ADR-0033: One crate-scoped licence exception, Apache-2.0 WITH LLVM-exception for ar_archive_writer, which the Python parser's build requires

• Status: Accepted (2026-09-24, by the maintainer)
• Date: 2026-09-24
• Derives from: ADR-0019 (licence policy), ADR-0013 (ruff_python_parser for Python), ADR-0025 (cargo deny gates the supply chain), ADR-0026 (the precedent for a crate-scoped exception)
• Constrains: deny.toml
• Implemented by: Wave 2 plan, sub-wave 2B
• Requirements: FR-EXT-PY-01

Context

ruff_python_parser 0.0.14, pinned by the Python extractor under ADR-0013, depends unconditionally on stacker (it grows the stack for deeply nested source), which depends on psm (MIT OR Apache-2.0). psm's build script uses ar_archive_writer 0.5, a build-time dependency published by the Rust project, whose licence is Apache-2.0 WITH LLVM-exception. The deny.toml allow-list from ADR-0019 names Apache-2.0 but not the LLVM exception, so the deny job refuses the build.

The LLVM exception only adds permissions to Apache-2.0: it waives the attribution requirements for object code that embeds parts of the licensed work. Apache-2.0 is already allowed, so the exception licence is at least as permissive as one the project accepts. The crate runs at build time to assemble an archive; it is not a parser and reads no untrusted input.

Decision

• deny.toml gains one exception, scoped to the crate: ar_archive_writer may use Apache-2.0 WITH LLVM-exception. The global allow-list does not change.
• The exception follows the pinned ruff_python_parser version. When a bump drops the dependency, the exception is removed in the same pull request; cargo deny reports an unused exception so it cannot linger.

Consequences

• The third-party notices list one Apache-2.0-with-LLVM-exception build-time component.
• Any further non-listed licence still fails the deny job and needs its own ADR.

Alternatives considered

• Add the expression to the global allow-list. Rejected, as in ADR-0026: it would admit the licence for every future dependency without review.
• Patch ruff_python_parser to drop stacker. Rejected: a patched or git dependency is an ADR-level change under ADR-0025, and removing the stack guard would let deeply nested input overflow the stack, which architecture § Security posture forbids.
• Another Python parser. Rejected: ADR-0013 chose ruff's parser; reversing it for a build-time licence with strictly more permissions than Apache-2.0 would not be proportionate.