retoorandClaude Sonnet 5
684c6d95de
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
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6.5 KiB
Markdown
116 lines
6.5 KiB
Markdown
# Contributing to PackFS
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## Before changing anything
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Read [`concept.md`](concept.md) in full. It is the project's specification,
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not background reading — every backend, lock, and on-disk field in `src/`
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exists because a section of `concept.md` requires it, and most functions'
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comments cite the section they implement rather than re-explaining it.
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`concept.md` is frozen (see [`CLAUDE.md`](CLAUDE.md)): if you believe the
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design itself needs to change, that belongs in a new document that amends
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or supersedes it, not in an edit to `concept.md`.
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Then read [`CLAUDE.md`](CLAUDE.md), which states the load-bearing constraints
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a change must respect (static linking, the write model, the concurrency
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model, path containment, pack integrity) and the documentation register this
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project is written in.
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## Workflow
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```sh
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make test # must pass before any PR
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cc ... -fsanitize=address,undefined # ASan/UBSan: see .github/workflows/ci.yml for exact flags
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cc ... -fsanitize=thread # TSan, for anything touching src/upper.c, src/overlay.c, or src/vfs.c
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```
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Any change to the concurrency-sensitive files (`upper.c`, `overlay.c`,
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`vfs.c`) must be run under ThreadSanitizer, not just the plain test suite —
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a data race there is exactly the class of bug Section 5 of `concept.md`
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exists to prevent, and the plain build will not surface it.
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**Known environment limitation, stated plainly rather than glossed over:**
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ThreadSanitizer cannot run at all in some sandboxed/containerized
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development environments — including the one this project's own commit
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history was largely developed in — because TSan requires disabling ASLR
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for itself via `personality(ADDR_NO_RANDOMIZE)`, and some sandboxes block
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that syscall outright (confirmed here: `personality()` returns `EPERM`,
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and even a trivial unrelated pthread program fails identically with
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`FATAL: ThreadSanitizer: unexpected memory mapping`, not just PackFS code).
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If your environment can't run TSan, say so rather than silently skipping
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it or claiming verification that didn't happen — ASan/UBSan still catch
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real bugs (they found and fixed a genuine heap-use-after-free during this
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project's development, see the `reclaim_gate` note in `internal.h`) but do
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not do TSan's happens-before race analysis, so they are not a substitute
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for it. CI (`.github/workflows/ci.yml`) runs on a normal, unrestricted
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GitHub Actions VM where TSan is expected to work; treat a change as
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TSan-verified only once it has actually passed there or on a local machine
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that can run it, not merely because ASan/UBSan passed.
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**A second, separate environment quirk, also observed directly rather than
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assumed:** in the same kind of sandboxed environment, an ASan/UBSan-built
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test binary occasionally (non-deterministically, roughly 1 run in 5–10 in
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this project's own experience) fails to start at all, printing
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`AddressSanitizer:DEADLYSIGNAL` — and, in its worse form, printing that same
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line in an unbounded loop rather than printing it once and exiting, which
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can run for minutes and consume unbounded CPU/memory if left unattended.
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This has been observed hitting different, unrelated test binaries from run
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to run (in one session: `test_dir` and `test_mem`, in another: nothing at
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all, in another: `test_pack_overlay`), which — together with the fact that
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every one of those binaries passes cleanly on a repeat run — indicates a
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sandbox startup race (plausibly in the same family as the ASLR/mapping
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restrictions behind the TSan limitation above), not a bug in the binary
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being tested. **Always wrap sanitizer-build test runs in `timeout` in such
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an environment** (e.g. `timeout 30 ./build/san/test_name`) so a stuck run
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fails loudly and bounded instead of hanging; treat a `DEADLYSIGNAL` exit as
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inconclusive, re-run, and only treat the run as a real finding if the output
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contains an actual `ERROR: AddressSanitizer` or `runtime error:` string —
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`DEADLYSIGNAL` alone, with neither of those strings present, is this flake,
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not a memory-safety bug in PackFS. As with the TSan limitation, the correct
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response is to say this plainly and re-run until a clean pass is obtained
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(or hand off to CI, which runs on an unrestricted VM where this has not been
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observed), not to suppress or silently ignore a `DEADLYSIGNAL` exit.
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A change to the index structure in `upper.c` (the `TreapNode`/`treap_*`/
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`node_*` functions, `snapshot_upsert`, `snapshot_remove`, or the
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`UpperSnapshot`/`UpperEntry` representation) should be run through
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`make bench` before and after, not just `make test`: `BENCH.md` records a
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real, measured O(n²) cost the flat array this index used to be had in bulk
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sequential create/unlink, and the persistent treap that replaced it exists
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specifically to avoid regressing back into it — a correctness-preserving
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change to this code can still reintroduce that regression (e.g. an
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implementation that silently degrades to a linked list under adversarial
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insertion order) in a way `make test` alone cannot detect, only `make bench`
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plus `tests/test_index_stress.c`'s randomized-order correctness check can.
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## Scope
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Changes that add functionality `concept.md` Section 10 lists as explicitly
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out of scope for v0 (full POSIX semantics, enforced permissions/symlinks/hard
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links, cross-process concurrency, content-defined chunking/delta
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compression) should discuss the tradeoff with a maintainer first — those
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exclusions were deliberate design decisions, not gaps waiting to be filled.
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**`zip` and `tar` import/export backends will not be accepted, full stop —
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not discussed, not behind a flag.** `concept.md` Section 11 recommends them,
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but that recommendation is permanently superseded; see `CLAUDE.md`, "Project
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decisions that supersede concept.md." This is a harder line than the v0
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exclusions above, which are open to future discussion — this one is not.
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## Static-linking constraint
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`concept.md` Section 11.1 is a hard constraint: the core must build with
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zero required third-party libraries and zero dynamic loading. In practice
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this project has no optional-dependency boundary at all: `concept.md`
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described one at the `zip`/`tar` backend for `miniz`/`libarchive`, but per
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the decision above, that backend will never exist, so there is no path by
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which a third-party dependency enters this codebase — a change proposing
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one, for any backend, will not be accepted.
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## Style
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Match the register already in the file you're editing — precise,
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constraint-labeled comments citing the `concept.md` section they implement,
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no filler. See the "Documentation standard" section of `CLAUDE.md` for the
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full rule; it applies to code comments and commit messages, not only to
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`.md` files.
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