ipn/ipnlocal: add (*LocalBackend).RefreshExitNode

In this PR, we add (*LocalBackend).RefreshExitNode which determines which exit node
to use based on the current prefs and netmap and switches to it if needed. It supports
both scenarios when an exit node is specified by IP (rather than ID) and needs to be resolved
once the netmap is ready as well as auto exit nodes.

We then use it in (*LocalBackend).SetControlClientStatus when the netmap changes,
and wherever (*LocalBackend).pickNewAutoExitNode was previously used.

Updates tailscale/corp#29969

Signed-off-by: Nick Khyl <nickk@tailscale.com>

ipn/ipnlocal/local.go

@@ -1627,16 +1627,6 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 	if applySysPolicy(prefs, b.overrideAlwaysOn) {
 		prefsChanged = true
 	}
-	if prefs.AutoExitNode.IsSet() {
-		// Re-evaluate exit node suggestion in case circumstances have changed.
-		_, err := b.suggestExitNodeLocked(curNetMap)
-		if err != nil && !errors.Is(err, ErrNoPreferredDERP) {
-			b.logf("SetControlClientStatus failed to select auto exit node: %v", err)
-		}
-	}
-	if setExitNodeID(prefs, b.lastSuggestedExitNode, curNetMap) {
-		prefsChanged = true
-	}
 
 	// Until recently, we did not store the account's tailnet name. So check if this is the case,
 	// and backfill it on incoming status update.
@@ -1653,6 +1643,8 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 		}); err != nil {
 			b.logf("Failed to save new controlclient state: %v", err)
 		}
+
+		b.sendToLocked(ipn.Notify{Prefs: ptr.To(prefs.View())}, allClients)
 	}
 
 	// initTKALocked is dependent on CurrentProfile.ID, which is initialized
@@ -1695,16 +1687,17 @@ func (b *LocalBackend) SetControlClientStatus(c controlclient.Client, st control
 	b.mu.Unlock()
 
 	// Now complete the lock-free parts of what we started while locked.
-	if prefsChanged {
-		b.send(ipn.Notify{Prefs: ptr.To(prefs.View())})
-	}
-
 	if st.NetMap != nil {
+		// Check and update the exit node if needed, now that we have a new netmap.
+		b.RefreshExitNode()
+
 		if envknob.NoLogsNoSupport() && st.NetMap.HasCap(tailcfg.CapabilityDataPlaneAuditLogs) {
 			msg := "tailnet requires logging to be enabled. Remove --no-logs-no-support from tailscaled command line."
 			b.health.SetLocalLogConfigHealth(errors.New(msg))
 			// Connecting to this tailnet without logging is forbidden; boot us outta here.
 			b.mu.Lock()
+			// Get the current prefs again, since we unlocked above.
+			prefs := b.pm.CurrentPrefs().AsStruct()
 			prefs.WantRunning = false
 			p := prefs.View()
 			if err := b.pm.SetPrefs(p, ipn.NetworkProfile{
@@ -1999,7 +1992,7 @@ func (b *LocalBackend) UpdateNetmapDelta(muts []netmap.NodeMutation) (handled bo
 			if !ok || n.StableID() != exitNodeID {
 				continue
 			}
-			b.goTracker.Go(b.pickNewAutoExitNode)
+			b.goTracker.Go(b.RefreshExitNode)
 			break
 		}
 	}
@@ -5898,30 +5891,50 @@ func (b *LocalBackend) setNetInfo(ni *tailcfg.NetInfo) {
 	}
 	cc.SetNetInfo(ni)
 	if refresh {
-		b.pickNewAutoExitNode()
+		b.RefreshExitNode()
 	}
 }
 
-// pickNewAutoExitNode picks a new automatic exit node if needed.
-func (b *LocalBackend) pickNewAutoExitNode() {
-	unlock := b.lockAndGetUnlock()
-	defer unlock()
+// RefreshExitNode determines which exit node to use based on the current
+// prefs and netmap and switches to it if needed.
+func (b *LocalBackend) RefreshExitNode() {
+	if b.resolveExitNode() {
+		b.authReconfig()
+	}
+}
 
-	newSuggestion, err := b.suggestExitNodeLocked(nil)
-	if err != nil {
-		b.logf("setAutoExitNodeID: %v", err)
-		return
+// resolveExitNode determines which exit node to use based on the current
+// prefs and netmap. It updates the exit node ID in the prefs if needed,
+// sends a notification to clients, and returns true if the exit node has changed.
+//
+// It is the caller's responsibility to reconfigure routes and actually
+// start using the selected exit node, if needed.
+//
+// b.mu must not be held.
+func (b *LocalBackend) resolveExitNode() (changed bool) {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+
+	nm := b.currentNode().NetMap()
+	prefs := b.pm.CurrentPrefs().AsStruct()
+	if prefs.AutoExitNode.IsSet() {
+		_, err := b.suggestExitNodeLocked(nil)
+		if err != nil && !errors.Is(err, ErrNoPreferredDERP) {
+			b.logf("failed to select auto exit node: %v", err)
 		}
-	if b.pm.CurrentPrefs().ExitNodeID() == newSuggestion.ID {
-		return
 	}
-	_, err = b.editPrefsLockedOnEntry(&ipn.MaskedPrefs{
-		Prefs:         ipn.Prefs{ExitNodeID: newSuggestion.ID},
-		ExitNodeIDSet: true,
-	}, unlock)
-	if err != nil {
-		b.logf("setAutoExitNodeID: failed to apply exit node ID preference: %v", err)
+	if !setExitNodeID(prefs, b.lastSuggestedExitNode, nm) {
+		return false // no changes
 	}
+
+	if err := b.pm.SetPrefs(prefs.View(), ipn.NetworkProfile{
+		MagicDNSName: nm.MagicDNSSuffix(),
+		DomainName:   nm.DomainName(),
+	}); err != nil {
+		b.logf("failed to save exit node changes: %v", err)
+	}
+	b.sendToLocked(ipn.Notify{Prefs: ptr.To(prefs.View())}, allClients)
+	return true
 }
 
 // setNetMapLocked updates the LocalBackend state to reflect the newly