[wp-trac] [WordPress Trac] #48257: REST API: post-process endpoint cannot be discovered from media creation endpoint

[WordPress Trac]

#48257: REST API: post-process endpoint cannot be discovered from media creation
endpoint
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 Reporter:  kadamwhite    |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Future Release
Component:  REST API      |    Version:  trunk
 Severity:  normal        |   Keywords:  needs-patch
  Focuses:  rest-api      |
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 Split out from #47987:

 In #47987 we introduced a new `/wp/v2/media/{id}/post-process` endpoint,
 which may be hit with a POST to resume media processing in the event the
 initial POST to `/wp/v2/media` fails. Clients may know to utilize this new
 endpoint through the presence of a new non-standard header `X-WP-Upload-
 Attachment-ID`, but @rmccue caught that we do not provide any Link-based
 method of discovering the existence of this endpoint from the initial
 media collection. @TimothyBlynJacobs proposes using the `edit-media`
 relation for this link.

 We should add an additional `Link` to the header of the failing POST
 response (or any POST response, potentially, with the understanding a link
 is only necessary in the event of failure). Implementing this Link was
 punted from 5.3 because the `$response` object on which we can call
 add_link was not available at the time we set the initial X-header, which
 we can work around but did not have time to resolve before the final 5.3
 beta.

 ''Aside:'' If we're able to resolve the order-of-operations issue, it's
 possible that the Link could become the preferred method of signalling
 that post-processing should occur, rather than the non-standard header.
 However the X-header is more appropriate for the other media paths in
 `media.php` and `admin-ajax.php`, so this may not be feasible.

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Ticket URL: <https://core.trac.wordpress.org/ticket/48257>
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