fix: handle None branch in build_canceller

[Mic92]

Fixes #522 where buildbot-master fails to start with AttributeError when sourcestamp has an explicit None branch value. dict.get() only uses the default when the key is missing, not when the value is None.

Summary by CodeRabbit

• Bug Fixes
  • Improved handling of builds with missing branch information to prevent potential errors.

[coderabbitai]

Walkthrough

Modified branch_key_for_pr in build_canceller.py to coerce None branch values to empty strings using the pattern build.get("branch") or "" instead of build.get("branch", ""). This prevents AttributeError when branch is None and .startswith() is called.

Changes

Cohort / File(s)	Change Summary
Branch None handling fix buildbot_nix/buildbot_nix/build_canceller.py	Updated branch retrieval to safely coerce None values to empty string, preventing AttributeError when calling .startswith() on a None branch during build cancellation logic

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~5–10 minutes

• Focus areas:
  • Verify that coercing None to "" does not alter the intended behavior of PR branch detection (refs/pull/*, refs/merge-requests/*)
  • Confirm that the fix addresses all code paths where branch may be None without introducing new edge cases
  • Check for any other locations in the codebase that may have similar None-handling vulnerabilities

Possibly related PRs

• Cancel old builds #501: Introduced the branch_key_for_pr function that is being modified in this PR to handle None branch values safely.

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues Check	⚠️ Warning	The PR addresses the core crash fix specified in issue #522 by updating branch retrieval logic to handle None values, preventing the AttributeError when branch.startswith() is called on None. However, linked issue #522 explicitly requires regression tests covering sourcestamp.branch == None and PR ref patterns (refs/pull/, refs/merge-requests/), which are not shown in the provided changeset summary. While the immediate crash is fixed, the absence of regression tests means the PR does not fully satisfy all coding-related requirements from the linked issue.	Add regression tests that specifically cover the case where sourcestamp.branch is None, as well as branch values that match PR reference patterns. This will prevent future regressions and verify that the fix handles all edge cases mentioned in issue #522. Include tests in the changeset to demonstrate that the fix works correctly and that canceller behavior is not affected.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	The PR title "fix: handle None branch in build_canceller" is clear, concise, and directly related to the main change in the changeset. It accurately summarizes the primary modification—handling None values for the branch field in the build_canceller module—in a way that a teammate scanning the commit history would immediately understand the purpose of the change.
Out of Scope Changes Check	✅ Passed	The changeset modifies only buildbot_nix/buildbot_nix/build_canceller.py to handle None branch values, which is directly aligned with the PR objective of fixing the startup crash from issue #522. The change is narrowly scoped to the specific function that was failing (branch_key_for_pr) and does not introduce unrelated modifications or changes to other areas of the codebase.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

✨ 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 fix-buildbot-canceler

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

Actionable comments posted: 1

📜 Review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 01dfc9a and 3ba0717.

📒 Files selected for processing (1)

• buildbot_nix/buildbot_nix/build_canceller.py (1 hunks)