Add cross-session file locking + concurrent build tests#11
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Move target and database lock files from per-session temp directories (/tmp/redo-<user>/<session>/) to a global location (/tmp/redo-<uid>-locks/). Previously, each top-level redo invocation used its own REDO_SESSION-scoped lock directory, meaning two separate `redo` commands could not coordinate locks on shared targets. This caused potential races when building the same target tree from multiple terminals or scripts. Now lock files are UID-namespaced in /tmp for auto-cleanup on reboot, and shared across all redo sessions for the same user. The flock() mechanism is unchanged, only the directory location moves.
Five tests exercising cross-session concurrency: 1. Same target from two processes simultaneously 2. Overlapping dependency chains with shared deep dep 3. Stress test: 4 processes building same target 4. Different targets with shared slow dependency 5. Rapid-fire rebuild after .do file invalidation Tests use sleep delays to create race windows and verify output correctness after concurrent builds complete.
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Two commits:
1. Cross-session file locking
Moves target and database lock files from per-session temp directories (
/tmp/redo-<user>/<session>/) to a global location (/tmp/redo-<uid>-locks/).Before: Each top-level
redoinvocation used its ownREDO_SESSION-scoped lock directory. Two separateredocommands could not coordinate locks on shared targets — they'd both build the same target simultaneously.After: Lock files are UID-namespaced in
/tmp(auto-cleanup on reboot) and shared across all redo sessions for the same user. Theflock()mechanism is unchanged — only the directory location moves.2. Concurrent build tests (
test/410-concurrent/)5 tests exercising cross-session concurrency:
Why
The existing locking only worked for intra-session parallelism (
-jflag). When two separateredoprocesses built overlapping targets, they couldn't see each other's locks. This is a real scenario in CI pipelines, multi-terminal development, and script-driven builds.