feature/relayserver: use eventbus.Monitor to simplify lifecycle management (#17234)

Instead of using separate channels to manage the lifecycle of the eventbus
client, use the recently-added eventbus.Monitor, which handles signaling the
processing loop to stop and waiting for it to complete.  This allows us to
simplify some of the setup and cleanup code in the relay server.

Updates #15160

Change-Id: Ia1a47ce2e5a31bc8f546dca4c56c3141a40d67af
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

feature/relayserver/relayserver.go

@@ -84,9 +84,9 @@ type extension struct {
 
 	mu       sync.Mutex // guards the following fields
 	shutdown bool
+
 	port                          *int                             // ipn.Prefs.RelayServerPort, nil if disabled
-	disconnectFromBusCh           chan struct{}                    // non-nil if consumeEventbusTopics is running, closed to signal it to return
-	busDoneCh                     chan struct{}                    // non-nil if consumeEventbusTopics is running, closed when it returns
+	eventSubs                     *eventbus.Monitor                // nil if not connected to eventbus
 	debugSessionsCh               chan chan []status.ServerSession // non-nil if consumeEventbusTopics is running
 	hasNodeAttrDisableRelayServer bool                             // tailcfg.NodeAttrDisableRelayServer
 }
@@ -119,15 +119,13 @@ func (e *extension) handleBusLifetimeLocked() {
 	if !busShouldBeRunning {
 		e.disconnectFromBusLocked()
 		return
-	}
-	if e.busDoneCh != nil {
+	} else if e.eventSubs != nil {
 		return // already running
 	}
-	port := *e.port
-	e.disconnectFromBusCh = make(chan struct{})
-	e.busDoneCh = make(chan struct{})
+
+	ec := e.bus.Client("relayserver.extension")
 	e.debugSessionsCh = make(chan chan []status.ServerSession)
-	go e.consumeEventbusTopics(port)
+	e.eventSubs = ptr.To(ec.Monitor(e.consumeEventbusTopics(ec, *e.port)))
 }
 
 func (e *extension) selfNodeViewChanged(nodeView tailcfg.NodeView) {
@@ -175,17 +173,15 @@ var overrideAddrs = sync.OnceValue(func() (ret []netip.Addr) {
 
 // consumeEventbusTopics serves endpoint allocation requests over the eventbus.
 // It also serves [relayServer] debug information on a channel.
-// consumeEventbusTopics must never acquire [extension.mu], which can be held by
-// other goroutines while waiting to receive on [extension.busDoneCh] or the
+// consumeEventbusTopics must never acquire [extension.mu], which can be held
+// by other goroutines while waiting to receive on [extension.eventSubs] or the
 // inner [extension.debugSessionsCh] channel.
-func (e *extension) consumeEventbusTopics(port int) {
-	defer close(e.busDoneCh)
-
-	eventClient := e.bus.Client("relayserver.extension")
-	reqSub := eventbus.Subscribe[magicsock.UDPRelayAllocReq](eventClient)
-	respPub := eventbus.Publish[magicsock.UDPRelayAllocResp](eventClient)
-	defer eventClient.Close()
+func (e *extension) consumeEventbusTopics(ec *eventbus.Client, port int) func(*eventbus.Client) {
+	reqSub := eventbus.Subscribe[magicsock.UDPRelayAllocReq](ec)
+	respPub := eventbus.Publish[magicsock.UDPRelayAllocResp](ec)
+	debugSessionsCh := e.debugSessionsCh
 
+	return func(ec *eventbus.Client) {
 		var rs relayServer // lazily initialized
 		defer func() {
 			if rs != nil {
@@ -194,11 +190,9 @@ func (e *extension) consumeEventbusTopics(port int) {
 		}()
 		for {
 			select {
-		case <-e.disconnectFromBusCh:
-			return
-		case <-eventClient.Done():
+			case <-ec.Done():
 				return
-		case respCh := <-e.debugSessionsCh:
+			case respCh := <-debugSessionsCh:
 				if rs == nil {
 					// Don't initialize the server simply for a debug request.
 					respCh <- nil
@@ -239,13 +233,12 @@ func (e *extension) consumeEventbusTopics(port int) {
 			}
 		}
 	}
+}
 
 func (e *extension) disconnectFromBusLocked() {
-	if e.busDoneCh != nil {
-		close(e.disconnectFromBusCh)
-		<-e.busDoneCh
-		e.busDoneCh = nil
-		e.disconnectFromBusCh = nil
+	if e.eventSubs != nil {
+		e.eventSubs.Close()
+		e.eventSubs = nil
 		e.debugSessionsCh = nil
 	}
 }
@@ -270,7 +263,7 @@ func (e *extension) serverStatus() status.ServerStatus {
 		UDPPort:  nil,
 		Sessions: nil,
 	}
-	if e.port == nil || e.busDoneCh == nil {
+	if e.port == nil || e.eventSubs == nil {
 		return st
 	}
 	st.UDPPort = ptr.To(*e.port)
@@ -281,7 +274,7 @@ func (e *extension) serverStatus() status.ServerStatus {
 		resp := <-ch
 		st.Sessions = resp
 		return st
-	case <-e.busDoneCh:
+	case <-e.eventSubs.Done():
 		return st
 	}
 }

feature/relayserver/relayserver_test.go

@@ -101,8 +101,8 @@ func Test_extension_profileStateChanged(t *testing.T) {
 			}
 			defer e.disconnectFromBusLocked()
 			e.profileStateChanged(ipn.LoginProfileView{}, tt.args.prefs, tt.args.sameNode)
-			if tt.wantBusRunning != (e.busDoneCh != nil) {
-				t.Errorf("wantBusRunning: %v != (e.busDoneCh != nil): %v", tt.wantBusRunning, e.busDoneCh != nil)
+			if tt.wantBusRunning != (e.eventSubs != nil) {
+				t.Errorf("wantBusRunning: %v != (e.eventSubs != nil): %v", tt.wantBusRunning, e.eventSubs != nil)
 			}
 			if (tt.wantPort == nil) != (e.port == nil) {
 				t.Errorf("(tt.wantPort == nil): %v != (e.port == nil): %v", tt.wantPort == nil, e.port == nil)
@@ -118,7 +118,7 @@ func Test_extension_handleBusLifetimeLocked(t *testing.T) {
 		name                          string
 		shutdown                      bool
 		port                          *int
-		busDoneCh                     chan struct{}
+		eventSubs                     *eventbus.Monitor
 		hasNodeAttrDisableRelayServer bool
 		wantBusRunning                bool
 	}{
@@ -157,13 +157,13 @@ func Test_extension_handleBusLifetimeLocked(t *testing.T) {
 				bus:                           eventbus.New(),
 				shutdown:                      tt.shutdown,
 				port:                          tt.port,
-				busDoneCh:                     tt.busDoneCh,
+				eventSubs:                     tt.eventSubs,
 				hasNodeAttrDisableRelayServer: tt.hasNodeAttrDisableRelayServer,
 			}
 			e.handleBusLifetimeLocked()
 			defer e.disconnectFromBusLocked()
-			if tt.wantBusRunning != (e.busDoneCh != nil) {
-				t.Errorf("wantBusRunning: %v != (e.busDoneCh != nil): %v", tt.wantBusRunning, e.busDoneCh != nil)
+			if tt.wantBusRunning != (e.eventSubs != nil) {
+				t.Errorf("wantBusRunning: %v != (e.eventSubs != nil): %v", tt.wantBusRunning, e.eventSubs != nil)
 			}
 		})
 	}