[change_lister] Unshallow fetch to fix git merge-base in shallow clones

[andrew-anyscale]

In CI environments like Buildkite, repos are cloned with --depth=1 and
PR heads are fetched with --depth=1, leaving both the master tip and PR
commit as grafted roots with no shared history. This causes
git merge-base origin/master to fail with exit status 1.

Fix by checking git rev-parse --is-shallow-repository before fetching.
If shallow, use --unshallow to fetch the full base branch history so
merge-base can find the common ancestor. This avoids masking unrelated
errors (network, permissions) that would previously be swallowed by the
fallback.

Add TestListChangedFiles_ShallowClone to reproduce the failure and
verify the fix.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com
Topic: fix-shallow
Signed-off-by: andrew andrew@anyscale.com

[andrew-anyscale]

Reviews in this chain:
└#457 [change_lister] Unshallow fetch to fix git merge-base in shallow clones

[andrew-anyscale]

#	head	base	diff	date	summary
0	346a6dbd	d036417e	diff	Mar 13 11:47 AM	2 files changed, 48 insertions(+), 3 deletions(-)
1	21ac8247	d036417e	diff	Mar 13 11:57 AM	1 file changed, 8 insertions(+), 4 deletions(-)

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves an issue where git merge-base commands fail in shallow Git clones, a common scenario in CI environments like Buildkite. The changes ensure that the Git history is sufficiently deep to find a common ancestor by attempting an unshallow fetch, thereby preventing errors and improving the reliability of change detection.

Highlights

• Git Fetch Logic: Modified the ListChangedFiles function to first attempt a git fetch --unshallow for the base branch to ensure a complete history for git merge-base operations.
• Shallow Clone Handling: Implemented a fallback mechanism where if git fetch --unshallow fails (e.g., in a complete repository), a standard git fetch is performed instead.
• Test Coverage: Added TestListChangedFiles_ShallowClone to reproduce and verify the fix for git merge-base failures in shallow clone environments, simulating CI setups like Buildkite.

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Changelog

• raycicmd/change_lister.go
  • Modified the ListChangedFiles function to include an --unshallow option in the initial git fetch command for the base branch.
  • Added error handling to fall back to a regular git fetch if the unshallow operation is not applicable or fails.
• raycicmd/change_lister_test.go
  • Introduced TestListChangedFiles_ShallowClone, a new test case that simulates a shallow Git clone environment.
  • The test verifies that ListChangedFiles correctly identifies changed files even when the repository is shallowly cloned and the base branch is fetched shallowly.

Activity

• No specific activity (comments, reviews, or progress updates) has been recorded for this pull request yet.

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[gemini-code-assist]

Code Review

This pull request addresses an issue with git merge-base failing in shallow clones, which is common in CI environments. The fix involves unshallowing the repository before trying to find the merge-base. The change is well-supported by a new test case that accurately reproduces the problem and verifies the solution.

My main feedback is a suggestion to improve the robustness of the implementation in raycicmd/change_lister.go. Instead of relying on error-based fallback, I recommend explicitly checking if the repository is shallow first. This will make the logic clearer and the error handling more precise.

Overall, this is a valuable fix for a real-world CI problem.