nix-tools: share the git-fetch cache between plan2nix and stack2nix (#1689)

[hamishmack]

Summary

Plan2Nix.Cache and Stack2nix.Cache were supposed to be identical (the
Plan2Nix copy literally opened with -- Note: this is identical to Stack2nix.Cache)
but had drifted apart in a way that mattered:

• Plan2Nix.Cache stored a NOHASH placeholder for empty hashes, so the
  whitespace-delimited cache format still round-trips through words — but it
  read and wrote the file lazily.
• Stack2nix.Cache forced the reads/writes strictly (<$!!> / $!) — but
  wrote a raw empty string for a missing hash. Under words an empty field
  simply disappears, so a local-package cache line loses a column and no longer
  parses back to the 6-tuple: latent cache corruption.

This externalises the logic into a single library module NixTools.Cache
that keeps the union of the correct behaviours — the NOHASH placeholder and
the strict IO — then repoints both Stack2nix and Plan2Nix at it and deletes
the two old modules. plan-to-nix already depends on the nix-tools library, so
no new dependency is needed.

Changes

• Add nix-tools/nix-tools/lib/NixTools/Cache.hs (merged, more-defensive impl).
• Delete lib/Stack2nix/Cache.hs and plan2nix/Plan2Nix/Cache.hs.
• import NixTools.Cache in lib/Stack2nix.hs and plan2nix/Plan2Nix.hs.
• cabal: expose NixTools.Cache in the library (replacing Stack2nix.Cache);
  drop Plan2Nix.Cache from plan-to-nix's other-modules.

Testing

The public API and type signatures are identical to both old modules
(readCache/appendCache/cacheHits), and both call sites only import
(appendCache, cacheHits), so they compile unchanged — this is a
signature-preserving merge. I typechecked the new module standalone with GHC
(ghc -fno-code -Wall): it compiles; the only warnings (non-exhaustive
toTuple, unused sha256 in the cacheHits comprehension) are present
verbatim in both original modules, so there is no new warning/regression.

A full from-source nix-tools build (needs ghc-9.6 + the hnix closure) was
not run in this environment.

Release caveat: haskell.nix consumes nix-tools as a prebuilt static release
binary, so this change reaches haskell.nix builds only once a new nix-tools
release is cut.

Closes #1689.

[Copilot]

Pull request overview

Consolidates the git-fetch cache implementation used by plan-to-nix and stack-to-nix into a single shared library module (NixTools.Cache) to prevent format drift and cache corruption (notably around missing hashes and strict IO).

Changes:

• Introduce NixTools.Cache as the unified cache implementation (NOHASH placeholder + strict read/write).
• Repoint Plan2Nix and Stack2nix to import NixTools.Cache instead of tool-specific cache modules.
• Update nix-tools.cabal to expose NixTools.Cache and remove the duplicated module wiring.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
nix-tools/nix-tools/lib/NixTools/Cache.hs	New shared cache module; merges correct behaviors from both prior implementations.
nix-tools/nix-tools/plan2nix/Plan2Nix.hs	Switch cache import to the shared NixTools.Cache.
nix-tools/nix-tools/lib/Stack2nix.hs	Switch cache import to the shared NixTools.Cache.
nix-tools/nix-tools/nix-tools.cabal	Expose NixTools.Cache from the library; drop the duplicate module from plan-to-nix’s other-modules.
nix-tools/nix-tools/lib/Stack2nix/Cache.hs	Removes the now-redundant cache module implementation.

Comments suppressed due to low confidence (1)

nix-tools/nix-tools/lib/NixTools/Cache.hs:29

• readCache uses a non-exhaustive pattern match in toTuple, so any malformed/blank cache line will throw a PatternMatchFail and abort the entire read. Making this total removes a -Wall warning and yields a clearer error if it ever happens (behaviour still fails fast).

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