Task 01: Add `GitOperator.IsAncestor` helper

Add a thin GitOperator method that answers "is A a strict ancestor of B?" so the commits-panel base resolver (task 02) can detect a stale stored base without shelling out inline.

Context / Inputs

Spec: docs/specs/platform/requirements/workspace-git-status.md → "Base-commit staleness and refresh" (a stored base is stale when it is a strict ancestor of merge-base(HEAD, origin/<base_branch>)).
Existing helpers to mirror for style/error handling: GetMergeBase and GetRevParse in apps/backend/internal/agentctl/server/process/git_log.go (both use runGitCommand).
git merge-base --is-ancestor A B exits 0 (true), 1 (false), or >1 (real error). The current runGitCommand returns a non-nil error on any non-zero exit, so this helper MUST inspect the exit code to distinguish "false" (exit 1) from a genuine failure.

Implementation

Add to apps/backend/internal/agentctl/server/process/git_log.go:

// IsAncestor reports whether `ancestor` is a strict ancestor of `descendant`
// (git merge-base --is-ancestor). Exit 0 => true, exit 1 => false, anything
// else => error. Note: git treats a commit as an ancestor of itself, so an
// equal SHA returns true; callers wanting a STRICT check compare SHAs first.
func (g *GitOperator) IsAncestor(ctx context.Context, ancestor, descendant string) (bool, error)

Use the lowest-level command runner that exposes exit status (check how runGitCommand surfaces *exec.ExitError; if it collapses exit codes, add a sibling that returns the exit code, or use errors.As on the returned error to read ExitError.ExitCode()).
Empty ancestor or descendant returns (false, nil) without invoking git.

Acceptance

IsAncestor returns (true, nil) for a real ancestor, (false, nil) for a non-ancestor / diverged commit, and a non-nil error only for genuine git failures (bad repo, unknown SHA that git reports as error).
Equal SHAs return (true, nil) (documented; strictness is enforced by the caller).

Verification

cd apps/backend && go test -run 'IsAncestor' ./internal/agentctl/server/process/...

Add a table-driven test in apps/backend/internal/agentctl/server/process/git_log_test.go using setupTestRepo covering: ancestor, non-ancestor (sibling branch), equal SHA, and unknown-SHA error.

Files likely touched

apps/backend/internal/agentctl/server/process/git_log.go
apps/backend/internal/agentctl/server/process/git_log_test.go

Dependencies

None.

Parallelism

sequential. (Task 02 imports this helper.)

Output contract

Report: helper signature added, exit-code handling approach, test cases, go test result, any blockers. Update this file's status to in_progress at start and done at finish, and tick the task-01 checkbox in plan.md.

3.0 KiB Raw Blame History

Task 01: Add GitOperator.IsAncestor helper