Function getOrCreateWorktree

source code/utils/worktree.ts:237–377

* Creates a new git worktree for the given slug, or resumes it if it already exists. * Named worktrees reuse the same path across invocations, so the existence check * prevents unconditionally running `git fetch` (which can hang waiting for credentials) * on every resume.

(
  repoRoot: string,
  slug: string,
  options?: { prNumber?: number },
)

 * on every resume.
 */
async function getOrCreateWorktree(
  repoRoot: string,
  slug: string,
  options?: { prNumber?: number },
): Promise<WorktreeCreateResult> {
  const worktreePath = worktreePathFor(repoRoot, slug)
  const worktreeBranch = worktreeBranchName(slug)

  // Fast resume path: if the worktree already exists skip fetch and creation.
  // Read the .git pointer file directly (no subprocess, no upward walk) — a
  // subprocess `rev-parse HEAD` burns ~15ms on spawn overhead even for a 2ms
  // task, and the await yield lets background spawnSyncs pile on (seen at 55ms).
  const existingHead = await readWorktreeHeadSha(worktreePath)
  if (existingHead) {
    return {
      worktreePath,
      worktreeBranch,
      headCommit: existingHead,
      existed: true,
    }
  }

  // New worktree: fetch base branch then add
  await mkdir(worktreesDir(repoRoot), { recursive: true })

  const fetchEnv = { ...process.env, ...GIT_NO_PROMPT_ENV }

  let baseBranch: string
  let baseSha: string | null = null
  if (options?.prNumber) {
    const { code: prFetchCode, stderr: prFetchStderr } =
      await execFileNoThrowWithCwd(
        gitExe(),
        ['fetch', 'origin', `pull/${options.prNumber}/head`],
        { cwd: repoRoot, stdin: 'ignore', env: fetchEnv },
      )
    if (prFetchCode !== 0) {
      throw new Error(
        `Failed to fetch PR #${options.prNumber}: ${prFetchStderr.trim() || 'PR may not exist or the repository may not have a remote named "origin"'}`,
      )
    }
    baseBranch = 'FETCH_HEAD'
  } else {
    // If origin/<branch> already exists locally, skip fetch. In large repos
    // (210k files, 16M objects) fetch burns ~6-8s on a local commit-graph
    // scan before even hitting the network. A slightly stale base is fine —
    // the user can pull in the worktree if they want latest.
    // resolveRef reads the loose/packed ref directly; when it succeeds we
    // already have the SHA, so the later rev-parse is skipped entirely.
    const [defaultBranch, gitDir] = await Promise.all([
      getDefaultBranch(),
      resolveGitDir(repoRoot),
    ])
    const originRef = `origin/${defaultBranch}`
    const originSha = gitDir
      ? await resolveRef(gitDir, `refs/remotes/origin/${defaultBranch}`)
      : null
    if (originSha) {