Implement HuffmanCodec with LiteSpeed (#12357)

Performance tooling highlights hpack decoding as an expensive operation
for us. This improves hpack performance by using the MIT licensed
LiteSpeed implementation.

Fixes: #12321

Author: bneradt

NOTICE

@@ -111,3 +111,10 @@ https://fossies.org/linux/systemtap/includes/sys/sdt.h
 
 tools/ebpf/trylock-stats is try lock version of lockstat-solution.py of goldshtn/linux-tracing-workshop
 Copyright (c) 2017 Sasha Goldshtein (MIT License)
+
+~~
+
+LS-HPACK provides functionality to encode and decode HTTP headers using
+HPACK compression mechanism specified in RFC 7541.
+Copyright (c) 2018 - 2023 LiteSpeed Technologies Inc, (MIT License)
+https://github.com/litespeedtech/ls-hpack.git

include/proxy/hdrs/HuffmanCodec.h

@@ -23,11 +23,7 @@
 
 #pragma once
 
-#include <cstddef>
 #include <cstdint>
 
-void     hpack_huffman_init();
-void     hpack_huffman_fin();
-int64_t  huffman_decode(char *dst_start, const uint8_t *src, uint32_t src_len);
-uint8_t *huffman_encode_append(uint8_t *dst, uint32_t src, int n);
-int64_t  huffman_encode(uint8_t *dst_start, const uint8_t *src, uint32_t src_len);
+int64_t huffman_decode(char *dst, uint32_t dst_len, uint8_t const *src, uint32_t src_len);
+int64_t huffman_encode(uint8_t *dst, uint32_t dst_len, uint8_t const *src, uint32_t src_len);

lib/CMakeLists.txt

@@ -44,3 +44,5 @@ target_include_directories(catch2::catch2 INTERFACE "${CMAKE_CURRENT_SOURCE_DIR}
 
 add_library(systemtap::systemtap INTERFACE IMPORTED GLOBAL)
 target_include_directories(systemtap::systemtap INTERFACE "${CMAKE_CURRENT_SOURCE_DIR}/systemtap")
+
+add_subdirectory(ls-hpack)

lib/ls-hpack/CMakeLists.txt

@@ -0,0 +1,25 @@
+#######################
+#
+#  Licensed to the Apache Software Foundation (ASF) under one or more contributor license
+#  agreements.  See the NOTICE file distributed with this work for additional information regarding
+#  copyright ownership.  The ASF licenses this file to you under the Apache License, Version 2.0
+#  (the "License"); you may not use this file except in compliance with the License.  You may obtain
+#  a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software distributed under the License
+#  is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+#  or implied. See the License for the specific language governing permissions and limitations under
+#  the License.
+#
+#######################
+
+add_library(lshpack STATIC)
+target_sources(lshpack PRIVATE lshpack.cc huff-tables.h lshpack.h)
+
+# lshpack.h uses C99 array designators (see the hencs array in huff-tables.h).
+# In theory this limits portability because very old compilers that are C89 only
+# won't support this. However, the supported compilers for ATS should all be
+# modern enough to support this.
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-c99-designator")

lib/ls-hpack/README.md

@@ -0,0 +1,26 @@
+# LiteSpeed HPACK in ATS
+
+Originally, ATS included a home-grown HPACK encoder and decoder for HTTP/2
+header compression, featuring a custom Huffman coding implementation. As HTTP/2
+adoption grew and later with HTTP/3 evolving to QPACK, the need for
+high-performance header decompression became critical. Our original Huffman
+decoder was simple but performed bit-by-bit processing, which proved to be a
+performance bottleneck under heavy traffic.
+
+To address this, we replaced the ATS home-grown version with the open-source,
+MIT-licensed LiteSpeed HPACK library. LiteSpeed's implementation trades static
+memory (a fixed-size, compile-time-allocated lookup table of 64K entries) for
+drastically reduced computation. In `perf top` testing with production traffic,
+the CPU usage of the `huffman_decode` function dropped from about 1% to
+effectively unmeasurable levels, delivering a significant performance
+improvement.
+
+This integration maintains full HPACK compliance for HTTP/2 and seamlessly
+supports header compression performance for HTTP/3 (QPACK), leveraging the same
+Huffman decoding acceleration.
+
+# LiteSpeed Version
+
+The current implementation pulled into ATS is based upon what is currently the
+latest version,
+[v2.3.4](https://github.com/litespeedtech/ls-hpack/releases/tag/v2.3.4).