[anishmehta24]

Issue number: closes #8710

Summary

Changes

When compression is enabled for the route or the response, the body depends on the request's Accept-Encoding, but responses had no Vary for it, so a shared cache could serve a gzipped body to a client without gzip (or the plain body to everyone). ResponseBuilder now adds Accept-Encoding to Vary in that case, on both the compressed response and the plain one sent to a client without gzip. A Vary value the route already set is kept, the field isn't added twice, and Vary: * is left as is. Compression turned off with Response(compress=False) adds nothing.

Added test_compress_sets_vary_accept_encoding (8 cases) in tests/functional/event_handler/required_dependencies/test_api_gateway.py; the four cases that expect a new Vary fail on develop. The event handler functional tests pass.

User experience

Before: a compress=True route returns Content-Encoding: gzip with no Vary. After: it also returns Vary: Accept-Encoding (or appends it to the route's own Vary), so caches keep the encodings apart.

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[leandrodamascena]

Thanks @anishmehta24, confirmed on the REST resolver with and without gzip, with CORS, with a Vary already set as a string, as a list and as *. I pushed one commit on top: the CORS code from #8638 had its own copy of the merge logic, so I made it call your _add_vary instead. That also stops develop from emitting Origin, Origin and *, Origin when the route already set Vary. I added the list case to your test.

I'm approving now. This goes out in 3.40.0, so it will sit here for a while before I merge it. No rebase needed in the meantime, and you'll be credited in the release notes.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.02%. Comparing base (30ebaa4) to head (8d1d30e).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8711      +/-   ##
===========================================
+ Coverage    97.01%   97.02%   +0.01%     
===========================================
  Files          321      321              
  Lines        16432    16443      +11     
  Branches      1484     1487       +3     
===========================================
+ Hits         15941    15954      +13     
+ Misses         350      349       -1     
+ Partials       141      140       -1     

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