Keep a merge a cut made history, and build a batch's walk at the branch (hq ADR 0276 review)

cmd/mesh-controller/batches.go

@@ -153,7 +153,11 @@ func (f following) hearMerge(ctx context.Context, m link.SourceMoved, now time.T
 			return notNow(err)
 		}
 	}
-	if from, packaging, already := mergeCandidates(m, entries, read); len(from) == 0 && len(packaging) == 0 && len(later) == 0 {
+	owed, err := owedLate(ctx, inv, m, merged, touches)
+	if err != nil {
+		return notNow(err)
+	}
+	if from, packaging, already := mergeCandidates(m, entries, read); len(from) == 0 && len(packaging) == 0 && len(later) == 0 && !owed {
 		// "Already built from it" and "nothing reads it" are different facts, and reading the first
 		// as the second sends somebody looking for a broken trigger when the mesh is up to date.
 		if already > 0 {
@@ -216,6 +220,23 @@ func (f following) hearMerge(ctx context.Context, m link.SourceMoved, now time.T
 	return cutBatchesHeld(ctx, f.open, now)
 }
 
+// owedLate says a merge that reads as history is still the record's to answer (review of this change): a cut
+// marks every module it moved seen at that moment, so a merge of that branch the bus lost and the catch-up
+// hands over later — made before the cut, heard after it — reads as history and was dropped. One that touches
+// a module the mesh holds, of a branch whose merges the controller keeps, made since it began keeping them,
+// is kept; one made before it began is old news, as before.
+func owedLate(ctx context.Context, inv *inventory.Inventory, m link.SourceMoved, merged time.Time,
+	touches []inventory.Entry) (bool, error) {
+	if len(touches) == 0 {
+		return false, nil
+	}
+	kept, first, err := inv.KeptOnBranch(ctx, m.Owner+"/"+m.Repo, m.Base)
+	if err != nil || !kept {
+		return false, err
+	}
+	return !merged.Before(first.Add(-mergeGrace)), nil
+}
+
 // answeredByALaterMerge is the plan that answers a merge heard after a later merge of its branch was put in
 // one (ADR 0276 decision 4): a batch not yet cut, whose walk plans every file of the repository's merges; an
 // open walk, or a walk done, that builds every module this merge would move — built from the branch after the
@@ -595,7 +616,7 @@ func walkAlone(ctx context.Context, open *stores, m inventory.BatchedMerge, now
 	p := inventory.Plan{ID: fmt.Sprintf("plan-%d", now.UnixNano()), Repository: m.Repository, Branch: m.Branch,
 		Commit: m.Commit, Merged: m.Merged, Created: now, State: inventory.PlanQueued, Tiers: [][]string{},
 		Modules: map[string]*inventory.PlanModule{}, Note: "walked alone after a failed walk carried it",
-		Delivery: &inventory.PlanDelivery{}}
+		Delivery: &inventory.PlanDelivery{Alone: true}}
 	if err := inv.SavePlan(ctx, &p); err != nil {
 		return err
 	}
@@ -678,6 +699,7 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 		plan.Delivery = &inventory.PlanDelivery{}
 	}
 	plan.Delivery.Merges = named
+	plan.Delivery.Alone = batch.Delivery != nil && batch.Delivery.Alone
 	if len(moved) == 0 {
 		plan.State = inventory.PlanDone
 		plan.Tiers = [][]string{}