bugfix: do not interrupt connwctions when syncing

sync.go

@@ -12,19 +12,48 @@ func Sync() {
 }
 
 func ConfigureDevice(conf *DsnetConfig) {
-	peers := conf.GetWgPeerConfigs()
-
-	wgConfig := wgtypes.Config{
-		PrivateKey:   &conf.PrivateKey.Key,
-		ListenPort:   &conf.ListenPort,
-		ReplacePeers: true,
-		Peers:        peers,
-	}
-
 	wg, err := wgctrl.New()
 	check(err)
 	defer wg.Close()
 
+	dev, err := wg.Device(conf.InterfaceName)
+
+	if err != nil {
+		ExitFail("Could not retrieve device '%s' (%v)", conf.InterfaceName, err)
+	}
+
+	peers := conf.GetWgPeerConfigs()
+
+	// compare peers to see if any exist on the device and not the config. If
+	// so, they should be removed by appending a dummy peer with Remove:true + pubkey.
+	knownKeys := make(map[wgtypes.Key]bool)
+
+	for _, peer := range peers {
+		knownKeys[peer.PublicKey] = true
+	}
+
+	// find deleted peers, and append dummy "remove" peers
+	for _, peer := range dev.Peers {
+		if !knownKeys[peer.PublicKey] {
+			peers = append(peers, wgtypes.PeerConfig{
+				PublicKey: peer.PublicKey,
+				Remove:    true,
+			})
+		}
+	}
+
+	wgConfig := wgtypes.Config{
+		PrivateKey: &conf.PrivateKey.Key,
+		ListenPort: &conf.ListenPort,
+		// ReplacePeers with the same peers results in those peers losing
+		// connection, so it's not possible to do declarative configuration
+		// idempotently with ReplacePeers like I had assumed. Instead, peers
+		// must be removed imperatively with Remove:true. Peers can still be
+		// added/updated with ConfigureDevice declaratively.
+		ReplacePeers: false,
+		Peers:        peers,
+	}
+
 	err = wg.ConfigureDevice(conf.InterfaceName, wgConfig)
 
 	if err != nil {