Two consumers may share a name; they must not share a delivery subject

novox/hq 04-ISSUES/146. A push consumer delivers onto an ordinary subject and
everything subscribed to it gets a copy. The controller holds a consumer called
'controller' on CONTROL and another called 'controller' on EVENTS, and both were
given _DELIVER.controller — so the one process, holding both subscriptions,
acted on every message twice.

Measured: one enrolment published, one message in the stream, one delivery, no
redelivery, and the controller enrolled the machine twice — the second minting a
credential that replaced the one the machine had just been handed, which is why
it then reconnected for ever as a user whose password the mesh had rotated. Every
report and every followed event doubled the same way, silently.

The stream goes in the subject because the pair is what identifies a consumer.
A subscriber's permission gains the same shape, keeping the bare name so an
existing consumer keeps working until the next assertion moves it.

internal/broker/streams_test.go

@@ -233,3 +233,30 @@ func containsStep(steps []string, want string) bool {
 	}
 	return false
 }
+
+// **Two consumers may share a name, and must not share a delivery subject** (novox/hq
+// 04-ISSUES/146).
+//
+// A push consumer delivers onto an ordinary subject and everything subscribed to it gets a copy.
+// The controller holds a consumer called `controller` on CONTROL and another called `controller` on
+// EVENTS; while both were given `_DELIVER.controller`, the one process holding both subscriptions
+// acted on every message twice — a joining machine enrolled twice from one request, with the second
+// enrolment minting a credential that replaced the one the machine had just been handed.
+//
+// Checked here rather than against a server because it is a property of what the mesh asks for, and
+// because the failure it produces is silent: every count is right, nothing is redelivered, and the
+// work simply happens twice.
+func TestNoTwoConsumersDeliverOntoTheSameSubject(t *testing.T) {
+	seen := map[string]string{}
+	for _, c := range MeshConsumers() {
+		if !c.Push && c.Queue == "" {
+			continue
+		}
+		subject := DeliverSubjectFor(c)
+		if other, taken := seen[subject]; taken {
+			t.Errorf("%s on %s and %s deliver onto %s, so whoever holds both acts on every "+
+				"message twice", c.Name, c.Stream, other, subject)
+		}
+		seen[subject] = c.Name + " on " + c.Stream
+	}
+}