A seat's holders pull one ask at a time from one shared worker (hq ADR 0190, issue 186)

The worker a holder bound was a push consumer in a queue group with one ask in flight: right for
one holder, and with two it would still be a queue of one — the server hands a pushed ask to
whichever subscriber it picks, busy or not, and the in-flight cap is per consumer, not per holder.
Now the worker is pulled: every machine holding the seat binds the same durable and fetches one
ask when it has finished the last, so an idle machine is the one that takes the next, the asks in
flight are bounded by the holders working, and nothing is delivered that nobody asked for — which
is also what ended the race issue 186 describes. A holder's grants trade the delivery subject for
MSG.NEXT on the worker; the ack grant and the heartbeat that keeps a long build alive stay.

Proven against a real bus: the build round trip, a backlog taken by a machine that arrives later,
and work handed back by one machine coming round again.

internal/broker/derived_test.go

@@ -88,15 +88,20 @@ func TestAModuleThatConsumesNothingGetsNoConsumer(t *testing.T) {
 	}
 }
 
-// The seat is authority and the queue group is delivery. Tie them together and the day somebody
-// allows two holders, every message is processed twice with nothing reporting it.
-func TestAHoldersWorkerUsesAQueueGroupAnyway(t *testing.T) {
+// The seat is authority and the worker is delivery (novox/hq ADR 0190): one worker per seat, shared
+// by every holder and pulled from, so a second holder takes the next ask rather than a copy of the
+// same one — which is what a queue group used to guard, and what pulling one durable gives outright.
+func TestAHoldersWorkerIsOneSharedByItsHolders(t *testing.T) {
 	c, ok := HolderConsumerFor("one", "telegram", telegramSeat())
 	if !ok {
 		t.Fatal("the holder of a seat with inbound work got no worker")
 	}
-	if c.Queue == "" {
-		t.Fatal("the worker is not in a queue group, so a second holder would double-process")
+	two, _ := HolderConsumerFor("two", "telegram", telegramSeat())
+	if c.Name != two.Name || c.Stream != two.Stream {
+		t.Fatal("two holders got two workers, so each would process every ask")
+	}
+	if c.Push || c.Queue != "" {
+		t.Fatal("the worker is pushed, so the server would hand an ask to a busy holder")
 	}
 	if c.Stream != "SEAT_TELEGRAM_SENDER" {
 		t.Fatalf("the worker reads %q, not the seat's own stream", c.Stream)
@@ -154,15 +159,23 @@ func TestANodesDeclarationConsumerIsWhatItsOwnGrantAllows(t *testing.T) {
 	has(t, perms.Subscribe, c.Filters[0])
 }
 
-// A holder works one ask at a time, so the server hands it one at a time (novox/hq issue 186):
-// asks queued behind the one being worked wait in the stream rather than being delivered,
-// left to expire and dropped after the fifth redelivery.
-func TestAHoldersWorkerTakesOneAskAtATime(t *testing.T) {
-	c, found := HolderConsumerFor("anchor", "builder", DeclaredSeat{Name: "mesh-build-machine", Accepts: []string{"build"}})
+// Every holder of a seat shares one worker and pulls from it (novox/hq ADR 0190): no queue group
+// and no delivery subject, because a push consumer hands the next ask to whichever subscriber the
+// server picks, busy or not; and no cap of one in flight, because pulling bounds the asks in flight
+// by the holders that are free — which is what ended the race of issue 186, where asks delivered
+// behind the one being worked expired and were dropped.
+func TestAHoldersWorkerIsPulledByEveryHolder(t *testing.T) {
+	c, found := HolderConsumerFor("anchor", "build-agent", DeclaredSeat{Name: "node-build-agent", Accepts: []string{"build"}})
 	if !found {
 		t.Fatal("a seat that accepts work has no worker")
 	}
-	if c.MaxAckPending != 1 {
-		t.Fatalf("the worker may have %d asks in flight; one, so a queue is a queue", c.MaxAckPending)
+	if c.Queue != "" || c.Push {
+		t.Fatalf("the worker is pushed (queue %q, push %v); a holder pulls when it is free", c.Queue, c.Push)
+	}
+	if c.MaxAckPending != 0 {
+		t.Fatalf("the worker caps asks in flight at %d; pulling bounds them by the holders working", c.MaxAckPending)
+	}
+	if c.Name != "SEAT_NODE_BUILD_AGENT_worker" || c.Stream != "SEAT_NODE_BUILD_AGENT" {
+		t.Fatalf("the worker is %s on %s; one per seat, shared by its holders", c.Name, c.Stream)
 	}
 }