Teach git refs migrate to support worktrees
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Remove the worktree limitation from `git refs migrate` by implementing migration support for repositories with linked worktrees. Previously, attempting to migrate a repository with worktrees would fail with "migrating repositories with worktrees is not supported yet". This limitation existed because each worktree has its own ref storage that needed to be migrated separately. Migration now uses a multi-phase approach to safely handle multiple worktrees: 1. Phase 1: Iterate through all worktrees and create temporary new ref storage for each in a staging directory. 2. Phase 2: For each worktree, backup the existing ref storage, then move the new storage into place. 3. Phase 3: Update the repository format config, clear cached ref stores, and delete all backups. On failure, restore from backups and report where the migrated refs can be found for manual recovery. This approach ensures that if migration fails partway through, the repository can be restored to its original state. Key implementation details: - For files backend: Create a commondir file in temp directories for linked worktrees so the files backend knows where the common git directory is located. - For linked worktrees: Use non-INITIAL transactions to avoid creating packed-refs files (linked worktrees should never have packed-refs). - Filter refs during iteration: Linked worktrees only migrate their per-worktree refs (refs/bisect/*, refs/rewritten/*, refs/worktree/*). Shared refs are migrated once in the main worktree. - Write per-worktree refs as loose files: The files backend's transaction_finish_initial() optimization writes most refs to packed-refs, but per-worktree refs must be stored as loose files to maintain proper worktree isolation. - Backup root refs: During Phase 2, backup all root refs (HEAD, ORIG_HEAD, etc.) by iterating files in the git directory and using is_root_ref() to identify them. This ensures safe rollback if migration fails. Tests are updated to expect migration with worktrees to succeed, and new tests verify: - Basic worktree migration in both directions (files ↔ reftable) - Migration with multiple worktrees - Dry-run mode with worktrees - Physical separation of per-worktree refs - Bare repository with worktrees Documentation is updated to reflect the worktree support and note that migration must be run from the main worktree. As the author's familiarity with git internals and C is limited, this change was made with the assistance of Claude Code. However, the author has carefully reviewed and iterated on the work to ensure quality to the best of their ability. Signed-off-by: Sam Bostock <sam.bostock@shopify.com> Co-Authored-By: Claude <noreply@anthropic.com>
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