Use ls-hpack's fast Huffman decoder for HPACK/QPACK strings

[phongn]

Summary

When the home-grown Huffman codec was replaced with vendored LiteSpeed ls-hpack code (#12357), only the conservative 4-bit FSM decoder (lshpack_dec_huff_decode_full) was ported — although huff-tables.h has carried the 64K-entry table for upstream's fast decoder all along. This PR ports the fast decoder (lshpack_dec_huff_decode, from ls-hpack v2.3.5) and switches huffman_decode() to it.

The fast decoder consumes 16 bits of input per table lookup and emits up to 3 bytes, falling back to the FSM decoder for the rare codes longer than 16 bits. HPACK and QPACK share the wrapper through xpack_decode_string(), so both HTTP/2 and HTTP/3 header decoding benefit. No new memory footprint: the hdecs table has been compiled into the binary since the original vendoring.

Performance

tools/benchmark/benchmark_HuffmanDecode.cc (new; build with -DENABLE_BENCHMARKS=ON), release build, Ice Lake:

Input	FSM decoder	fast decoder	speedup
8B value (text/css)	103 ns	73 ns	1.4x
86B Accept value	1003 ns	505 ns	2.0x
113B User-Agent	1296 ns	659 ns	2.0x
10-value mixed corpus (459B)	5.32 µs	2.80 µs	1.9x

RFC 7541 strictness (deliberate divergence from upstream ls-hpack)

Differential testing revealed that upstream's fast decoder accepts padding of 8–10 bits when it follows the final symbol near the end of the input. RFC 7541 §5.2 requires that "a padding strictly longer than 7 bits MUST be treated as a decoding error", and the FSM decoder has always enforced this. The ported decoder adds a guard in the tail check to keep the strict behavior, so this PR does not loosen what ATS accepts. The divergence is documented in lib/ls-hpack/README.md and pinned by the deterministic decode_overlong_padding test, which fails if a future re-sync drops the guard.

Semantics and compatibility

• Validity and output are otherwise identical to the FSM decoder. Verified by: exhaustive parity over all 1- and 2-byte inputs, 100k seeded differential fuzz iterations (with out-of-bounds-write sentinel checks), 20k encode/decode roundtrips, and the RFC 7541 vectors. Offline, the port was additionally validated four-ways against upstream's own two decoders over exhaustive 1–3-byte inputs (16.8M cases) plus a 134M-case destination-size sweep: bit-identical to upstream's fast decoder apart from the strictness guard above.
• The one observable boundary is an exactly-sized destination (dst_len == decoded length), where either decoder may report LSHPACK_ERR_MORE_BUF depending on how trailing padding falls on nibble boundaries. One byte of headroom guarantees success; ATS sizes destinations at 2× the encoded length (strictly larger than any decoded result, since Huffman expansion is at most 8/5), so callers are unaffected. The sizing contract is now documented on huffman_decode().
• Error returns remain negative-on-failure as callers expect (xpack_decode_string() checks len < 0).

[Copilot]

Pull request overview

This PR ports LiteSpeed ls-hpack’s 16-bit table (“fast”) Huffman decoder into ATS’s vendored lib/ls-hpack and switches ATS’s huffman_decode() wrapper (used by both HPACK and QPACK via xpack_decode_string()) to use it, aiming to reduce header decompression CPU cost while preserving ATS’s stricter RFC 7541 padding validation.

Changes:

• Add lshpack_dec_huff_decode() (fast decoder) to the vendored ls-hpack code and wire huffman_decode() to use it instead of the 4-bit FSM decoder.
• Add extensive parity / fuzz / strict-padding regression tests to ensure the fast decoder matches the existing behavior (including ATS’s deliberate stricter padding rule).
• Add a Catch2 benchmark target to compare full vs fast decoder performance.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/benchmark/CMakeLists.txt	Adds the new Huffman decode micro-benchmark executable target.
tools/benchmark/benchmark_HuffmanDecode.cc	New Catch2 benchmark comparing lshpack_dec_huff_decode_full vs lshpack_dec_huff_decode on representative inputs.
src/proxy/hdrs/unit_tests/test_Huffmancode.cc	Adds known-vector decode coverage, exhaustive/parity tests, fuzzing, and strict padding regression coverage for the fast decoder.
src/proxy/hdrs/HuffmanCodec.cc	Switches ATS huffman_decode() wrapper to call the fast ls-hpack decoder.
src/proxy/hdrs/CMakeLists.txt	Ensures unit tests can include vendored ls-hpack headers (lib/).
lib/ls-hpack/README.md	Updates referenced upstream version and documents ATS’s deliberate strict-padding divergence.
lib/ls-hpack/lshpack.h	Exposes LSHPACK_ERR_MORE_BUF and declares lshpack_dec_huff_decode().
lib/ls-hpack/lshpack.cc	Implements the fast decoder and adds ATS-specific guard rejecting padding ≥ 8 bits.
include/proxy/hdrs/HuffmanCodec.h	Documents huffman_decode() sizing expectations (dst must be strictly larger than decoded output).