The mesh bus seat is one per mesh, and the amqp broker does not contend

Step 2.3 of novox/hq ADR 0116. The refusal is the resolver's existing one;
these pin it for this seat, including that a different bus implementation is
refused for the same reason — the property that lets the bus be replaced.

internal/catalogue/broker_seat_test.go

@@ -0,0 +1,65 @@
+package catalogue
+
+import (
+	"strings"
+	"testing"
+)
+
+// The mesh's bus is one per mesh, read from the catalogue beside this checkout.
+//
+// **This is step 2's claim, and it is checked here rather than in a bed** (novox/hq ADR 0116):
+// adoption puts the NATS server into the `mesh-broker` seat on a mesh that is already running,
+// and the property that matters is that a second one anywhere is refused *when it is assigned*,
+// not discovered later as two servers holding different halves of the mesh's traffic. A second
+// bus is not a degraded mesh; it is two meshes that both believe they are the one.
+func TestASecondMeshBusAnywhereIsRefusedByName(t *testing.T) {
+	nats := catalogueManifest(t, "nats")
+
+	if _, err := Resolve(shelf(nats), []string{"nats"}, workstation(), World{}); err != nil {
+		t.Fatalf("the bus alone does not resolve: %v", err)
+	}
+
+	elsewhere := World{Held: []Held{{Claim: "mesh-broker", Scope: ScopeMesh,
+		Node: "anchor", Module: "nats"}}}
+	other := workstation()
+	other.Name = "laptop"
+	_, err := Resolve(shelf(nats), []string{"nats"}, other, elsewhere)
+	if err == nil {
+		t.Fatal("a second bus was accepted on another machine")
+	}
+	if !strings.Contains(err.Error(), "mesh-broker") || !strings.Contains(err.Error(), "one per mesh") {
+		t.Fatalf("refused without naming the seat: %v", err)
+	}
+}
+
+// The seat is the server's role, not the product's name (novox/hq ADR 0079). A different
+// implementation of the bus claims the same seat, and the mesh refuses it for the same reason —
+// which is the property that lets the bus be replaced at all.
+func TestTheSeatRefusesADifferentBusToo(t *testing.T) {
+	nats := catalogueManifest(t, "nats")
+	held := World{Held: []Held{{Claim: "mesh-broker", Scope: ScopeMesh,
+		Node: "anchor", Module: "some-other-broker"}}}
+	other := workstation()
+	other.Name = "laptop"
+	if _, err := Resolve(shelf(nats), []string{"nats"}, other, held); err == nil {
+		t.Fatal("the seat admitted a second holder because the module's name differed")
+	}
+}
+
+// The old broker no longer claims the seat: it is an ordinary provider of `amqp`
+// (novox/hq ADR 0119), so it can sit on the same mesh as the bus without contending for it.
+func TestTheAmqpBrokerDoesNotContendForTheSeat(t *testing.T) {
+	lavinmq := catalogueManifest(t, "lavinmq")
+	for _, c := range lavinmq.Claims {
+		if c.Name == "mesh-broker" {
+			t.Fatal("the amqp broker still claims mesh-broker; it is a provider, not foundation")
+		}
+	}
+	busHeld := World{Held: []Held{{Claim: "mesh-broker", Scope: ScopeMesh,
+		Node: "anchor", Module: "nats"}}}
+	other := workstation()
+	other.Name = "laptop"
+	if _, err := Resolve(shelf(lavinmq), []string{"lavinmq"}, other, busHeld); err != nil {
+		t.Fatalf("the amqp broker was refused beside the mesh bus: %v", err)
+	}
+}