feat(cache): classify auto-tracked inputs and outputs from access order

[wan9chi]

Motivation

Automatic input/output tracking cannot currently classify a path that a task
both reads and writes, and it has no notion of an output directory being cleaned
and republished. Two consequences show up on real workloads:

• Atomic publishing looks like nothing happened. A task that stages into
  dist.tmp and renames the directory into place has all of its writes recorded
  against a path that no longer exists at archive time. The archive comes out
  empty, so a later cache hit restores nothing and leaves a wrong tree.
• A task that reads its own derived state never settles. Generated files
  under a gitignored output directory get recorded as inputs, so the fingerprint
  changes every run and the task can never hit.

Together these are why a workspace like emdash
(20 packages, mostly plain tsdown) could not cache its build without
hand-written input/output globs. With this change it caches with no cache
configuration at all.

Approach

Classification is driven by the order of accesses to each path plus whether
the path is gitignored:

• write-only, or written before it was read → output
• read before it was written → output if gitignored, otherwise a modified input
• a rename destination counts as a write, and renaming a directory
  re-attributes the writes recorded under the old subtree
• an output that does not exist at archive time is dropped, but only if its
  absence is explained by an ancestor rename; otherwise the task is not cached
• a gitignored path underneath a directory this task wrote into is the task's
  own derived state, and is not an input

The last two matter for safety: a missed signal must degrade into a missed cache
entry, never a wrong tree.

To make this observable, fspy gains mutation intent. AccessMode widens to
u16 and adds CREATE, TRUNCATE, EXCLUSIVE, RENAME_FROM, RENAME_TO,
DELETED and IS_DIR. Rename and delete are now intercepted on all three
backends — on Windows both travel through NtSetInformationFile rather than
dedicated syscalls. CreateDisposition / open flags supply create and truncate,
which is what distinguishes a handle merely opened for writing from one that
actually replaces content.

Every flag is derived from a call's arguments and never from its result. The
Linux seccomp supervisor responds with SECCOMP_USER_NOTIF_FLAG_CONTINUE, so it
is notified before the syscall runs and can never learn the outcome. An
outcome-dependent rule would silently hold on macOS and Windows but not Linux.

CACHE_SCHEMA_VERSION goes 18 → 19, so existing entries are invalidated.

Notes for review

• DECISIONS.md records the rationale and the points where the design changed
  during development, including which earlier decisions the
  arguments-not-outcomes constraint invalidated. Happy to drop it from the
  branch if it is not wanted in the repo.
• Two commits fix pre-existing Windows-only clippy failures (vite_powershell,
  fspy::windows). Both items are behind cfg(windows), so the native lint runs
  never reach them and just lint-windows is red on main without them. They
  are unrelated to tracking.
• One pre-existing bug is not addressed here: pnpm's SEA build aborts with
  Assertion failed: (magic) == (kMagic) when run under fspy. It reproduces on
  main and blocks one emdash package.
• INVESTIGATE.md records two tracking issues these rules do not fix, both
  found while verifying against emdash. --parallel reproducibly loses cache hits
  because a reader fingerprints a dependency's output directory mid-write, and
  three emdash tasks never hit. Both trace to pnpm self-linking a workspace
  package, so packages/core/dist and node_modules/emdash/dist are one
  directory under two names and the own-derived-state rule only recognises the
  first. Neither is a regression; the second is a case these rules were meant to
  cover but attribute by the wrong path.

[wan9chi]

• feat(cache): classify auto-tracked inputs and outputs from access order #571 👈 (View in Graphite)
• main

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[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	cargo/​ignore@​0.4.31

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