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feat(merger): auto-rehome FF-recoverable orphan commits in contamination recovery

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Follow-up to bf4428c00 (FF-only ref advance). After the prevention fix
new orphans can't form, but pre-fix orphans like f6358ce4 on
fusion/fn-5419 still need a path back onto the integration branch.

Adds an `orphan-our-advance` classification to contamination recovery:
a "unique" foreign commit whose Fusion-Task-Id trailer points at a
`done` task AND that is unreachable from refs/heads/<integrationBranch>
is treated as a stranded merger output.

For these, the executor attempts a fast-forward rehome onto the
integration branch via advanceIntegrationBranchRef (which still enforces
the FF-only invariant). When successful, the orphan sha is added to the
existing `shasToDrop` set so the same recovery pass that drops
already-upstream/misrouted commits also drops the now-upstream orphan.

Non-FF orphans (diverged from current integration tip) are refused.
Doing a cherry-pick onto the integration branch from inside automated
recovery would introduce conflict-resolution surface that's too high
blast radius for a never-event recovery path. The refusal log line
includes the exact `git cherry-pick <sha>` command an operator can run
manually.

Two new GitMutationType audit events:
  - merger:orphan-rehome-ff (successful FF rehome)
  - merger:orphan-rehome-refused (non-FF, manual cherry-pick required)

Tests in merger-orphan-rehome.test.ts cover classification (orphan,
not-done, already-reachable, no-trailer) and the rehome operation
(FF success advances the ref + emits the audit event; non-FF refusal
emits the hint and leaves the ref untouched).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
gsxdsm
2026-05-23 09:59:50 -07:00
parent bf4428c00c
commit ec1269fd84
5 changed files with 564 additions and 2 deletions
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"@fusion/engine": patch
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feat(merger): auto-rehome FF-recoverable orphan commits during contamination recovery
Follow-up to the FF-only ref advance fix: contamination recovery now classifies a fourth bucket — `orphan-our-advance` — and fast-forwards the integration branch onto pre-fix orphan commits when safe.
When the executor's contamination handler sees a "unique" foreign commit, it now also asks:
- Does the commit's `Fusion-Task-Id` trailer point at a `done` task?
- Is the commit unreachable from `refs/heads/<integrationBranch>`?
If both, the commit is an orphan from the pre-fix non-FF ref-advance bug. Recovery attempts to fast-forward the integration branch onto the orphan:
- **FF possible** (integration tip is an ancestor of the orphan): advance via `advanceIntegrationBranchRef`, then drop the orphan from the task branch alongside `already-upstream` commits. Emits `merger:orphan-rehome-ff`.
- **Non-FF** (orphan diverges from integration tip — would require cherry-pick): refuse to auto-rehome. The commit stays in `genuinelyUnique` for human adjudication, but the recovery log line now includes the exact `git cherry-pick <sha>` command an operator can run to unstick it. Emits `merger:orphan-rehome-refused`.
The non-FF refusal is intentional: cherry-pick into the integration branch from inside automated recovery introduces conflict-resolution surface that's too high blast radius for a never-event recovery path.