[client] Fix ios route notify ordering (#6454)

* [client] fix iOS route-update reordering that black-holed IPv6 on exit-node disable

On iOS the route notifier delivered each prefix update from its own
fire-and-forget goroutine (notify -> `go func`), so Go provided no ordering
guarantee between consecutive updates. It also read currentPrefixes inside
that goroutine without holding the lock, racing the next OnNewPrefixes write.

On exit-node disable the core removes the default routes as two separate
prefix updates (0.0.0.0/0, then the synthesized ::/0). When the two
goroutines were reordered, the stale snapshot still containing ::/0 was
delivered last and clobbered the correct default-free one. iOS then kept the
::/0 default route on the tunnel with no exit node to carry it, black-holing
all IPv6 traffic while IPv4 recovered correctly.

Fix: deliver updates through a single worker goroutine fed by a buffered
channel, preserving production order, and snapshot the joined prefix string
under the mutex so it can't race a concurrent update. Buffered so producers
(which run under the route manager lock) don't block on the listener callback.

* [client] close iOS notifier delivery goroutine on Stop, unbounded queue

The delivery goroutine was never stopped, leaking on every engine
restart. Add Notifier.Close, called from the route manager Stop after
routing cleanup.

Replace the buffered update channel with a cond-driven linked-list
queue so route-update producers (running under the route manager lock)
never block when the listener callback is slow.

client/internal/routemanager/manager.go

@@ -333,6 +333,8 @@ func (m *DefaultManager) Stop(stateManager *statemanager.Manager) {
 		}
 	}
 
+	m.notifier.Close()
+
 	m.mux.Lock()
 	defer m.mux.Unlock()
 	m.clientRoutes = nil

client/internal/routemanager/notifier/notifier_android.go

@@ -16,7 +16,7 @@ import (
 type Notifier struct {
 	initialRoutes []*route.Route
 	currentRoutes []*route.Route
-	fakeIPRoutes []*route.Route
+	fakeIPRoutes  []*route.Route
 
 	listener    listener.NetworkChangeListener
 	listenerMux sync.Mutex
@@ -119,3 +119,7 @@ func (n *Notifier) GetInitialRouteRanges() []string {
 	sort.Strings(initialStrings)
 	return initialStrings
 }
+
+func (n *Notifier) Close() {
+	// unused
+}

client/internal/routemanager/notifier/notifier_ios.go

@@ -3,6 +3,7 @@
 package notifier
 
 import (
+	"container/list"
 	"net/netip"
 	"slices"
 	"sort"
@@ -14,19 +15,26 @@ import (
 )
 
 type Notifier struct {
+	mu              sync.Mutex
+	cond            *sync.Cond
 	currentPrefixes []string
-
+	listener        listener.NetworkChangeListener
-	listener    listener.NetworkChangeListener
+	queue           *list.List
-	listenerMux sync.Mutex
+	closed          bool
 }
 
 func NewNotifier() *Notifier {
-	return &Notifier{}
+	n := &Notifier{
+		queue: list.New(),
+	}
+	n.cond = sync.NewCond(&n.mu)
+	go n.deliverLoop()
+	return n
 }
 
 func (n *Notifier) SetListener(listener listener.NetworkChangeListener) {
-	n.listenerMux.Lock()
+	n.mu.Lock()
-	defer n.listenerMux.Unlock()
+	defer n.mu.Unlock()
 	n.listener = listener
 }
 
@@ -43,32 +51,52 @@ func (n *Notifier) OnNewRoutes(route.HAMap) {
 }
 
 func (n *Notifier) OnNewPrefixes(prefixes []netip.Prefix) {
-	newNets := make([]string, 0)
+	newNets := make([]string, 0, len(prefixes))
 	for _, prefix := range prefixes {
 		newNets = append(newNets, prefix.String())
 	}
 
 	sort.Strings(newNets)
 
+	n.mu.Lock()
 	if slices.Equal(n.currentPrefixes, newNets) {
+		n.mu.Unlock()
 		return
 	}
-
 	n.currentPrefixes = newNets
-	n.notify()
+	routes := strings.Join(n.currentPrefixes, ",")
+	n.queue.PushBack(routes)
+	n.cond.Signal()
+	n.mu.Unlock()
 }
-func (n *Notifier) notify() {
-	n.listenerMux.Lock()
-	defer n.listenerMux.Unlock()
-	if n.listener == nil {
-		return
-	}
 
-	go func(l listener.NetworkChangeListener) {
+func (n *Notifier) Close() {
-		l.OnNetworkChanged(strings.Join(n.currentPrefixes, ","))
+	n.mu.Lock()
-	}(n.listener)
+	n.closed = true
+	n.cond.Signal()
+	n.mu.Unlock()
 }
 
 func (n *Notifier) GetInitialRouteRanges() []string {
 	return nil
 }
+
+func (n *Notifier) deliverLoop() {
+	for {
+		n.mu.Lock()
+		for n.queue.Len() == 0 && !n.closed {
+			n.cond.Wait()
+		}
+		if n.closed && n.queue.Len() == 0 {
+			n.mu.Unlock()
+			return
+		}
+		routes := n.queue.Remove(n.queue.Front()).(string)
+		l := n.listener
+		n.mu.Unlock()
+
+		if l != nil {
+			l.OnNetworkChanged(routes)
+		}
+	}
+}

client/internal/routemanager/notifier/notifier_other.go

@@ -38,3 +38,7 @@ func (n *Notifier) OnNewPrefixes(prefixes []netip.Prefix) {
 func (n *Notifier) GetInitialRouteRanges() []string {
 	return []string{}
 }
+
+func (n *Notifier) Close() {
+	// unused
+}