Bug: Precompressed and dynamic compression choose gzip over br/zstd when browser sends default Accept-Encoding order

[edehan]

Ferron currently appears to use the order of values in Accept-Encoding as the tie-breaker when selecting a precompressed or dynamically compressed response.

This makes modern browsers receive gzip instead of br/zstd, because Chrome/Chromium and Firefox commonly send:

Accept-Encoding: gzip, deflate, br, zstd

With all variants available, Ferron selects .gz first.

Environment

• Ferron: 2.7.0
• Branch checked: develop-2.x
• Observed browser defaults:
  • Chromium 148: gzip, deflate, br, zstd
  • Firefox 150: gzip, deflate, br, zstd

Reproduction

Prepare a static file with all precompressed variants:

  mkdir -p /tmp/ferron-ae-test
  cp README.md /tmp/ferron-ae-test/big.js
  ferron-precompress /tmp/ferron-ae-test/big.js

Run Ferron:

  ferron --config-string ':18081 {
    root "/tmp/ferron-ae-test"
    precompressed
  }'

Test:

 curl -sI -H "Accept-Encoding: gzip, deflate, br, zstd" \
    http://localhost:18081/big.js | grep -i content-encoding

 curl -sI -H "Accept-Encoding: br" \
    http://localhost:18081/big.js | grep -i content-encoding

Actual:

  content-encoding: gzip
  content-encoding: br

Observed behavior

When all precompressed variants exist:

  gzip, deflate, br, zstd                       -> gzip
  br, gzip, deflate, zstd                       -> br
  zstd, br, gzip, deflate                       -> zstd
  gzip;q=0.1, br;q=1, zstd;q=0.9, deflate;q=0.2 -> br

Dynamic static compression without precompressed shows the same ordering behavior.

There also appears to be a related q=0 issue. For example, if .gz is missing but .br exists:

Accept-Encoding: gzip, br;q=0 -> br

br should not be selected when its qvalue is 0.

Expected behavior

Ferron should perform Accept-Encoding negotiation according to qvalues, not raw header order.

Expected behavior:

• Higher valid q values win.
• q=0 means the encoding is not acceptable.
• When multiple supported encodings have the same qvalue, Ferron should use its own server-side preference order.
• By default, when the client supports modern encodings with equal qvalues, Ferron should prefer modern compression over older encodings.

For example, with all variants available:

Accept-Encoding: gzip, deflate, br, zstd

Ferron should choose br or zstd according to its documented server-side preference, not gzip just because it appears first in the browser header.

---

This appears to affect at least:

• precompressed static file selection
• dynamic static compression
• the dcompress module

[DorianNiemiecSVRJS]

The current apporach used by Ferron 2 (and Ferron 3) seems technically RFC-compliant, but suboptimal... I might be going to fix that...

[DorianNiemiecSVRJS]

I think I have just fixed the suboptimal compression algorithm selection in Ferron 2 (b086e90) and Ferron 3 beta (38e3ce6). So the next version of Ferron 2 will have the fix.