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Every merge opened a walk and the next merge of the branch superseded it: two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 One walk at a time; a started walk is never superseded, a waiting one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/watchdogs.go

@@ -53,6 +53,8 @@ type signalFacts struct {
 	plansErr error
 	// waits are the walks waiting for their delivery's word (S16, novox/hq ADR 0239).
 	waits []waitFacts
+	// batches are the batches not yet cut (S18, S19, novox/hq ADR 0276).
+	batches []batchFacts
 
 	loop    loopFacts
 	loopErr error
@@ -159,6 +161,8 @@ type waitFacts struct {
 	since                          time.Time
 	// modules are the modules the walk moves, as a person names the delivery.
 	modules []string
+	// merges are the merges it answers (novox/hq ADR 0276), as the deliveries to read are found.
+	merges []inventory.PlanMerge
 }
 
 type loopFacts struct {
@@ -326,6 +330,9 @@ func (w *watchdogs) gather(ctx context.Context) *signalFacts {
 	f.machines, f.machinesErr = w.gatherMachines(ctx, inv, now)
 	f.bus, f.busErr = gatherBus(ctx, inv)
 	f.plans, f.waits, f.plansErr = gatherPlans(ctx, inv, now, f.bus.heldByTheBus())
+	if f.plansErr == nil {
+		f.batches, f.plansErr = gatherBatches(ctx, inv, now)
+	}
 	f.loop, f.loopErr = w.gatherLoop()
 	f.mergesPassed, f.merges, f.mergesErr = watchedMerges.last()
 	if f.mergesErr == nil && !f.mergesPassed.IsZero() && now.Sub(f.mergesPassed) > 3*mergeCatchUpEvery {
@@ -480,7 +487,7 @@ func gatherPlans(ctx context.Context, inv *inventory.Inventory, now time.Time, b
 		// S16's, whatever mesh-delivery says or does not say.
 		if p.Waiting() {
 			waits = append(waits, waitFacts{id: p.ID, repository: p.Repository, commit: p.Commit,
-				awaits: p.Delivery.Awaits, since: p.Created, modules: planModules(p)})
+				awaits: p.Delivery.Awaits, since: p.Created, modules: planModules(p), merges: p.Delivery.Merges})
 			continue
 		}
 		_, paused := pausedWaiting(p, pause, now)