[wp-trac] [WordPress Trac] #39883: Code hooking on `image_downsize` can no longer assume the file is an image

[WordPress Trac]

#39883: Code hooking on `image_downsize` can no longer assume the file is an image
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 Reporter:  stephdau      |      Owner:
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Media         |    Version:  trunk
 Severity:  normal        |   Keywords:
  Focuses:                |
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 In r38949, Core  pretty drastically changed the expectations that any code
 hooking onto the `image_downsize` filter could make until now, potentially
 leading to issues for integrators.

 We went from having the `image_downsize()` function immediately return
 `false` if a file wasn't an image, not getting to that filter application
 at all, to only setting a variable with the result of the
 `wp_attachment_is_image( $id )` test and now applying the filter (not even
 passing said result, for that matter).

 This was a pretty big safe assumption to take away from under integrators'
 feet.

 Coupled to this, a wise integrator that might have picked up on this
 change could have wanted to have its own code have `image_downsize()`
 still returning `false` if it does not want to have the file further
 processed by the function, but there is no opportunity to, since returning
 `false` will cause `image_downsize()` to keep on with its processing
 instead of proceeding with `return $out`.

 Returning anything but false or null will cause `image_downsize()` to
 return, but that might not always be desirable to preserve the way WP
 worked prior to 4.7.

 What's better here for general use isn't as clear cut as what r38949 made
 it to be. I've discussed the case with @mikeschroder, and we've agreed to
 open this ticket so we can further discuss what should be done, if
 anything.

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Ticket URL: <https://core.trac.wordpress.org/ticket/39883>
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