jax_fingerprint: Reduce allocations and gate methods at build time

[maskit]

Summary

Reduces per-connection memory work in the jax_fingerprint plugin and makes the set of compiled-in fingerprint methods a configure-time choice.

• ContextMap is now an inline fixed-size table of (method-name, JAxContext*) pairs, replacing the std::unordered_map<std::string, JAxContext*> used previously. Lookup is a short linear scan; the bucket array and node allocations that the map performed on every TLS connection are gone.

• set_fingerprint takes std::string_view, dropping the temporary std::string that was materialised at every call site purely to bind a const std::string & parameter.

• ENABLE_JAX_METHODS is a new CMake cache variable (default "ja3;ja4;ja4h") that drives, from a single list:

  • which method subdirectories contribute sources to the plugin and to test_jax,
  • which ENABLE_JAX_METHOD_<NAME> preprocessor defines are set, and
  • the value of JAX_FINGERPRINT_MAX_METHODS (the inline context-table size, derived from the list length).

  Build slim: cmake -B build -DENABLE_JAX_METHODS="ja4". Empty list or unknown directory name causes a FATAL_ERROR at configure time.

• plugin.cc dispatcher is now a constexpr table of Method const * entries, each gated by the per-method ENABLE_JAX_METHOD_* macro. The dispatcher and the method #includes are the only place in the plugin that names concrete methods; method.h stays method-agnostic.

• README in the plugin directory documents the per-method file layout, the build-time switches, and the file-naming rules the CMake glob relies on (including the tls_client_hello_summary.cc historical carve-out and how to handle similar cases going forward).

Test plan

• cmake --build my-b-bssl -t jax_fingerprint — default (ja3;ja4;ja4h) builds cleanly.
• cmake --build my-b-bssl -t test_jax and run test_jax — 67 assertions across 5 Catch2 test cases pass.
• cmake --build my-b-bssl -t format — clang-format / cmake-format clean.
• AuTest jax_fingerprint* passes against the installed plugin.
• Slim build: cmake -B my-b-bssl -DENABLE_JAX_METHODS="ja4" — only ja4/*.cc compiled into the plugin.
• Negative configure paths: -DENABLE_JAX_METHODS="" and -DENABLE_JAX_METHODS="ja99" both fail at configure time with the intended messages.

Notes for reviewers

• ContextMap keys are std::string_views that alias Method::name (which is itself a string_view pointing at a string literal). The slot table stays valid as long as the Method struct does, i.e. for the lifetime of the process.
• The plugin's other lookup against a heterogeneous-lookup hash (the SNI server-name set in plugin.cc) is unchanged. The __cpp_lib_generic_unordered_lookup #ifdef there is still needed and is unrelated to ContextMap.
• JAX_FINGERPRINT_MAX_METHODS is not a separate user knob; CMake derives it from the length of ENABLE_JAX_METHODS. The #ifndef ... #define ... 8 in context_map.h is a fallback so the header is valid when compiled standalone (e.g. IDE indexing). A static_assert in context_map.h enforces a minimum of 1, and set() TSReleaseAsserts on overflow.

[Copilot]

Pull request overview

This PR optimizes the experimental jax_fingerprint plugin by removing per-connection container allocations and making the set of compiled fingerprinting methods a CMake-time choice, reducing runtime overhead and enabling slimmer builds.

Changes:

• Replaces the per-connection method→context hash map with a fixed-size inline slot table (ContextMap) and updates fingerprint-setting to accept std::string_view.
• Adds ENABLE_JAX_METHODS CMake cache variable to select method subdirectories, drive per-method compile definitions, and derive JAX_FINGERPRINT_MAX_METHODS.
• Updates method dispatch to a macro-gated constexpr method table and adds a developer README documenting method layout and build rules.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
plugins/experimental/jax_fingerprint/README	New developer-focused documentation for method layout, build-time switches, and file naming rules.
plugins/experimental/jax_fingerprint/plugin.cc	Gates method includes/dispatch entries at compile time via ENABLE_JAX_METHOD_* macros and uses a table-driven --method lookup.
plugins/experimental/jax_fingerprint/context.h	Changes set_fingerprint to accept std::string_view.
plugins/experimental/jax_fingerprint/context.cc	Implements the std::string_view signature and assignment into the stored std::string.
plugins/experimental/jax_fingerprint/context_map.h	Replaces unordered_map with a fixed-size (method_name, ctx*) slot table and linear scan lookup.
plugins/experimental/jax_fingerprint/CMakeLists.txt	Introduces ENABLE_JAX_METHODS to select sources/defines and derives JAX_FINGERPRINT_MAX_METHODS.

[Copilot]

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