subscribe: add new subscribe package

This commit creates a new package 'subscribe', that exposes a common
Client-Server subscription system, that can be shared among packages.

subscribe/subscribe_test.go

@@ -0,0 +1,110 @@
+package subscribe_test
+
+import (
+	"testing"
+	"time"
+
+	"github.com/lightningnetwork/lnd/subscribe"
+)
+
+// TestSubscribe tests that the subscription clients receive the updates sent
+// to them after they subscribe, and that cancelled clients don't get more
+// updates.
+func TestSubscribe(t *testing.T) {
+	t.Parallel()
+
+	server := subscribe.NewServer()
+	if err := server.Start(); err != nil {
+		t.Fatalf("unable to start server")
+	}
+
+	const numClients = 300
+	const numUpdates = 1000
+
+	var clients [numClients]*subscribe.Client
+
+	// Start by registering two thirds the clients.
+	for i := 0; i < numClients*2/3; i++ {
+		c, err := server.Subscribe()
+		if err != nil {
+			t.Fatalf("unable to subscribe: %v", err)
+		}
+
+		clients[i] = c
+	}
+
+	// Send half the updates.
+	for i := 0; i < numUpdates/2; i++ {
+		if err := server.SendUpdate(i); err != nil {
+			t.Fatalf("unable to send update")
+		}
+	}
+
+	// Register the rest of the clients.
+	for i := numClients * 2 / 3; i < numClients; i++ {
+		c, err := server.Subscribe()
+		if err != nil {
+			t.Fatalf("unable to subscribe: %v", err)
+		}
+
+		clients[i] = c
+	}
+
+	// Cancel one third of the clients.
+	for i := 0; i < numClients/3; i++ {
+		clients[i].Cancel()
+	}
+
+	// Send the rest of the updates.
+	for i := numUpdates / 2; i < numUpdates; i++ {
+		if err := server.SendUpdate(i); err != nil {
+			t.Fatalf("unable to send update")
+		}
+	}
+
+	// Now ensure the clients got the updates we expect.
+	for i, c := range clients {
+
+		var from, to int
+		switch {
+
+		// We expect the first third of the clients to quit, since they
+		// were cancelled.
+		case i < numClients/3:
+			select {
+			case <-c.Quit():
+				continue
+			case <-time.After(1 * time.Second):
+				t.Fatalf("cancelled client %v did not quit", i)
+			}
+
+		// The next third should receive all updates.
+		case i < numClients*2/3:
+			from = 0
+			to = numUpdates
+
+		// And finally the last third should receive the last half of
+		// the updates.
+		default:
+			from = numUpdates / 2
+			to = numUpdates
+		}
+
+		for cnt := from; cnt < to; cnt++ {
+			select {
+			case upd := <-c.Updates():
+				j := upd.(int)
+				if j != cnt {
+					t.Fatalf("expected %v, got %v, for "+
+						"client %v", cnt, j, i)
+				}
+
+			case <-time.After(1 * time.Second):
+				t.Fatalf("did not receive expected update %v "+
+					"for client %v", cnt, i)
+			}
+		}
+
+	}
+
+}