packfs/CONTRIBUTING.md

Contributing to PackFS

Before changing anything

Read concept.md in full. It is the project's specification, not background reading — every backend, lock, and on-disk field in src/ exists because a section of concept.md requires it, and most functions' comments cite the section they implement rather than re-explaining it. concept.md is frozen (see CLAUDE.md): if you believe the design itself needs to change, that belongs in a new document that amends or supersedes it, not in an edit to concept.md.

Then read CLAUDE.md, which states the load-bearing constraints a change must respect (static linking, the write model, the concurrency model, path containment, pack integrity) and the documentation register this project is written in.

Workflow

make test                              # must pass before any PR
cc ... -fsanitize=address,undefined    # ASan/UBSan: see .gitea/workflows/ci.yml for exact flags
cc ... -fsanitize=thread               # TSan, for anything touching src/upper.c, src/overlay.c, or src/vfs.c

Any change to the concurrency-sensitive files (upper.c, overlay.c, vfs.c) must be run under ThreadSanitizer, not just the plain test suite — a data race there is exactly the class of bug Section 5 of concept.md exists to prevent, and the plain build will not surface it.

Known environment limitation, stated plainly rather than glossed over: ThreadSanitizer cannot run at all in some sandboxed/containerized development environments — including the one this project's own commit history was largely developed in — because TSan requires disabling ASLR for itself via personality(ADDR_NO_RANDOMIZE), and some sandboxes block that syscall outright (confirmed here: personality() returns EPERM, and even a trivial unrelated pthread program fails identically with FATAL: ThreadSanitizer: unexpected memory mapping, not just PackFS code). If your environment can't run TSan, say so rather than silently skipping it or claiming verification that didn't happen — ASan/UBSan still catch real bugs (they found and fixed a genuine heap-use-after-free during this project's development, see the reclaim_gate note in internal.h) but do not do TSan's happens-before race analysis, so they are not a substitute for it. CI (.gitea/workflows/ci.yml, Gitea Actions — this project is hosted on Gitea, not GitHub) runs on this project's own self-hosted runner; whether that runner can execute TSan depends on that runner's own environment, which is not controlled by this repository the way a hosted-runner VM would be — do not assume CI's TSan step passing means what it would on an unrestricted machine without having actually checked what environment the registered runner provides. Treat a change as TSan-verified only once it has actually passed on a machine confirmed able to run it, not merely because a CI step reported success.

A second, separate environment quirk, also observed directly rather than assumed: in the same kind of sandboxed environment, an ASan/UBSan-built test binary occasionally (non-deterministically, roughly 1 run in 5–10 in this project's own experience) fails to start at all, printing AddressSanitizer:DEADLYSIGNAL — and, in its worse form, printing that same line in an unbounded loop rather than printing it once and exiting, which can run for minutes and consume unbounded CPU/memory if left unattended. This has been observed hitting different, unrelated test binaries from run to run (in one session: test_dir and test_mem, in another: nothing at all, in another: test_pack_overlay), which — together with the fact that every one of those binaries passes cleanly on a repeat run — indicates a sandbox startup race (plausibly in the same family as the ASLR/mapping restrictions behind the TSan limitation above), not a bug in the binary being tested. Always wrap sanitizer-build test runs in timeout in such an environment (e.g. timeout 30 ./build/san/test_name) so a stuck run fails loudly and bounded instead of hanging; treat a DEADLYSIGNAL exit as inconclusive, re-run, and only treat the run as a real finding if the output contains an actual ERROR: AddressSanitizer or runtime error: string — DEADLYSIGNAL alone, with neither of those strings present, is this flake, not a memory-safety bug in PackFS. As with the TSan limitation, the correct response is to say this plainly and re-run until a clean pass is obtained (or hand off to CI, which runs on an unrestricted VM where this has not been observed), not to suppress or silently ignore a DEADLYSIGNAL exit.

A change to the index structure in upper.c (the TreapNode/treap_*/ node_* functions, snapshot_upsert, snapshot_remove, or the UpperSnapshot/UpperEntry representation) should be run through make bench before and after, not just make test: BENCH.md records a real, measured O(n²) cost the flat array this index used to be had in bulk sequential create/unlink, and the persistent treap that replaced it exists specifically to avoid regressing back into it — a correctness-preserving change to this code can still reintroduce that regression (e.g. an implementation that silently degrades to a linked list under adversarial insertion order) in a way make test alone cannot detect, only make bench plus tests/test_index_stress.c's randomized-order correctness check can.

Scope

Changes that add functionality concept.md Section 10 lists as explicitly out of scope for v0 (full POSIX semantics, enforced permissions/symlinks/hard links, cross-process concurrency, content-defined chunking/delta compression) should discuss the tradeoff with a maintainer first — those exclusions were deliberate design decisions, not gaps waiting to be filled.

zip and tar import/export backends will not be accepted, full stop — not discussed, not behind a flag. concept.md Section 11 recommends them, but that recommendation is permanently superseded; see CLAUDE.md, "Project decisions that supersede concept.md." This is a harder line than the v0 exclusions above, which are open to future discussion — this one is not.

Static-linking constraint

concept.md Section 11.1 is a hard constraint: the core must build with zero required third-party libraries and zero dynamic loading. In practice this project has no optional-dependency boundary at all: concept.md described one at the zip/tar backend for miniz/libarchive, but per the decision above, that backend will never exist, so there is no path by which a third-party dependency enters this codebase — a change proposing one, for any backend, will not be accepted.

Style

Match the register already in the file you're editing — precise, constraint-labeled comments citing the concept.md section they implement, no filler. See the "Documentation standard" section of CLAUDE.md for the full rule; it applies to code comments and commit messages, not only to .md files.