fetchTree: Implicitly set __final = true when a narHash is supplied

[edolstra]

Motivation

This makes fetchTree substitutable, which is good for flake-compat. It also means that Git fetches will be implicitly shallow (since we don't need to get a revCount).

Alternative to NixOS#14634.

Context

Summary by CodeRabbit

• Breaking Changes

  • Removed the legacy final-tree public API; use the standard tree fetch interface instead.

• Improvements

  • Finality is now inferred from presence of a NAR hash rather than an explicit flag, simplifying tree resolution.

• Tests

  • Updated assertions for final behavior and NAR-hash error messages; removed an older test block that verified non-final substitution behavior.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Walkthrough

Removed the fetchFinalTree public API and the isFinal parameter; final-tree marking is now determined solely by presence of a NAR hash, with corresponding updates to primops, flake evaluation call sites, and tests.

Changes

Cohort / File(s)	Summary
Primop: fetchTree final API removal src/libexpr/primops/fetchTree.cc	Deleted prim_fetchFinalTree and its RegisterPrimOp; removed bool isFinal from FetchTreeParams; replaced explicit final-flag guard with final detection driven by presence of a narHash (sets __final when present) and removed else-final guard logic.
Flake evaluation changes src/libflake/call-flake.nix, src/libflake/flake.cc, src/libflake/include/nix/flake/flake.hh	Removed local fetchTreeFinal binding and retrieval/use of internal prim_fetchFinalTree; call-flake pathways no longer accept or pass a fetchFinalTree primitive and now rely on builtins.fetchTree; header declaration of prim_fetchFinalTree removed.
Tests: assertions and removals tests/functional/tarball.sh, tests/nixos/github-flakes.nix, tests/nixos/fetchers-substitute.nix	Added assertion that fetchTree with a narHash is final and does not expose lastModified; adjusted expected error wording for incorrect NAR hash; removed a test block in fetchers-substitute.nix that verified non-substitution behavior of fetchTree (test and exit-code assertion deleted).

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20–25 minutes

• Inspect src/libexpr/primops/fetchTree.cc for correctness of the new final-flagging via narHash and any remaining references to isFinal.
• Verify call sites in src/libflake/flake.cc and src/libflake/call-flake.nix no longer expect or pass the removed primitive.
• Check header cleanup in src/libflake/include/nix/flake/flake.hh for matching declarations.
• Confirm tests reflect runtime behavior and error-message wording changes.

Suggested reviewers

• grahamc
• cole-h

Poem

🐰 A NAR hash gleamed beneath the log,

"I'll tuck the flag inside my cog."
No extra gate, no dangling string,
The rabbit hops — a cleaner thing. 🥕

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: removing explicit fetchFinalTree API and making final-flagging implicit based on narHash presence.

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch eelcodolstra/fh-1214-make-fetchtree-final

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📜 Recent review details

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 36c91b0 and b378e48.

📒 Files selected for processing (1)

• tests/nixos/fetchers-substitute.nix (0 hunks)

💤 Files with no reviewable changes (1)

• tests/nixos/fetchers-substitute.nix

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (2)

• GitHub Check: build_aarch64-darwin / manual
• GitHub Check: build_aarch64-darwin / test

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[github-actions]

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