Avoid installing middleware if Content-Encoding is set at all

[adamchainz]

The previous check for "gzip" ignored the possibility of other Content-Encoding values, such as Brotli’s br, which the middleware could also not touch. Instead, do not attempt to alter the content for any Content-Encoding.

I tested by installing django-brotli in the example app below the toolbar middleware and hitting it with:

curl http://localhost:8000/ -H 'Accept-Encoding: br'

Without the change, it would crash with:

  File "/.../debug_toolbar/middleware.py", line 87, in __call__
    content = response.content.decode(response.charset)
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xc7 in position 1: invalid continuation byte

After, it is fixed.

[matthiask]

Thanks! It would be relatively easy to write a test for this but.... that's basically already a concern and not introduced by this proposal (following holacracy's governance rules)

[adamchainz]

I am happy to find a test but couldn't pin down where instantly - if you can point me I will.

[tim-schilling]

Duplicating test_middleware_response_injection where it has django.middleware.gzip.GZipMiddleware applied before sending it to the toolbar's middleware would cover the regression case of it. Alternatively manipulating the Content-Encoding directly without the additional middleware.

[adamchainz]

Thanks Tim, test PR in #1562.