ipn/ipnlocal: rebuild allowed suggested exit nodes when syspolicy changes

In this PR, we update LocalBackend to rebuild the set of allowed suggested exit nodes whenever
the AllowedSuggestedExitNodes syspolicy setting changes. Additionally, we request a new suggested
exit node when this occurs, enabling its use if the ExitNodeID syspolicy setting is set to auto:any.

Updates #12687

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

ipn/ipnlocal/local.go

@@ -87,7 +87,6 @@ import (
 	"tailscale.com/types/dnstype"
 	"tailscale.com/types/empty"
 	"tailscale.com/types/key"
-	"tailscale.com/types/lazy"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/logid"
 	"tailscale.com/types/netmap"
@@ -356,6 +355,12 @@ type LocalBackend struct {
 	// avoid unnecessary churn between multiple equally-good options.
 	lastSuggestedExitNode tailcfg.StableNodeID
 
+	// allowedSuggestedExitNodes is a set of exit nodes permitted by the most recent
+	// [syspolicy.AllowedSuggestedExitNodes] value. The allowedSuggestedExitNodesMu
+	// mutex guards access to this set.
+	allowedSuggestedExitNodesMu sync.Mutex
+	allowedSuggestedExitNodes   set.Set[tailcfg.StableNodeID]
+
 	// refreshAutoExitNode indicates if the exit node should be recomputed when the next netcheck report is available.
 	refreshAutoExitNode bool
 
@@ -1724,6 +1729,7 @@ func (b *LocalBackend) registerSysPolicyWatch() (unregister func(), err error) {
 	if prefs, anyChange := b.applySysPolicy(); anyChange {
 		b.logf("syspolicy: changed initial profile prefs: %v", prefs.Pretty())
 	}
+	b.refreshAllowedSuggestions()
 	return unregister, nil
 }
 
@@ -1743,7 +1749,20 @@ func (b *LocalBackend) applySysPolicy() (_ ipn.PrefsView, anyChange bool) {
 
 // sysPolicyChanged is a callback triggered by syspolicy when it detects
 // a change in one or more syspolicy settings.
-func (b *LocalBackend) sysPolicyChanged(*rsop.PolicyChange) {
+func (b *LocalBackend) sysPolicyChanged(policy *rsop.PolicyChange) {
+	if policy.HasChanged(syspolicy.AllowedSuggestedExitNodes) {
+		b.refreshAllowedSuggestions()
+		// Re-evaluate exit node suggestion now that the policy setting has changed.
+		b.mu.Lock()
+		_, err := b.suggestExitNodeLocked(nil)
+		b.mu.Unlock()
+		if err != nil && !errors.Is(err, ErrNoPreferredDERP) {
+			b.logf("failed to select auto exit node: %v", err)
+		}
+		// If [syspolicy.ExitNodeID] is set to `auto:any`, the suggested exit node ID
+		// will be used when [applySysPolicy] updates the current profile's prefs.
+	}
+
 	if prefs, anyChange := b.applySysPolicy(); anyChange {
 		b.logf("syspolicy: changed profile prefs: %v", prefs.Pretty())
 	}
@@ -7197,7 +7216,7 @@ func (b *LocalBackend) suggestExitNodeLocked(netMap *netmap.NetworkMap) (respons
 	lastReport := b.MagicConn().GetLastNetcheckReport(b.ctx)
 	prevSuggestion := b.lastSuggestedExitNode
 
-	res, err := suggestExitNode(lastReport, netMap, prevSuggestion, randomRegion, randomNode, getAllowedSuggestions())
+	res, err := suggestExitNode(lastReport, netMap, prevSuggestion, randomRegion, randomNode, b.getAllowedSuggestions())
 	if err != nil {
 		return res, err
 	}
@@ -7211,6 +7230,22 @@ func (b *LocalBackend) SuggestExitNode() (response apitype.ExitNodeSuggestionRes
 	return b.suggestExitNodeLocked(nil)
 }
 
+// getAllowedSuggestions returns a set of exit nodes permitted by the most recent
+// [syspolicy.AllowedSuggestedExitNodes] value. Callers must not mutate the returned set.
+func (b *LocalBackend) getAllowedSuggestions() set.Set[tailcfg.StableNodeID] {
+	b.allowedSuggestedExitNodesMu.Lock()
+	defer b.allowedSuggestedExitNodesMu.Unlock()
+	return b.allowedSuggestedExitNodes
+}
+
+// refreshAllowedSuggestions rebuilds the set of permitted exit nodes
+// from the current [syspolicy.AllowedSuggestedExitNodes] value.
+func (b *LocalBackend) refreshAllowedSuggestions() {
+	b.allowedSuggestedExitNodesMu.Lock()
+	defer b.allowedSuggestedExitNodesMu.Unlock()
+	b.allowedSuggestedExitNodes = fillAllowedSuggestions()
+}
+
 // selectRegionFunc returns a DERP region from the slice of candidate regions.
 // The value is returned, not the slice index.
 type selectRegionFunc func(views.Slice[int]) int
@@ -7220,8 +7255,6 @@ type selectRegionFunc func(views.Slice[int]) int
 // choice.
 type selectNodeFunc func(nodes views.Slice[tailcfg.NodeView], last tailcfg.StableNodeID) tailcfg.NodeView
 
-var getAllowedSuggestions = lazy.SyncFunc(fillAllowedSuggestions)
-
 func fillAllowedSuggestions() set.Set[tailcfg.StableNodeID] {
 	nodes, err := syspolicy.GetStringArray(syspolicy.AllowedSuggestedExitNodes, nil)
 	if err != nil {