packfs/CONTRIBUTING.md

# Contributing to PackFS

## Before changing anything

Read [`concept.md`](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`](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`](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

```sh
make test                              # must pass before any PR
cc ... -fsanitize=address,undefined    # ASan/UBSan: see .github/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.

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.