Improve path display

[edolstra]

Motivation

This fixes various cases where Nix would show store paths instead of source trees in error messages, e.g.

error: path '/nix/store/v57qvdsi3n14hlzdrxkbv3dxj8781818-source/READMEE.md' does not exist

instead of

error: path '«file:///home/eelco/Dev/nix/nixpkgs.tar.gz»/READMEE.md' does not exist

It also removes now unnecessary substitution code in fetchTarball as a followup to #468.

Context

Summary by CodeRabbit

• Bug Fixes

  • Better distinction of missing-file errors (throws FileNotFound vs other system errors) and clearer "path … does not exist" messages.
  • Improved path display for substituted/impure sources so missing-file errors reference original source paths.

• New Features

  • Tarball inputs now set explicit type/name and mark content-addressed inputs as final to enable later substitution.

• Tests

  • Added/updated unit and functional tests covering impure vs pure fetch/read behavior and new utility helpers.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a311a954-605c-41db-a44e-2574a3291548

📥 Commits

Reviewing files that changed from the base of the PR and between 468a7cd and e99f276.

📒 Files selected for processing (1)

• tests/functional/flakes/source-paths.sh

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📝 Walkthrough

Walkthrough

This PR adds a mark-last range adaptor, standardizes FileNotFound errors across source accessors, extends UnionSourceAccessor to accept a separate display accessor, updates callers to use the new factory signature, removes the SubstitutedSourceAccessor wrapper, and enables tarball fetch substitution with tests.

Changes

Source Accessor Display and Error Handling

Layer / File(s)	Summary
Range utility and exception standardization src/libutil/include/nix/util/util.hh, src/libutil-tests/util.cc, src/libutil/posix-source-accessor.cc, src/libutil/memory-source-accessor.cc, src/libfetchers/git-utils.cc, src/libutil-tests/source-accessor.cc	markLast range adaptor pairs each element with a boolean indicating if it is last. POSIX accessor now distinguishes ENOENT from other open errors to throw FileNotFound. Memory and Git accessors updated to throw FileNotFound consistently with "path does not exist" message. Test expectations adjusted.
Union accessor display support src/libutil/include/nix/util/source-accessor.hh, src/libutil/union-source-accessor.cc	UnionSourceAccessor constructor accepts displayAccessor parameter; readFile and readLink iterate with markLast and return early on final accessor; showPath conditionally delegates to displayAccessor; factory signature updated to require displayAccessor.
Consumer integration and cleanup src/libexpr/eval.cc, src/libfetchers/fetchers.cc	EvalState passes displayAccessor to new union accessor factory. SubstitutedSourceAccessor wrapper struct removed; accessor creation uses store.requireStoreObjectAccessor directly.
Tarball fetch substitution and test coverage src/libexpr/primops/fetchTree.cc, tests/functional/flakes/source-paths.sh	Tarball fetch unpack path sets __final = true and narHash when expectedHash is provided, enabling store substitution. Functional tests expanded to verify impure-mode error messages reference original Git paths and test narHash behavior in pure vs. impure modes.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant UnionSourceAccessor
  participant displayAccessor
  participant accessors as accessors vector
  Caller->>UnionSourceAccessor: readFile(path)
  UnionSourceAccessor->>accessors: iterate with markLast
  loop for each accessor until last
    accessors->>UnionSourceAccessor: maybeLstat result
  end
  Note over UnionSourceAccessor: early return on last accessor
  Caller->>UnionSourceAccessor: showPath(path)
  alt displayAccessor present
    UnionSourceAccessor->>displayAccessor: showPath(path)
  else
    UnionSourceAccessor->>UnionSourceAccessor: fallback SourceAccessor::showPath
  end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• DeterminateSystems/nix-src#468: Updates tarball fetch unpack behavior and substitution logic in src/libexpr/primops/fetchTree.cc alongside this PR's changes.

Suggested reviewers

• cole-h

Poem

🐰 I hopped through code and tuned each path,

FileNotFound now speaks with a clearer math;
Union shows the source it means to say,
markLast whispers which step ends the play,
Tarballs mark final, tests confirm the way.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 4.35% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Improve path display' directly relates to the main objective of fixing error message path displays to show original source trees instead of store paths, which is the core change across all modified files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch improve-show-path

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