External GitHub fork PRs need their contributor commit SHA in main's history
for GitHub's "merged" badge to fire. Cherry-pick rewrites the SHA, breaking
that detection. New _merge_no_ff_external function preserves the SHA via a
true merge commit.
Mechanics (mirrors _cherry_pick_onto_main shape):
1. Fetch origin/main + origin/{branch}
2. Detached worktree at origin/main, git merge --no-ff origin/{branch}
with verbose message: "Merge external GitHub PR #{N}: {branch_slug}"
3. Force-push merge commit M as origin/{branch}, replacing branch tip
4. Dispatch's existing ff-push origin/{branch} → main propagates M to main
M has parents [main_sha, contributor_sha]. M is a fast-forward descendant
of main_sha (first-parent chain), so the ff-push to main is valid without
--force. Contributor SHA reachable from main → GitHub recognizes merged.
Conflict handling: same auto-resolve as cherry-pick — entity-only conflicts
take main's version (--ours = current worktree HEAD = main), other conflicts
abort with detail.
Backout: config.EXTERNAL_PR_NO_FF_MERGE = True (default). Set False to fall
back to cherry-pick if no-ff destabilizes throughput one week pre-Accelerate.
Branch dispatch in _merge_domain_queue:
- reweave/* → _merge_reweave_pr (existing)
- gh-pr-N/* AND config.EXTERNAL_PR_NO_FF_MERGE → _merge_no_ff_external (new)
- everything else → _cherry_pick_onto_main (existing default)
Verified end-to-end in scratch repo:
- merge commit M has [main_sha, contributor_sha] as parents
- contributor SHA is ancestor of M
- after ff-push, contributor SHA is in main's history (GitHub badge fires)
- regex parses 8 cases correctly (real fork PR + edge cases reject cleanly)
Architecture per Ship Msg 3 / doc v3 (537cfd5 on epimetheus/external-merge-flow-design).
Phase 1 (sync-mirror self-heal) deployed yesterday. Phase 3 (FwazB PR #90 cleanup)
queued behind this deploy.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
m3taversal
992b4ee36f