nix flake prefetch-inputs: Skip build-time inputs

[edolstra]

Motivation

Build-time inputs can already be fetched in parallel, so prefetching them is usually not what you want.

Context

Summary by CodeRabbit

• New Features

  • The flake prefetch-inputs command now skips inputs marked with buildTime = true.

• Documentation

  • Updated documentation to clarify that flake prefetch-inputs recursively fetches transitive inputs and explicitly skips build-time inputs.

• Tests

  • Added test coverage to verify build-time inputs are properly skipped during prefetch operation.

[coderabbitai]

Walkthrough

The changes add support for skipping build-time inputs in the flake prefetch-inputs command. This includes a guard in the implementation that returns early when buildTime is set, updated documentation explaining recursive behavior and build-time input filtering, and test coverage verifying the feature works correctly.

Changes

Cohort / File(s)	Summary
Implementation src/nix/flake-prefetch-inputs.cc	Adds guard in visit function to return early and skip fetchToStore when lockedNode->buildTime is truthy, preventing fetching of build-time inputs
Documentation src/nix/flake-prefetch-inputs.md	Clarifies recursive nature of operation and adds explanation that build-time inputs (marked with buildTime = true) are skipped
Test Coverage tests/functional/flakes/build-time-flake-inputs.sh	Adds comprehensive test to verify prefetch-inputs ignores build-time inputs, including dependency flake setup, lockfile generation, cache clearing, and verification

Sequence Diagram

sequenceDiagram
    participant prefetch as prefetch-inputs
    participant visitor as LockedNode Visitor
    participant fetcher as fetchToStore
    
    prefetch->>visitor: visit(lockedNode)
    alt buildTime is set
        visitor->>visitor: early return
        Note over visitor: Skip this input
    else buildTime not set
        visitor->>fetcher: fetchToStore(...)
        fetcher-->>visitor: fetched
    end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Implementation requires careful verification of the guard logic and understanding of buildTime semantics
• Test setup involves git repo manipulation and multiple execution steps to verify correct behavior
• Documentation changes are straightforward but should be validated against implementation

Suggested reviewers

• cole-h

Poem

🐰 Build-time inputs, we skip with grace,
No fetching fuss in the prefetch race,
A guard stands tall, returns right quick,
When buildTime is true—that's the trick!
Tests verify this dance so bright,
Our flakes now prefetch just right! ✨

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 clearly and specifically describes the main change: making nix flake prefetch-inputs skip build-time inputs, which aligns with the modifications across all three files.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch eelcodolstra/fh-994-make-nix-flake-prefetch-inputs-ignore-build-time-inputs

---

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[github-actions]

🚀 Deployed on https://69171922fdb31446fa8944c8--determinate-nix.netlify.app

[coderabbitai]

Actionable comments posted: 1

🧹 Nitpick comments (1)

src/nix/flake-prefetch-inputs.cc (1)

47-48: Consider adding debug logging for skipped build-time inputs.

For better observability, consider logging when build-time inputs are skipped. This would help users understand why certain inputs are not being prefetched.

 if (lockedNode->buildTime)
+    logger->cout(lvlDebug, fmt("skipping build-time input '%s'", lockedNode->lockedRef));
     return;

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between ef054dc and dc4f411.

📒 Files selected for processing (3)

• src/nix/flake-prefetch-inputs.cc (1 hunks)
• src/nix/flake-prefetch-inputs.md (1 hunks)
• tests/functional/flakes/build-time-flake-inputs.sh (1 hunks)

🧰 Additional context used

🧬 Code graph analysis (1)

tests/functional/flakes/build-time-flake-inputs.sh (1)

tests/functional/flakes/common.sh (2)

• createGitRepo (127-136)
• createSimpleGitFlake (42-48)

⏰ 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 / build
• GitHub Check: build_x86_64-linux / build

🔇 Additional comments (2)

src/nix/flake-prefetch-inputs.cc (1)

47-48: LGTM! The guard correctly skips build-time inputs.

The early return ensures build-time inputs are not prefetched, which aligns with the PR objective. The placement after the done check and before the fetchToStore call is appropriate.

src/nix/flake-prefetch-inputs.md (1)

15-15: LGTM! Documentation accurately describes the new behavior.

The documentation clearly explains that the operation is recursive and that build-time inputs (with buildTime = true) are skipped. This aligns with the implementation changes.