GithubStatusNotificationHandler: Branches[^1] throws IndexOutOfRangeException on empty Branches list

[dnyw4l3n13]

Problem

In GithubStatusNotificationHandler.BuildStatusMessage(), the code unconditionally accesses message.Branches[^1]:

Branch lastBranch = message.Branches[^1];

If the GitHub status payload arrives with an empty Branches list, this throws IndexOutOfRangeException, crashing the handler.

Location

src/BuildBot.GitHub/Publishers/GithubStatusNotificationHandler.cs — BuildStatusMessage method

Fix

Guard with a null/empty check, or use a safe accessor such as:

Branch? lastBranch = message.Branches.Count > 0 ? message.Branches[^1] : null;

And handle the null case gracefully in the embed title.

Discovery

Identified during code review of PR #364 (coverage task). Not introduced by that PR — pre-existing gap.

[dnyw4l3n13]

Implementation Plan

Files to change

• src/BuildBot.GitHub/Publishers/GithubStatusNotificationHandler.cs — Fix BuildStatusMessage (line 56) to guard against an empty Branches list using message.Branches.Count > 0 ? message.Branches[^1] : (Branch?)null, and update the embed title interpolation to use lastBranch?.Name ?? string.Empty. Also fix AddBranchEmbed to apply a non-null fallback (?? string.Empty) on the FirstOrDefault() string result so WithValue never receives null when branches are empty.
• src/BuildBot.GitHub.Tests/Publishers/GithubStatusNotificationHandlerTests.cs — Add parameterised test cases covering the empty-branches path for each non-pending state (success, failure, error), verifying that Handle completes without throwing and that the mediator still receives exactly one Publish call.

Approach

In BuildStatusMessage, replace the unchecked Branch lastBranch = message.Branches[^1]; with a null-guarded form (Branch? lastBranch = message.Branches.Count > 0 ? message.Branches[^1] : null;) and update the embed title string interpolation to use lastBranch?.Name ?? string.Empty so an empty branches list produces a valid (if branch-less) title rather than an IndexOutOfRangeException. In AddBranchEmbed, append ?? string.Empty to the FirstOrDefault() call so WithValue never receives null when the list is empty. Existing test cases for the non-empty path are unchanged; new parameterised tests exercise the empty-branches path for each non-pending state.

Test strategy

Extend GithubStatusNotificationHandlerTests with a new helper MakeStatusWithNoBranches(string state) (or a parameter on the existing MakeStatus) that passes branches: []. Add a [Theory] / [InlineData] set covering "success", "failure", and "error" states with an empty branches list, asserting that Handle completes successfully and the mediator receives exactly one Publish call. This directly covers the previously-crashing code path.

Assumptions

• An empty string (string.Empty) is an acceptable fallback for the branch name in both the embed title and the branch embed field value (Discord accepts empty-string field values). If a non-empty placeholder such as "(unknown)" is preferred, that can be substituted without changing the logic.
• The fix must not alter existing behaviour for the non-empty branches case.

Open questions

None — ready to proceed pending approval.