This was originally reported as an an ​issue in the ​Customize Posts feature plugin for #34923. The plugin currently previews post data via the the_posts filter. This is problematic, however, because this filter is not called in queries with suppress_filters on or in calls to get_post():

When querying posts via get_posts() or when using new WP_Query( array( 'suppress_filters' => true ) ), the the_posts filters which apply the previewed changes to the posts is not run. This is a problem with Core in that there are not always filters applied to post data. This necessitates, for example, for there to be a comments_open filter in addition to a the_posts filter, since get_post() will return a pristine copy of the post without any hooks for Customize Posts to apply the previewed changes. In reality, get_post() should be returning the dirty post as opposed to the pristine post, and so an additional comments_open filter wouldn't be needed.

So, what is needed is a new filter to run whenever getting a post via get_post() to allow the array or object to be mutated. This filter can only be applied if is_customize_preview() since applying such a filter generally could be too far reaching.

[TimothyBlynJacobs]

Related: #30292.

[ocean90]

I don't think that this filter belongs directly into WP_Post. At least not with an is_customize_preview() conditional. Should this be a more general filter which makes it a duplicate of #30292?

[westonruter]

Indeed. I wasn't familiar with #30292.