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fix(engine): auto-reattach HEAD at handoff when branch ref is authoritative

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The merge handoff refused with head-branch-mismatch whenever the reused
worktree's HEAD wasn't on fusion/<id> (detached, recycled to main, or on
a sibling branch), even when the branch ref itself still held a clean,
task-attributed lineage. That wedged FN-5339-class tasks in review for
no good reason.

Add isBranchAuthoritativeForTask in branch-conflicts.ts (branch ref
exists, tip carries Fusion-Task-Id trailer, base..branch is foreign-
contamination-free) and use it in acquireReuseHandoff: when HEAD drifts
but the branch ref is authoritative, run a plain `git checkout <branch>`
inside the already-asserted-clean worktree, re-read HEAD, and emit a
branch:auto-reattach-authoritative audit. Refusal still fires unchanged
when the branch ref is missing, missing the trailer, or contaminated,
so FN-5363 strict-lease and foreign-commit guards remain authoritative.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
gsxdsm
2026-05-21 19:03:35 -07:00
parent 025683ca60
commit d947197c2a
4 changed files with 184 additions and 5 deletions
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"@runfusion/fusion": patch
---
Engine reliability: auto-recover merge handoff from HEAD drift when the branch ref is authoritative.
- `acquireReuseHandoff` previously refused outright with `head-branch-mismatch / unexpected-branch` whenever the worktree's HEAD pointed at anything other than `fusion/<id>` (detached, recycled to `main`, on a sibling branch). The existing case-mismatch autocorrect did not cover these states, leaving FN-5339-class tasks wedged in review even though their branch ref still held a clean, task-attributed lineage.
- New `isBranchAuthoritativeForTask` helper (in `branch-conflicts.ts`) confirms the expected branch ref exists, its tip carries the `Fusion-Task-Id: <id>` trailer, and the `base..branch` range has no foreign FN-attributed commits.
- When that probe passes, the handoff now performs a plain `git checkout <branch>` (not `-B`, which would clobber the ref) inside the already-asserted-clean worktree, re-reads HEAD, and emits a `branch:auto-reattach-authoritative` audit. Refusal still fires unchanged when the branch ref itself is missing, missing a trailer, or contaminated — so the FN-5363 strict-lease and foreign-commit protections remain authoritative.