fix: an inherited GIT_DIR silently redirected every baseline lookup

[pftg]

Found while triaging a test failure another lane reported as environment-specific noise. It is not noise — it is a silent false pass.

git -C <dir> sets the working directory, but GIT_DIR overrides it:

$ git -C /tmp/probe show HEAD:f.txt
hi
$ GIT_DIR=/elsewhere/.git git -C /tmp/probe show HEAD:f.txt
fatal: path 'f.txt' exists on disk, but not in 'HEAD'

So when a suite runs with GIT_DIR in the environment — any git hook exports it — checkout_vcs reads the wrong repository, every baseline lookup fails, and need_to_compare? is false. Since fail_if_new defaults to !ENV["CI"], the screenshot is recorded as new and the test passes. A visual regression suite that cannot find a single baseline reports success.

This is the same failure class as the missing-baseline false green, reached by a different route, and it is why this repo's own pre-push hook cannot run the suite.

The fix scrubs GIT_DIR, GIT_WORK_TREE and GIT_INDEX_FILE from the child environment at all four git call sites, so -C means what the surrounding code already assumes.

Guard: test/unit/vcs_test.rb runs checkout_vcs with GIT_DIR pointed at an unrelated repository and asserts the baseline is still checked out at the right size. Mutation-checked — emptying the scrubbed-env hash reds it; restored, green.

590 unit runs / 0 failures, standardrb clean.

Summary by Sourcery

Ensure Git baseline lookups honor their requested repository even when invoked from environments that define Git repository overrides.

Bug Fixes:

• Prevent inherited Git environment variables from redirecting baseline lookups to an unrelated repository and causing false-positive visual regression tests.

Tests:

• Add coverage confirming baseline checkout remains tied to the requested repository when GIT_DIR points elsewhere.

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📥 Commits

Reviewing files that changed from the base of the PR and between 4bd7416 and 54237c4.

📒 Files selected for processing (2)

• lib/snap_diff/vcs.rb
• test/unit/vcs_test.rb

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Reviewer's Guide

Scrubs Git-related environment variables from all Git invocations so baseline lookups always use the intended repository, and adds a unit test ensuring checkout_vcs ignores an inherited GIT_DIR.

Sequence diagram for repository-safe baseline checkout

sequenceDiagram
    participant Suite
    participant SnapDiff
    participant Git
    participant Repository

    Suite->>SnapDiff: checkout_vcs(root, screenshot_path, checkout_path)
    SnapDiff->>Git: system(GIT_ENV, git, -C, root, show, HEAD:file)
    Note over SnapDiff,Git: GIT_ENV clears GIT_DIR, GIT_WORK_TREE, and GIT_INDEX_FILE
    Git->>Repository: Resolve HEAD from root
    Repository-->>Git: Baseline content
    Git-->>SnapDiff: Write checkout_path
    SnapDiff-->>Suite: Baseline available for comparison

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Flow diagram for preventing inherited Git environment redirection

flowchart LR
    A[Inherited GIT_DIR or related Git variables] --> B[GIT_ENV clears Git environment overrides]
    B --> C[git -C root show HEAD:file]
    C --> D[Correct repository baseline]
    D --> E[need_to_compare? can compare screenshot]

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File-Level Changes

Change	Details	Files
Ensure all Git commands run with a scrubbed environment so -C root_path is not overridden by inherited Git variables.	Introduce a frozen GIT_ENV hash that clears GIT_DIR, GIT_WORK_TREE, and GIT_INDEX_FILE in child processes. Pass GIT_ENV as the environment argument to system calls that perform git show and git lfs smudge in checkout_vcs. Pass GIT_ENV as the environment argument to Open3.capture3 when resolving the Git root via rev-parse --show-toplevel.	lib/snap_diff/vcs.rb
Add a regression test verifying checkout_vcs uses its own root even when GIT_DIR points at an unrelated repository.	Create a temporary unrelated Git repository used to poison GIT_DIR during the test. Call SnapDiff::Vcs.checkout_vcs under a modified ENV where GIT_DIR points to the unrelated repo. Assert the baseline screenshot file exists and matches the size of the source screenshot, confirming correct baseline checkout. Introduce a with_env helper to temporarily override and restore ENV variables for tests.	test/unit/vcs_test.rb

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