#506 land: perf-spawn-without-fork

Accepted agent delivery; branch-keyed search descriptor included.

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Summary by cubic

Stops formatter, checker, and git spawns in the daemon from forking the process: they now spawn through posix_spawn, and the per-edit git root-commit probe is reused until the repository's git files change. Forking the large multi-threaded daemon took every malloc lock and stalled allocation process-wide on macOS.

Details

• isolate_in_process_group sets process_group(0) instead of a pre_exec setsid, keeping child spawns on the posix_spawn path; timeout kills still reach grandchildren through the process group, and unreaped zombies count as exited, not alive.
• A bare program name whose effective PATH differs from the process PATH is resolved against that PATH in the parent and spawned by absolute path with the same argv[0] and child environment; relative or empty PATH entries keep the child-side lookup.
• Root-commit results are cached keyed on HEAD and the git files that decide it; the cache holds only when a stamp re-captured after probing matches. Nothing is cached for git env overrides, reftable, symlink identities, or non-branch HEADs.
• Background bash keeps its fork path.

Confidence Score: 3/5

Fix the tool-search fallback and stale root keys before merging.

Findings

1.  Working tools are skipped ▶
2.  History changes keep old keys ▶

Summary

The PR reduces daemon forks and reuses Git root probes.

• Formatters and Git helpers avoid fork-based launches.
• Repeated lookups reuse a Git root-commit result while repository history stays unchanged.

_{Reviews (3) · Last reviewed commit: "mason: validate root-commit cache stamps..." · Reviewed by Greptile}