ipn/ipnlocal: use atomic instead of mutex for captive context

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: If1d593e2f3c300790a69e7a74bf6b4b2f4bfe5a4

ipn/ipnlocal/local.go

@@ -346,15 +346,11 @@ type LocalBackend struct {
 	// refreshAutoExitNode indicates if the exit node should be recomputed when the next netcheck report is available.
 	refreshAutoExitNode bool
 
-	// captiveCtx and captiveCancel are used to control captive portal
+	// captiveCtx contain a [context.Context] used to allow cancelation
-	// detection. They are protected by 'mu' and can be changed during the
+	// when sending to the needsCaptiveDetection channel, along with the
-	// lifetime of a LocalBackend.
+	// [context.CancelFunc] for that context. It can be changed during the
-	//
+	// lifetime of the backend, and will always be non-nil.
-	// captiveCtx will always be non-nil, though it might a canceled
+	captiveCtx syncs.AtomicValue[contextAndCancel]
-	// context. captiveCancel is non-nil if checkCaptivePortalLoop is
-	// running, and is set to nil after being canceled.
-	captiveCtx    context.Context
-	captiveCancel context.CancelFunc
 	// needsCaptiveDetection is a channel that is used to signal either
 	// that captive portal detection is required (sending true) or that the
 	// backend is healthy and captive portal detection is not required
@@ -362,6 +358,11 @@ type LocalBackend struct {
 	needsCaptiveDetection chan bool
 }
 
+type contextAndCancel struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+}
+
 // HealthTracker returns the health tracker for the backend.
 func (b *LocalBackend) HealthTracker() *health.Tracker {
 	return b.health
@@ -415,7 +416,8 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 	clock := tstime.StdClock{}
 
 	// Until we transition to a Running state, use a canceled context for
-	// our captive portal detection.
+	// the context that we use when sending to the needsCaptiveDetection
+	// channel.
 	captiveCtx, captiveCancel := context.WithCancel(ctx)
 	captiveCancel()
 
@@ -440,12 +442,15 @@ func NewLocalBackend(logf logger.Logf, logID logid.PublicID, sys *tsd.System, lo
 		clock:                 clock,
 		selfUpdateProgress:    make([]ipnstate.UpdateProgress, 0),
 		lastSelfUpdateState:   ipnstate.UpdateFinished,
-		captiveCtx:            captiveCtx,
-		captiveCancel:         nil, // so that we start checkCaptivePortalLoop when Running
 		needsCaptiveDetection: make(chan bool),
 	}
+	b.captiveCtx.Store(contextAndCancel{captiveCtx, captiveCancel})
 	mConn.SetNetInfoCallback(b.setNetInfo)
 
+	// Start our captive portal detection loop; this does nothing until
+	// triggered by needsCaptiveDetection.
+	go b.checkCaptivePortalLoop(ctx)
+
 	if sys.InitialConfig != nil {
 		if err := b.setConfigLocked(sys.InitialConfig); err != nil {
 			return nil, err
@@ -756,28 +761,27 @@ func (b *LocalBackend) onHealthChange(w *health.Warnable, us *health.UnhealthySt
 		}
 	}
 
-	// captiveCtx can be changed, and is protected with 'mu'; grab that
+	// captiveCtx can be changed; grab that before we start our select,
-	// before we start our select, below.
+	// below.
-	//
+	ctx := b.captiveCtx.Load().ctx
-	// It is guaranteed to be non-nil.
-	b.mu.Lock()
-	ctx := b.captiveCtx
-	b.mu.Unlock()
-
-	if isConnectivityImpacted {
-		b.logf("health: connectivity impacted; triggering captive portal detection")
 
+	sendCaptiveDetectionNeeded := func(val bool) {
+		if ctx.Err() != nil {
+			return
+		}
 		// Ensure that we select on captiveCtx so that we can time out
 		// triggering captive portal detection if the backend is shutdown.
 		select {
-		case b.needsCaptiveDetection <- true:
+		case b.needsCaptiveDetection <- val:
 		case <-ctx.Done():
 		}
+	}
+
+	if isConnectivityImpacted {
+		b.logf("health: connectivity impacted; triggering captive portal detection")
+		sendCaptiveDetectionNeeded(true)
 	} else {
-		select {
+		sendCaptiveDetectionNeeded(false)
-		case b.needsCaptiveDetection <- false:
-		case <-ctx.Done():
-		}
 	}
 }
 
@@ -791,10 +795,8 @@ func (b *LocalBackend) Shutdown() {
 	}
 	b.shutdownCalled = true
 
-	if b.captiveCancel != nil {
+	b.logf("canceling captive portal context")
-		b.logf("canceling captive portal context")
+	b.captiveCtx.Load().cancel()
-		b.captiveCancel()
-	}
 
 	if b.loginFlags&controlclient.LoginEphemeral != 0 {
 		b.mu.Unlock()
@@ -2174,16 +2176,33 @@ func (b *LocalBackend) updateFilterLocked(netMap *netmap.NetworkMap, prefs ipn.P
 
 func (b *LocalBackend) checkCaptivePortalLoop(ctx context.Context) {
 	var tmr *time.Timer
+
+	maybeStartTimer := func() {
+		// If there's an existing timer, nothing to do; just continue
+		// waiting for it to expire. Otherwise, create a new timer.
+		if tmr == nil {
+			tmr = time.NewTimer(captivePortalDetectionInterval)
+		}
+	}
+	maybeStopTimer := func() {
+		if tmr == nil {
+			return
+		}
+		if !tmr.Stop() {
+			<-tmr.C
+		}
+		tmr = nil
+	}
+
 	for {
-		// First, see if we have a signal on our "healthy" channel, which
+		// First, see if we have a signal on our start/stop channel,
-		// takes priority over an existing timer.
+		// which takes priority over an existing timer.
 		select {
 		case needsCaptiveDetection := <-b.needsCaptiveDetection:
-			if !needsCaptiveDetection && tmr != nil {
+			if needsCaptiveDetection {
-				if !tmr.Stop() {
+				maybeStartTimer()
-					<-tmr.C
+			} else {
-				}
+				maybeStopTimer()
-				tmr = nil
 			}
 		default:
 		}
@@ -2195,9 +2214,7 @@ func (b *LocalBackend) checkCaptivePortalLoop(ctx context.Context) {
 		select {
 		case <-ctx.Done():
 			// All done; stop the timer and then exit.
-			if tmr != nil && !tmr.Stop() {
+			maybeStopTimer()
-				<-tmr.C
-			}
 			return
 		case <-timerChan:
 			// Kick off captive portal check
@@ -2206,18 +2223,10 @@ func (b *LocalBackend) checkCaptivePortalLoop(ctx context.Context) {
 			tmr = nil
 		case needsCaptiveDetection := <-b.needsCaptiveDetection:
 			if needsCaptiveDetection {
-				// If there's an existing timer, nothing to do; just
+				maybeStartTimer()
-				// continue waiting for it to expire. Otherwise, create
-				// a new timer.
-				if tmr == nil {
-					tmr = time.NewTimer(captivePortalDetectionInterval)
-				}
 			} else {
 				// Healthy; cancel any existing timer
-				if tmr != nil && !tmr.Stop() {
+				maybeStopTimer()
-					<-tmr.C
-				}
-				tmr = nil
 			}
 		}
 	}
@@ -4656,26 +4665,18 @@ func (b *LocalBackend) enterStateLockedOnEntry(newState ipn.State, unlock unlock
 		b.authURL = ""
 		b.authURLTime = time.Time{}
 
-		// Start a captive portal detection loop if none has been
+		// Create a new context for sending to the captive portal
-		// started. Create a new context if none is present, since it
+		// detection loop, and cancel the old one.
-		// can be shut down if we transition away from Running.
+		captiveCtx, captiveCancel := context.WithCancel(b.ctx)
-		if b.captiveCancel == nil {
+		oldCC := b.captiveCtx.Swap(contextAndCancel{captiveCtx, captiveCancel})
-			b.captiveCtx, b.captiveCancel = context.WithCancel(b.ctx)
+		oldCC.cancel()
-			go b.checkCaptivePortalLoop(b.captiveCtx)
-		}
 	} else if oldState == ipn.Running {
 		// Transitioning away from running.
 		b.closePeerAPIListenersLocked()
 
-		// Stop any existing captive portal detection loop.
+		// Cancel our captiveCtx to unblock anything trying to send to
-		if b.captiveCancel != nil {
+		// the captive portal detection loop.
-			b.captiveCancel()
+		b.captiveCtx.Load().cancel()
-			b.captiveCancel = nil
-
-			// NOTE: don't set captiveCtx to nil here, to ensure
-			// that we always have a (canceled) context to wait on
-			// in onHealthChange.
-		}
 	}
 	b.pauseOrResumeControlClientLocked()