#24452: Staticfiles backends using HashedFilesMixin don't update CSS files' hash
when referenced media changes
-------------------------------------+--------------------
     Reporter:  pmclanahan           |      Owner:  nobody
         Type:  Bug                  |     Status:  new
    Component:  contrib.staticfiles  |    Version:  1.7
     Severity:  Normal               |   Keywords:
 Triage Stage:  Unreviewed           |  Has patch:  0
Easy pickings:  0                    |      UI/UX:  0
-------------------------------------+--------------------
 Steps to reproduce when using the Cached or Manifest staticfiles storages:

 1. Have an image ({{{a.jpg}}}) and a CSS file that references this image
 in a {{{url()}}} call.
 2. Run {{{collectstatic}}} and observe that copies of said files now have
 hashed names.
 3. Change the content of {{{a.jpg}}} (the color was clearly wrong).
 4. Run {{{collectstatic}}} again
 5. Observe that {{{a.jpg}}} now has a new hashed name version
 6. Obesrve that the CSS file refers to this new version, but the hashed
 name of the CSS file has '''not''' changed.

 Because the CSS file's hash name was determined before processing the
 file's contents, changes in the referenced assets (images, fonts, etc.)
 will not result in a new hashed CSS file name. Thus when using a CDN and
 very long cache expire headers the CSS will never update, and the new
 version of the image will never be seen on the site.

 We're currently working around this by making insignificant changes to the
 CSS whenever we need to make changes to only the content of referenced
 media, but this is error prone and clearly suboptimal.

 This was observed in 1.6 using the {{{CachedStaticFilesStorage}}}, but I
 testing in 1.7 as well. A fix could be backported to the 1.6 line should
 it come before 1.6 is unsupported and a patch release is desired.

--
Ticket URL: <https://code.djangoproject.com/ticket/24452>
Django <https://code.djangoproject.com/>
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