Dynamic image preview fails when WILLOW_OPTIMIZERS is set

[tomkins]

Issue Summary

Currently when you've got an optimizer enabled in WILLOW_OPTIMIZERS, and use the Image URL Generator, it'll come up with a 500:

Internal Server Error: /admin/images/26/preview/original/
Traceback (most recent call last):
  ...
  File "/Users/tomkins/Python/wagtail/wagtail/images/views/images.py", line 295, in preview
    image = Filter(spec=filter_spec).run(image, response)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomkins/Python/wagtail/wagtail/images/models.py", line 1031, in run
    return willow.save_as_png(output, optimize=True)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/tomkins/Python/willow/willow/plugins/pillow.py", line 313, in save_as_png
    self.optimize(f, "png")
  File "/Users/tomkins/Python/willow/willow/image.py", line 178, in optimize
    optimizer.process(file_path)
                      ^^^^^^^^^
UnboundLocalError: cannot access local variable 'file_path' where it is not associated with a value

Slightly obscure (not sure how many people actually the combination of Willow optimizers and the dynamic image serve view!).

Steps to Reproduce

• Install optipng if you don't have it already (brew install optipng)

• Add ServeView into your URL config (see https://docs.wagtail.org/en/v6.1.2/advanced_topics/images/image_serve_view.html#setup)

• Edit your settings file, add: WILLOW_OPTIMIZERS = "optipng"

• Upload a PNG into Wagtail images (eg. https://github.com/wagtail/wagtail/blob/d44808ac532a96c60b16377fbb25d29d7eae704c/docs/logo.png)

• Go to the edit image view, click on URL Generator on the right hand side

• Page loads, but the image fails to appear and generates the above 500

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.12 (any)
• Django version: 4.2 (any)
• Wagtail version: Current main
• Browser version: n/a

Working on this

Pull request coming shortly!

[RealOrangeOne]

Transferred this issue from Wagtail, as whilst we can avoid the issue in Wagtail, there's still a bug in Willow to fix.

The root cause is in this block: https://github.com/wagtail/Willow/blob/7e49ca06dc5211a3e85bfc89314ba8ae3914e854/willow/image.py#L150-L173

The if doesn't have an else case, and so if an unexpected type is passed, file_path never gets defined.

The solution will either be to attempt to read whatever object we get into a SpooledTemporaryFile, or explicitly raise a type error.