CompressionFilter compresses with an encoding the client sent with q=0 in Accept-Encoding #668

CompressionFilterUtils::determineCompressionType (the check CompressionFilterFactory::onRequest uses through getFilterParams) chooses the first gzip or zstd token in Accept-Encoding and ignores its qvalue. A request that says "not gzip" therefore gets a gzip response:

(enableZstd = true. The output is from the repro below on main @ e0cef2c.)

RFC 9110 §12.5.3 says a listed content coding "is acceptable unless it is accompanied by a qvalue of 0", and §12.4.2 defines a qvalue of 0 as "not acceptable". proxygen's own RFC2616::acceptsEncoding already returns false for q=0 (AcceptEncodingTest.QValues in lib/http/test/RFC2616Test.cpp). Only the codec selection in lib/utils/CompressionFilterUtils.h (the find_if in determineCompressionType) skips the check. parseQvalues already returns the qvalue in TokenQPair::second.

Repro (public API only):

CompressionFilterUtils::FactoryOptions opts;
opts.enableZstd = true;
opts.compressibleContentTypes = std::make_shared<std::set<std::string>>(
    std::set<std::string>{"text/html"});
HTTPMessage req;
req.setURL("/index.html");
req.getHeaders().set(HTTP_HEADER_ACCEPT_ENCODING, "gzip;q=0");
auto params = CompressionFilterUtils::getFilterParams(req, opts);
// params->headerEncoding == "gzip"; expected: no FilterParams (folly::none)

Suggested fix: in the find_if predicate, skip tokens whose qvalue is <= 0. This is a four-line change, including a comment. The existing tests are unchanged, including GzipWithQuality (gzip; q=5.0). Picking the highest-q coding instead of the first acceptable one is a separate question, and this fix does not touch it.

Environment: x86_64 Linux, GCC 13.3, proxygen built with getdeps.py build proxygen. The code involved is unchanged on main @ 48f1ccc.

A fix with a regression test is ready. Could this issue be assigned to me? I can then open a pull request.

This report was drafted with Claude Code (claude-opus-5-5); the reproducer output above is from the runs described.