Add new behaviours to improve Etag compatibility

[jlopex]

Flask-Compress always mutates the ETag when it's present and compresses a response. That’s reasonable, but it can lead to surprising behavior (see #65): conditional requests (If-None-Match, ranges) are evaluated against a different representation than the one actually sent, because compression and ETag mutation happen after the app or other middleware ran make_conditional.

This PR add one new default behavioir and an opt-in behavior to make things predictable while preserving backwards compatibility.

What’s new

Preserve weak ETags across encodings

The response already has a weak ETag (W/"..."), Flask-Compress will not append :{algorithm}.

Why: per RFC 7232, weak ETags are semantic validators and can be reused across transfer encodings. With Vary: Accept-Encoding in place, caches and clients behave correctly without per-encoding tag mutations.

Evaluate conditionals on the final, compressed representation

Config: COMPRESS_EVALUATE_CONDITIONAL_REQUEST (default: False)

When True, the extension calls response.make_conditional(request) after it sets the compressed body, Content-Encoding, Content-Length, and (if applicable) the ETag.

Why: this ensures 304 Not Modified and 206 Partial Content are computed against the bytes actually sent on the wire. It fixes the “mutated-after-the-fact” mismatch described in #65 without forcing apps to add their own “finisher” hook.

Both switches are off by default (or “legacy behaviour” by default), so existing apps won’t break.

Notes

Vary: Accept-Encoding is still enforced.

We use response.get_etag() / response.set_etag() (Werkzeug) instead of string slicing, so weak/strong handling is correct and future-proof.

make_conditional() is gated to safe methods and runs only after the final body and headers are set.

Tests

Added unit tests showing:

• Weak ETag and conditional GET returns 304 after this change.
• With COMPRESS_EVALUATE_CONDITIONAL_REQUEST=True, conditional GETs are evaluated on the compressed representation (304 with empty body, no Content-Encoding on 304, ETag echoed).

[jlopex]

@alexprengere here is a PR to try to help with conditional requests and compression with flask-compress. I did include some small tests to validate basic behaviour work as expected, I would appreciate any feedback on this.

I hope this little contribution can help a few guys that were raising issues at least in two approaches while not breaking the default behaviour of the package.

[alexprengere]

Thanks a lot! I will take some time to review all this, I will also ask the original authors of the etag-related issues their opinion on this.

[jlopex]

Thanks a lot! I will take some time to review all this, I will also ask the original authors of the etag-related issues their opinion on this.

I tried my best trying to make it retro-compatible and being the less intrusive possible but maybe there are possible improvements on top of it! Looking forward for the feedback!

[alexprengere]

This can be removed 😉

[jlopex]

Good catch @alexprengere I did remove the reference at the README.md, thanks for your review! Looking forward to start testing this in real world!