p2p: add network simulation framework (#14982)

This commit introduces a network simulation framework which
can be used to run simulated networks of devp2p nodes. The
intention is to use this for testing protocols, performing
benchmarks and visualising emergent network behaviour.

rpc/client_test.go

@@ -251,6 +251,38 @@ func TestClientSubscribe(t *testing.T) {
 	}
 }
 
+func TestClientSubscribeCustomNamespace(t *testing.T) {
+	namespace := "custom"
+	server := newTestServer(namespace, new(NotificationTestService))
+	defer server.Stop()
+	client := DialInProc(server)
+	defer client.Close()
+
+	nc := make(chan int)
+	count := 10
+	sub, err := client.Subscribe(context.Background(), namespace, nc, "someSubscription", count, 0)
+	if err != nil {
+		t.Fatal("can't subscribe:", err)
+	}
+	for i := 0; i < count; i++ {
+		if val := <-nc; val != i {
+			t.Fatalf("value mismatch: got %d, want %d", val, i)
+		}
+	}
+
+	sub.Unsubscribe()
+	select {
+	case v := <-nc:
+		t.Fatal("received value after unsubscribe:", v)
+	case err := <-sub.Err():
+		if err != nil {
+			t.Fatalf("Err returned a non-nil error after explicit unsubscribe: %q", err)
+		}
+	case <-time.After(1 * time.Second):
+		t.Fatalf("subscription not closed within 1s after unsubscribe")
+	}
+}
+
 // In this test, the connection drops while EthSubscribe is
 // waiting for a response.
 func TestClientSubscribeClose(t *testing.T) {