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Commits
| Author | SHA1 | Message | Date | |
|---|---|---|---|---|
  retoorandClaude Sonnet 5
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684c6d95de |
Fix pack_write's O(n^2) dedup + correctness bug; document mount table O(n^2)
A audit for other instances of the file-index O(n^2) shape (fixed previously via the persistent treap) found two more real issues: 1. pack_write's Section 9.2 exact-duplicate elimination was a linear scan of every previously-seen (hash, size) pair per entry -- O(n^2) total, invisible in the existing benchmark because its identical-content test files made every scan match on the first comparison. Measured with unique content instead: 80,000 entries took 1.74s, with a 20,000->80,000 step showing 15.6x for a 4x-N step, matching O(n^2)'s 16x prediction. The same scan also trusted a (hash, size) match without ever comparing actual bytes -- a latent correctness bug, since FNV-1a64 is explicitly not collision-resistant. Fixed both at once with an open-addressing hash table (load factor 1/2, linear probing) plus a memcmp verification before ever reusing a data_off. Post-fix: 80,000 entries in 0.044s (39.6x faster), ratio drops to 3.35x (consistent with O(n)). Covered permanently by two new/extended tests: a white-box assertion in test_pack_overlay.c that duplicate-content entries share one data_off and distinct-content entries do not, and a new tests/test_pack_write_perf.c regression tripwire against 10,000 unique entries. 2. vfs.c's mount table uses the same full-array-copy-per-write pattern the file index used to, confirmed O(n^2) via a new bench/bench.c category (500/2,000/8,000 mounts, both 4x-N steps showing 15-20x). Deliberately NOT rewritten: mount points are created by a program's own source code, not workload-driven, so realistic mount counts never reach the scale that made the file index's O(n^2) a real problem. Documented with full reasoning in BENCH.md and CLAUDE.md rather than silently left as an undocumented gap. Also fixes a real CI gap the new tests exposed: ci.yml's sanitizer-build steps never passed -D_GNU_SOURCE when compiling test files (only the library .o's got it), which was harmless while no test included internal.h and became a link failure once two did (internal.h needs _GNU_SOURCE for pthread_rwlock_t). And documents, in CONTRIBUTING.md and CLAUDE.md, a sandbox flake observed directly during this work's own sanitizer runs: ASan/UBSan test binaries occasionally fail to start with AddressSanitizer:DEADLYSIGNAL (sometimes looping rather than exiting), non-deterministically hitting different unrelated binaries across runs -- a startup race, not a memory-safety bug, confirmed by clean passes on retry; sanitizer runs in such an environment should be timeout-wrapped. BENCH.md's "After" table and Appendix B are replaced with the current, complete 54-measurement bench/bench.c run (the original 45 plus the new mount-scaling category); the pre-fix 45-measurement "Before" table is kept as the historical record, per this project's documentation standard. Verified: make test (all 6 binaries, including the 2 new/changed), a clean make all, and repeated ASan+UBSan runs (0 real findings; the DEADLYSIGNAL flake above was observed and correctly distinguished from a real finding by re-running until a clean pass). TSan could not be run in this sandbox (pre-existing, documented environment limitation). Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01UqJpkdJ6Njnt1pw3CbghzB |