feat: add Git-AI trailer with UUID to commits and authorship notes

[svarlamov]

feat: add Git-AI trailer with UUID to commits and authorship notes

Summary

Adds a Git-AI: <uuid> trailer to every commit message and stores the same UUID as a new id field in the authorship note metadata, creating a link between the two. Also ensures the id is preserved or generated across all rewrite flows (rebase, cherry-pick, CI squash/rebase).

How it works (commit flow):

1. Pre-commit: Generate UUID v4, write to .git/ai/pending_note_id temp file via RepoStorage
2. Wrapper mode: Inject --trailer "Git-AI: <uuid>" into the git commit args forwarded to real git (inserted before any -- pathspec separator). A -c trailer.Git-AI.ifExists=replace global arg ensures --amend replaces the original trailer rather than appending a duplicate.
3. Hooks mode: In prepare-commit-msg, use git interpret-trailers --in-place --if-exists replace to append (or replace) the trailer in the commit message file. The needs_prepare_commit_msg_handling() gate was updated to also trigger when a pending_note_id file exists. A cherry-pick guard prevents stale UUIDs from aborted commits leaking into cherry-picked messages.
4. Post-commit / amend: Read the pending UUID from the temp file, set authorship_log.metadata.id, then clean up the temp file

How it works (rewrite flows):

• Rebase (rewrite_authorship_after_rebase_v2): On the full recompute path, reads the original commit's note id via get_reference_as_authorship_log_v3 and carries it to the rewritten commit's note. Uses .and_then() chaining to always reset id each loop iteration, preventing stale id leakage when a source note lookup fails. The fast-path remap already preserved id since it only touches base_commit_sha.
• Cherry-pick (rewrite_authorship_after_cherry_pick): Same pattern — source commit's id is read and set on the cherry-picked commit's note during recomputation. Fresh authorship_log per iteration means no cross-iteration leak risk.
• CI squash/rebase (rewrite_authorship_after_squash_or_rebase): Generates a new UUID v4 for the merge note since there's no single source commit to inherit from.
• Metadata-only notes (build_metadata_only_authorship_log_from_source_notes): Also generates a new UUID v4 for notes that carry only prompt metadata without file attestations.

The id field on AuthorshipMetadata is Option<String> with #[serde(default, skip_serializing_if = "Option::is_none")] for backward compatibility with existing notes.

Worktree support: pending_note_id_exists() delegates to the canonical worktree_storage_ai_dir() helper (made pub(crate)) for path resolution. A new resolve_git_common_dir() reads git's commondir file from linked worktree git directories, matching how git itself resolves the common directory — no manual path reconstruction needed.

Review & Testing Checklist for Human

• Verify --amend produces exactly one Git-AI trailer: Run git commit --amend --no-edit (and repeated amends) and confirm only one Git-AI trailer is present via git log --format='%(trailers:key=Git-AI)'. The fix relies on trailer.Git-AI.ifExists=replace (wrapper) and --if-exists replace (hooks) — verify these work with the project's minimum supported git version (flags added in git 2.15).
• Verify rebase id preservation on the recompute path: The rebase tests mostly exercise the fast-path remap (which trivially preserves id). To test the full recompute path, rebase commits that modify AI-touched files with content conflicts, then verify note id via git notes --ref=refs/notes/ai show <new-commit>. Pay particular attention to the .and_then() reset pattern at rebase_authorship.rs:729-735 — this prevents inter-iteration id leakage when a source note lookup fails.
• Verify cherry-pick id preservation uses fresh authorship_log per iteration: The cherry-pick path still uses nested if let (not .and_then()), relying on a fresh authorship_log being created each loop iteration. Confirm this assumption holds by reviewing the loop structure at rebase_authorship.rs:947-955.
• Verify the cherry-pick guard in prepare-commit-msg: The new !is_cherry_pick_in_progress(&repo) guard at git_hook_handlers.rs:2455 prevents stale pending_note_id files (from aborted commits) from leaking into cherry-picked messages. This relies on CHERRY_PICK_HEAD existing when prepare-commit-msg fires during cherry-pick — standard git behavior, but worth manual verification. Note: there is no automated test for the abort→cherry-pick stale trailer scenario; only the guard itself was added.
• Confirm CI squash/rebase UUID generation semantics: Merge commits get a new UUID rather than inheriting from any source commit. Verify this is desired behavior — the merge note id will NOT match source commits.
• Test plan: Run task test in both wrapper (default) and hooks mode (GIT_AI_TEST_GIT_MODE=hooks). All 27 tests pass locally (16 original trailer tests + 11 new rewrite tests). Manually test with git-ai installed: make commits, amend (multiple times), rebase, cherry-pick, and CI-style merges in both main and linked worktrees. Verify trailers in git log and note id matches via git notes --ref=refs/notes/ai show <commit>. For rebase/cherry-pick, confirm id preservation across rewrites. Test the cherry-pick guard by aborting a commit (close editor) then cherry-picking — verify no stale trailer appears.

Notes

• Tests added: 16 original trailer UUID tests (8 base + 8 worktree variants) + 11 new rewrite tests (covering rebase, cherry-pick, CI squash paths, including worktree variants) = 27 total, all passing in both wrapper and hooks mode
• Insta snapshots: 3 snapshots updated to include the new id: None field
• Existing tests: commit_hooks_comprehensive.rs tests updated for the &mut signature change on commit_pre_command_hook
• Git-compat whitelist: Expanded significantly to cover trailer-induced commit message format changes (38 new entries across 9 test files)
• Missing from original spec: test_dry_run_commit_no_trailer (test 8 of 9) was not implemented. Dry-run commits correctly skip trailer injection but lack a dedicated test.
• Signature change: commit_pre_command_hook signature changed to &mut ParsedGitInvocation to allow in-place modification of args for trailer injection
• Git version requirement: The --if-exists flag for git interpret-trailers and -c trailer.<key>.ifExists config were added in git 2.15 (Oct 2017). Verify this aligns with the project's minimum supported git version.

Updates since last revision:

• Added id preservation during rebase rewrites (rewrite_authorship_after_rebase_v2)
• Added id preservation during cherry-pick rewrites (rewrite_authorship_after_cherry_pick)
• Added UUID generation for CI squash/rebase merges (rewrite_authorship_after_squash_or_rebase, build_metadata_only_authorship_log_from_source_notes)
• Added 11 comprehensive tests in tests/rewrite_trailer_uuid.rs covering all rewrite flows using TDD methodology
• Fixed rebase loop id leak bug: Changed from nested if let to .and_then() chain to ensure current_authorship_log.metadata.id always resets when source note lookup fails, preventing inter-iteration leakage
• Fixed stale UUID leak in cherry-pick: Added !is_cherry_pick_in_progress(&repo) guard to prepare-commit-msg trailer injection to prevent stale pending_note_id from aborted commits leaking into cherry-picked messages
• Fixed duplicate Git-AI trailers on --amend: Both wrapper mode (via -c trailer.Git-AI.ifExists=replace) and hooks mode (via --if-exists replace) now replace existing trailers instead of appending duplicates, preventing accumulation of stale trailers on repeated amends
• Applied cargo fmt formatting

Link to Devin Session: https://app.devin.ai/sessions/ecd79d3689984605abd6bfd6b01280ad
Requested by: @svarlamov

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[devin-ai-integration]

Addressed the stale pending_note_id issue from Devin Review Comment 17 in commit 688e886 — added !is_cherry_pick_in_progress(&repo) guard to the prepare-commit-msg trailer injection. This prevents a stale UUID from an aborted commit leaking into cherry-picked commits.

Full code review summary:

All 5 Devin Review issues have been addressed:

1. prepare-commit-msg skipped for normal commits in hooks mode (fixed in 9fe1c6a)
2. --trailer args after -- separator (fixed in 9fe1c6a)
3. Wrong worktree path in pending_note_id check (fixed in a99732a, refactored in 098db92)
4. Rebase loop id leak (fixed in 0810637)
5. Stale pending_note_id during cherry-pick (fixed in 688e886)

Additional review findings (no issues):

• Insert order for --trailer args is correct
• No double-trailer risk (hooks mode discards wrapper args)
• No race condition risk (git index.lock prevents concurrent commits; worktrees have separate storage)
• Cherry-pick id preservation uses fresh authorship_log per iteration — no leak risk
• .and_then() chain in rebase correctly resets id each iteration
• resolve_git_common_dir correctly reads git's commondir file for linked worktrees

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