Walk a stopped walk's merges again in the open batch, in one act, and say what stays on the machines (review of #232, hq issue 459)

A new batch beside the open one walked an older commit after a newer one, and a
stop that failed partway could leave a merge on two plans. A module shared with
the stopped merge, built at its branch, would deliver it; it is left out or
withheld. A batch not yet cut can drop a merge, so mesh-delivery can stop a
merge walked again. The stopped walk names what it left on which machines.

cmd/mesh-controller/batches.go

@@ -367,7 +367,13 @@ func keepBatch(ctx context.Context, inv *inventory.Inventory, b *inventory.Plan,
 	}
 	closes, latest := windowOf(merges, b.Created, window, atMost)
 	b.Delivery.Merges = named
-	b.Delivery.Batch = &inventory.PlanBatch{ClosesAt: closes, AtMost: latest, Behind: behind, Own: b.OwnPath()}
+	var closed bool
+	var withheld []string
+	if was := b.Delivery.Batch; was != nil {
+		closed, withheld = was.Closed, was.Withheld // a stop's, which no look takes back (novox/hq ADR 0299)
+	}
+	b.Delivery.Batch = &inventory.PlanBatch{ClosesAt: closes, AtMost: latest, Behind: behind, Own: b.OwnPath(),
+		Closed: closed, Withheld: withheld}
 	b.State = inventory.PlanAssembling
 	if behind != "" && windowClosed(*b, now) {
 		b.State = inventory.PlanQueued
@@ -402,7 +408,7 @@ func windowClosed(b inventory.Plan, now time.Time) bool {
 		return true
 	}
 	w := b.Delivery.Batch
-	return !now.Before(w.ClosesAt) || !now.Before(w.AtMost)
+	return w.Closed || !now.Before(w.ClosesAt) || !now.Before(w.AtMost)
 }
 
 // carriedOf is a set of merges as a walk carries them: the latest merge of each repository's branch, and
@@ -763,6 +769,15 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	sort.Strings(also)
 	ordered := groupOrderEdges(members, entries, read, edges)
 	plan := planOfMoves(moved, append(append([]inventory.Edge{}, edges...), ordered...))
+	// **Withheld, a stopped merge's modules** (novox/hq ADR 0299): a module is built at its branch, which holds the
+	// merge a stop left out of the walk these merges were walked again from — built here only as a dependent, it
+	// would deliver that merge. One a merge of this batch moves is built: that merge carries the stopped change,
+	// and the stop named it.
+	if batch.Delivery != nil && batch.Delivery.Batch != nil {
+		if out := withhold(&plan, batch.Delivery.Batch.Withheld, moved); len(out) > 0 {
+			fmt.Printf("  withheld, a stopped merge's modules built only as dependents: %s\n", strings.Join(out, ", "))
+		}
+	}
 	plan.ID, plan.Revision, plan.Created, plan.Epoch = batch.ID, batch.Revision, now, batch.Epoch
 	plan.Commits = commits
 	newest := newestCommit(commits)
@@ -823,6 +838,39 @@ func planBatch(ctx context.Context, open *stores, batch *inventory.Plan, carry [
 	return nil
 }
 
+// withhold takes from a plan every module named that its merges do not move, and the tiers left empty: the
+// modules taken.
+func withhold(plan *inventory.Plan, names, moved []string) []string {
+	var out []string
+	for _, n := range names {
+		if _, in := plan.Modules[n]; in && !slices.Contains(moved, n) {
+			delete(plan.Modules, n)
+			out = append(out, n)
+		}
+	}
+	if len(out) == 0 {
+		return nil
+	}
+	var tiers [][]string
+	for _, t := range plan.Tiers {
+		var kept []string
+		for _, n := range t {
+			if !slices.Contains(out, n) {
+				kept = append(kept, n)
+			}
+		}
+		if len(kept) > 0 {
+			tiers = append(tiers, kept)
+		}
+	}
+	if tiers == nil {
+		tiers = [][]string{}
+	}
+	plan.Tiers = tiers
+	sort.Strings(out)
+	return out
+}
+
 // walkTimesAtCut is a walk's own moments as it is cut (novox/hq ADR 0282 decision 6): when its batch's window
 // closed — no merge for the window's length, or its maximum, whichever came first — when it was cut, and its
 // class. A merge walked alone had no window.