Assemble merges in a rolling window and walk each batch once (hq ADR 0276, issue 362)

Every merge opened a walk and the next merge of the branch superseded it: two
catalogue merges 18 s apart left a walk no delivery held, and the operator
started it by hand 58 minutes later. A merge now joins the open batch, kept in
the store (migration 0089), which is cut into one walk when no merge came for
merge-window (90 s) or at merge-window-at-most (10 min): one commit per
repository, the latest of its branch, with every file the batch's merges
changed. One walk at a time; a started walk is never superseded, a waiting one
is folded into the next. The walk names every merge it answers on the wire
(delivery.merges, taken_over_by, batch). A failed walk walks its earlier merges
alone, newest first, until one is delivered. A delivery group's order becomes
tier edges inside the walk. plans shows the batch assembling; S18 and S19
bound its waits; S16 names the merges a waiting walk answers.

cmd/mesh-controller/signals.go

@@ -210,6 +210,22 @@ var signalsTable = []signalRow{
 		newest: func(f *signalFacts) time.Time {
 			return newestOf(f.waits, func(w waitFacts) time.Time { return w.since })
 		}},
+	{Row: "S18", Signal: "a batch of merges is cut into its walk", Emitter: "controller's merge window",
+		Trigger: "each batch (novox/hq ADR 0276)",
+		Bound:   "a minute past merge-window-at-most, while no walk is open: the controller failed to cut it",
+		Kind:    kindBatchNotCut, Severity: conditions.Warning, Phase: 3,
+		needs: func(f *signalFacts) error { return f.plansErr }, watch: watchBatchesNotCut,
+		newest: func(f *signalFacts) time.Time {
+			return newestOf(f.batches, func(b batchFacts) time.Time { return b.atMost })
+		}},
+	{Row: "S19", Signal: "a batch waiting behind an open walk is cut when it ends", Emitter: "controller's merge window",
+		Trigger: "each batch closed while a walk is open (novox/hq ADR 0276)",
+		Bound:   "the walk's bound, a tier's bound for each of its tiers, from when the batch closed: naming the walk",
+		Kind:    kindBatchBehindWalk, Severity: conditions.Warning, Phase: 3,
+		needs: func(f *signalFacts) error { return f.plansErr }, watch: watchBatchesBehind,
+		newest: func(f *signalFacts) time.Time {
+			return newestOf(f.batches, func(b batchFacts) time.Time { return b.closed })
+		}},
 	{Row: "S17", Signal: "a send held for the bus's planned step is told to a person", Emitter: "controller's plan",
 		Trigger: "each send refused because it would replace the bus outside its step (novox/hq issue 336)",
 		Bound: "none: raised at the first refusal, for the operator, naming what waits, the bus build from and to, " +
@@ -372,8 +388,8 @@ func watchWaits(f *signalFacts) []conditions.Observation {
 		out = append(out, conditions.Observation{Scope: conditions.ScopePlan, ID: w.id, Kind: kindWalkWaiting,
 			Severity: severity,
 			Summary: fmt.Sprintf("the walk of %s %s has waited %s for %s's word to start: `mesh-delivery.show` for the "+
-				"delivery that landed as %s says why; `plans go %s --why …` starts it by hand", w.repository,
-				short(w.commit), ago(in), w.awaits, short(w.commit), w.id),
+				"deliveries that landed as %s says why; `plans go %s --why …` starts it by hand", w.repository,
+				short(w.commit), ago(in), w.awaits, mergesWords(w), w.id),
 			Said:        fmt.Sprintf("waiting since %s for %s", w.since.UTC().Format(time.RFC3339), w.awaits),
 			Headline:    deliveryName(w.modules, w.repository) + " waiting to start",
 			Explanation: walkWaitingWords(w, in, severity),
@@ -384,6 +400,19 @@ func watchWaits(f *signalFacts) []conditions.Observation {
 	return out
 }
 
+// mergesWords is the merges a waiting walk answers (novox/hq ADR 0276): every delivery it carries, not one
+// commit that a supersession may already have ended (issue 362). Its own commit for a walk naming none.
+func mergesWords(w waitFacts) string {
+	if len(w.merges) == 0 {
+		return short(w.commit)
+	}
+	var out []string
+	for _, m := range w.merges {
+		out = append(out, m.Repository+"@"+short(m.Commit))
+	}
+	return readableList(out)
+}
+
 func watchLoop(f *signalFacts) []conditions.Observation {
 	if f.loop.pending == 0 {
 		return nil