fix(github): ensure local branch exists for fork PR checkout

[jschwxrz]

summary:

• fix PR review worktree setup for forked PRs when direct fallback is needed
• remove --detach from fallback gh pr checkout
• after checkout explicitly verify refs/heads/ exists
• If missing create the local branch ref from HEAD

this ensures downstream worktree creation can reliably use the expected local branch for fork PR review flows.

[vercel]

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[greptile-apps]

Greptile Summary

This PR fixes fork PR checkout in the fallback path of ensurePullRequestBranch by removing --detach from the gh pr checkout command and adding an explicit local-branch-ref guard. After the checkout, git show-ref --verify confirms that refs/heads/<branch> exists; if it does not (a known gh/fork edge case), git branch --force <branch> HEAD creates it so that the downstream git worktree add call in WorktreeService.createWorktreeFromBranch can reliably reference the branch by name.

• The core logic is correct: the verification + fallback creation runs inside the outer try block, so any failure in git branch --force still propagates to the outer catch and is rethrown.
• The finally block correctly restores the previous HEAD ref regardless of outcome, unchanged from before.
• A stale comment on line 900 still says "with detach to avoid working tree conflicts", which is the opposite of the new behaviour — it should be updated.
• The error logged in the catch block at line 934 ("Failed to checkout pull request branch via gh:") can now be emitted for failures originating from git branch --force, which may make debugging harder; a more neutral message would improve observability.

Confidence Score: 4/5

• Safe to merge — the fix correctly addresses the missing local branch ref edge case for fork PRs with no logic regressions.
• The change is narrow and well-targeted: it removes --detach and adds a post-checkout guard that creates the local branch ref when gh pr checkout leaves HEAD detached. The finally block that restores the previous branch is unaffected. The only issues are a stale comment and a slightly misleading error log message — neither affects runtime correctness.
• No files require special attention beyond the stale comment on line 900 of src/main/services/GitHubService.ts.

Important Files Changed

Filename	Overview
src/main/services/GitHubService.ts	Removes --detach from gh pr checkout fallback and adds a post-checkout guard that creates a local branch ref via git branch --force if git show-ref shows the ref is missing. Logic is sound; one stale comment and a slightly misleading error message remain.

Sequence Diagram

sequenceDiagram
    participant Caller as githubIpc.ts
    participant GHS as GitHubService
    participant Git as git (exec)
    participant GH as gh CLI

    Caller->>GHS: ensurePullRequestBranch(path, prNumber, branchName)

    GHS->>Git: git fetch origin refs/pull/N/head:refs/heads/<branch> --force
    alt fetch succeeds
        Git-->>GHS: ok
    else fetch fails (fork PR / no direct access)
        Git-->>GHS: error
        GHS->>Git: git rev-parse --abbrev-ref HEAD
        Git-->>GHS: previousRef

        GHS->>GH: gh pr checkout <N> --branch <branch> --force
        GH-->>GHS: ok (working tree updated)

        GHS->>Git: git show-ref --verify --quiet refs/heads/<branch>
        alt ref exists
            Git-->>GHS: exit 0
        else ref missing (gh/fork edge case)
            Git-->>GHS: exit 1
            GHS->>Git: git branch --force <branch> HEAD
            Git-->>GHS: branch created
        end

        Note over GHS,Git: finally block
        GHS->>Git: git checkout <previousRef> --force
        Git-->>GHS: working tree restored
    end

    GHS-->>Caller: safeBranch (string)

    Caller->>GHS: WorktreeService.createWorktreeFromBranch(path, ..., branchName)
    GHS->>Git: git worktree add <worktreePath> <branchName>
    Git-->>GHS: worktree created

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Comments Outside Diff (1)

1. src/main/services/GitHubService.ts, line 900 (link)

   Stale comment still references --detach

   The comment on line 900 still says "try gh pr checkout with detach to avoid working tree conflicts", but this PR removes --detach from the command below. The comment now describes the opposite of what the code does — gh pr checkout without --detach explicitly will touch the working tree. The comment should be updated to reflect the new intent.

_{Last reviewed commit: 60cfd4a}

[arnestrickmann]

Thanks! good catch