lib/discover: Add instance ID to local discovery (fixes #3278)

A random "instance ID" is generated on each start of the local discovery
service. The instance ID is included in the announcement. When we see a
new instance ID we treat is a new device and respond with an
announcement of our own. Hence devices get to know each other quickly on
restart.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3385

lib/discover/local_test.go

@@ -0,0 +1,75 @@
+package discover
+
+import (
+	"net"
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+func TestRandomLocalInstanceID(t *testing.T) {
+	c, err := NewLocal(protocol.LocalDeviceID, ":0", &fakeAddressLister{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	go c.Serve()
+	defer c.Stop()
+
+	lc := c.(*localClient)
+
+	p0 := lc.announcementPkt()
+	p1 := lc.announcementPkt()
+	if p0.InstanceID == p1.InstanceID {
+		t.Error("each generated packet should have a new instance id")
+	}
+}
+
+func TestLocalInstanceIDShouldTriggerNew(t *testing.T) {
+	c, err := NewLocal(protocol.LocalDeviceID, ":0", &fakeAddressLister{})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	lc := c.(*localClient)
+	src := &net.UDPAddr{IP: []byte{10, 20, 30, 40}, Port: 50}
+
+	new := lc.registerDevice(src, Announce{
+		ID:         []byte{10, 20, 30, 40, 50, 60, 70, 80, 90},
+		Addresses:  []string{"tcp://0.0.0.0:22000"},
+		InstanceID: 1234567890,
+	})
+
+	if !new {
+		t.Fatal("first register should be new")
+	}
+
+	new = lc.registerDevice(src, Announce{
+		ID:         []byte{10, 20, 30, 40, 50, 60, 70, 80, 90},
+		Addresses:  []string{"tcp://0.0.0.0:22000"},
+		InstanceID: 1234567890,
+	})
+
+	if new {
+		t.Fatal("second register should not be new")
+	}
+
+	new = lc.registerDevice(src, Announce{
+		ID:         []byte{42, 10, 20, 30, 40, 50, 60, 70, 80, 90},
+		Addresses:  []string{"tcp://0.0.0.0:22000"},
+		InstanceID: 1234567890,
+	})
+
+	if !new {
+		t.Fatal("new device ID should be new")
+	}
+
+	new = lc.registerDevice(src, Announce{
+		ID:         []byte{10, 20, 30, 40, 50, 60, 70, 80, 90},
+		Addresses:  []string{"tcp://0.0.0.0:22000"},
+		InstanceID: 91234567890,
+	})
+
+	if !new {
+		t.Fatal("new instance ID should be new")
+	}
+}