routeselector: make exit-node reconciliation atomic

enforceSingleExitNode took the RouteSelector lock three separate times
(IsDeselectAll, then DeselectRoutes, then SelectRoutes), so a concurrent
DeselectAllRoutes could interleave and be silently undone: SelectRoutes on
its deselectAll branch clears the flag and re-selects the preferred exit
node, overriding the user's "all off".

Move the whole reconciliation into a single locked RouteSelector method
(SetExclusiveExitNode) that checks deselectAll inside the critical section,
so a deselect-all either fully precedes the reconcile (left untouched) or
fully follows it (honoured). No interleaving is possible.

client/internal/routemanager/exit_node_selection_test.go

@@ -0,0 +1,191 @@
+package routemanager
+
+import (
+	"net/netip"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/internal/routeselector"
+	"github.com/netbirdio/netbird/route"
+)
+
+func newExitNodeTestManager() *DefaultManager {
+	return &DefaultManager{routeSelector: routeselector.NewRouteSelector()}
+}
+
+func exitRoute(netID, peer string, skipAutoApply bool) *route.Route {
+	return &route.Route{
+		NetID:         route.NetID(netID),
+		Network:       netip.MustParsePrefix("0.0.0.0/0"),
+		Peer:          peer,
+		SkipAutoApply: skipAutoApply,
+	}
+}
+
+func TestPickPreferredExitNode(t *testing.T) {
+	tests := []struct {
+		name string
+		info exitNodeInfo
+		want route.NetID
+	}{
+		{
+			name: "persisted user selection wins over management",
+			info: exitNodeInfo{
+				allIDs:               []route.NetID{"a", "b", "c"},
+				userSelected:         []route.NetID{"b"},
+				selectedByManagement: []route.NetID{"a"},
+			},
+			want: "b",
+		},
+		{
+			name: "multiple user-selected self-heal to deterministic min",
+			info: exitNodeInfo{
+				allIDs:       []route.NetID{"a", "b", "c"},
+				userSelected: []route.NetID{"c", "a"},
+			},
+			want: "a",
+		},
+		{
+			name: "explicit opt-out keeps none",
+			info: exitNodeInfo{
+				allIDs:         []route.NetID{"a", "b"},
+				userDeselected: []route.NetID{"a", "b"},
+			},
+			want: "",
+		},
+		{
+			name: "fresh defaults to management auto-apply pick",
+			info: exitNodeInfo{
+				allIDs:               []route.NetID{"a", "b", "c"},
+				selectedByManagement: []route.NetID{"b"},
+			},
+			want: "b",
+		},
+		{
+			name: "no user pick and no management auto-apply selects none",
+			info: exitNodeInfo{
+				allIDs: []route.NetID{"c", "a", "b"},
+			},
+			want: "",
+		},
+		{
+			name: "user-deselect does not block a management auto-apply sibling",
+			info: exitNodeInfo{
+				allIDs:               []route.NetID{"a", "b"},
+				userDeselected:       []route.NetID{"a"},
+				selectedByManagement: []route.NetID{"b"},
+			},
+			want: "b",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, pickPreferredExitNode(tt.info), "preferred exit node")
+		})
+	}
+}
+
+func TestEnforceSingleExitNode(t *testing.T) {
+	m := newExitNodeTestManager()
+	all := []route.NetID{"a", "b", "c"}
+
+	m.enforceSingleExitNode("b", all)
+	assert.False(t, m.routeSelector.IsSelected("a"), "a should be deselected")
+	assert.True(t, m.routeSelector.IsSelected("b"), "b should be the only selected exit node")
+	assert.False(t, m.routeSelector.IsSelected("c"), "c should be deselected")
+
+	// Switching the preferred node moves the single selection.
+	m.enforceSingleExitNode("c", all)
+	assert.False(t, m.routeSelector.IsSelected("a"), "a stays deselected")
+	assert.False(t, m.routeSelector.IsSelected("b"), "b should now be deselected")
+	assert.True(t, m.routeSelector.IsSelected("c"), "c should now be selected")
+
+	// Empty preferred turns every exit node off.
+	m.enforceSingleExitNode("", all)
+	for _, id := range all {
+		assert.False(t, m.routeSelector.IsSelected(id), "no exit node should be selected")
+	}
+}
+
+func TestEnforceSingleExitNode_RespectsDeselectAll(t *testing.T) {
+	m := newExitNodeTestManager()
+	m.routeSelector.DeselectAllRoutes()
+
+	m.enforceSingleExitNode("b", []route.NetID{"a", "b"})
+
+	assert.True(t, m.routeSelector.IsDeselectAll(), "global deselect-all must stay in effect")
+	assert.False(t, m.routeSelector.IsSelected("b"), "no exit node should be forced on while deselect-all is set")
+}
+
+func TestUpdateRouteSelectorFromManagement_FreshSelectsOne(t *testing.T) {
+	m := newExitNodeTestManager()
+	routes := route.HAMap{
+		"exitA|0.0.0.0/0":    {exitRoute("exitA", "p1", false)},
+		"exitB|0.0.0.0/0":    {exitRoute("exitB", "p2", false)},
+		"lan|192.168.1.0/24": {{NetID: "lan", Network: netip.MustParsePrefix("192.168.1.0/24"), Peer: "p3"}},
+		"exitC|0.0.0.0/0":    {exitRoute("exitC", "p4", false)},
+	}
+
+	m.updateRouteSelectorFromManagement(routes)
+
+	// Exactly one exit node (the deterministic first) is selected.
+	assert.True(t, m.routeSelector.IsSelected("exitA"), "exitA is the deterministic default")
+	assert.False(t, m.routeSelector.IsSelected("exitB"), "exitB must not also be selected")
+	assert.False(t, m.routeSelector.IsSelected("exitC"), "exitC must not also be selected")
+	// Non-exit routes are left at their default-on state.
+	assert.True(t, m.routeSelector.IsSelected("lan"), "non-exit route selection is untouched")
+}
+
+func TestUpdateRouteSelectorFromManagement_HonorsPersistedPick(t *testing.T) {
+	m := newExitNodeTestManager()
+	routes := route.HAMap{
+		"exitA|0.0.0.0/0": {exitRoute("exitA", "p1", false)},
+		"exitB|0.0.0.0/0": {exitRoute("exitB", "p2", false)},
+	}
+	all := []route.NetID{"exitA", "exitB"}
+
+	// Simulate the state the runtime select path leaves behind: exactly one
+	// exit node explicitly selected, its sibling deselected.
+	require.NoError(t, m.routeSelector.SelectRoutes([]route.NetID{"exitB"}, true, all))
+	require.NoError(t, m.routeSelector.DeselectRoutes([]route.NetID{"exitA"}, all))
+
+	m.updateRouteSelectorFromManagement(routes)
+
+	assert.True(t, m.routeSelector.IsSelected("exitB"), "persisted pick must stay selected")
+	assert.False(t, m.routeSelector.IsSelected("exitA"), "the other exit node stays deselected")
+}
+
+func TestUpdateRouteSelectorFromManagement_OptOutKeepsNone(t *testing.T) {
+	m := newExitNodeTestManager()
+	routes := route.HAMap{
+		"exitA|0.0.0.0/0": {exitRoute("exitA", "p1", false)},
+		"exitB|0.0.0.0/0": {exitRoute("exitB", "p2", false)},
+	}
+	all := []route.NetID{"exitA", "exitB"}
+
+	// User deselected exit nodes and selected none.
+	require.NoError(t, m.routeSelector.DeselectRoutes(all, all))
+
+	m.updateRouteSelectorFromManagement(routes)
+
+	assert.False(t, m.routeSelector.IsSelected("exitA"), "opt-out keeps exitA off")
+	assert.False(t, m.routeSelector.IsSelected("exitB"), "opt-out keeps exitB off")
+}
+
+func TestUpdateRouteSelectorFromManagement_NoAutoApplySelectsNone(t *testing.T) {
+	m := newExitNodeTestManager()
+	// SkipAutoApply=true: management offers the exit nodes but doesn't request
+	// auto-activation, so none should be selected until the user picks one.
+	routes := route.HAMap{
+		"exitA|0.0.0.0/0": {exitRoute("exitA", "p1", true)},
+		"exitB|0.0.0.0/0": {exitRoute("exitB", "p2", true)},
+	}
+
+	m.updateRouteSelectorFromManagement(routes)
+
+	assert.False(t, m.routeSelector.IsSelected("exitA"), "no auto-apply keeps exitA off")
+	assert.False(t, m.routeSelector.IsSelected("exitB"), "no auto-apply keeps exitB off")
+}

client/internal/routemanager/manager.go

@@ -701,7 +701,13 @@ func resolveURLsToIPs(urls []string) []net.IP {
 	return ips
 }
 
-// updateRouteSelectorFromManagement updates the route selector based on the isSelected status from the management server
+// updateRouteSelectorFromManagement reconciles exit-node selection on every
+// network map: it keeps at most one exit node selected — the user's persisted
+// pick, else whatever management marks for auto-apply (SkipAutoApply=false),
+// else none. We never auto-activate an exit node the map doesn't request; it
+// stays off until the user picks it. Exit nodes are mutually exclusive, but the
+// RouteSelector stores routes with default-on semantics, so without this every
+// available exit node would report selected at once.
 func (m *DefaultManager) updateRouteSelectorFromManagement(clientRoutes route.HAMap) {
 	m.mirrorV6ExitPairSelections(clientRoutes)
 
@@ -712,13 +718,14 @@ func (m *DefaultManager) updateRouteSelectorFromManagement(clientRoutes route.HA
 		return
 	}
 
-	exitNodeInfo := m.collectExitNodeInfo(clientRoutes)
-	if len(exitNodeInfo.allIDs) == 0 {
+	info := m.collectExitNodeInfo(clientRoutes)
+	if len(info.allIDs) == 0 {
 		return
 	}
 
-	m.updateExitNodeSelections(exitNodeInfo)
-	m.logExitNodeUpdate(exitNodeInfo)
+	preferred := pickPreferredExitNode(info)
+	m.enforceSingleExitNode(preferred, info.allIDs)
+	m.logExitNodeUpdate(info, preferred)
 }
 
 // mirrorV6ExitPairSelections keeps every synthesized "-v6" exit route's selection
@@ -746,6 +753,10 @@ type exitNodeInfo struct {
 	userDeselected       []route.NetID
 }
 
+// collectExitNodeInfo categorises the available exit nodes by their persisted
+// selection state. It keys on the base (v4) NetID and skips the synthesized
+// "-v6" partner, which inherits its base's selection through the RouteSelector
+// — counting it separately would double-count the pair.
 func (m *DefaultManager) collectExitNodeInfo(clientRoutes route.HAMap) exitNodeInfo {
 	var info exitNodeInfo
 
@@ -755,6 +766,9 @@ func (m *DefaultManager) collectExitNodeInfo(clientRoutes route.HAMap) exitNodeI
 		}
 
 		netID := haID.NetID()
+		if strings.HasSuffix(string(netID), route.V6ExitSuffix) {
+			continue
+		}
 		info.allIDs = append(info.allIDs, netID)
 
 		if m.routeSelector.HasUserSelectionForRoute(netID) {
@@ -791,45 +805,52 @@ func (m *DefaultManager) checkManagementSelection(routes []*route.Route, netID r
 	}
 }
 
-func (m *DefaultManager) updateExitNodeSelections(info exitNodeInfo) {
-	routesToDeselect := m.getRoutesToDeselect(info.allIDs)
-	m.deselectExitNodes(routesToDeselect)
-	m.selectExitNodesByManagement(info.selectedByManagement, info.allIDs)
+// pickPreferredExitNode chooses the single exit node to keep selected. In order:
+//   - a persisted user selection wins (deterministic if several survive from
+//     legacy state, so the set self-heals down to one);
+//   - otherwise activate only what management marks for auto-apply
+//     (SkipAutoApply=false); the lexicographically first if it marks several.
+//
+// Returns "" when neither holds — we never force an arbitrary exit node on. A
+// route the map doesn't auto-apply stays off until the user selects it.
+// info.userDeselected is informational only: an explicit deselect simply keeps
+// that route out of both lists above, so it can't be picked.
+func pickPreferredExitNode(info exitNodeInfo) route.NetID {
+	if len(info.userSelected) > 0 {
+		return minNetID(info.userSelected)
+	}
+	if len(info.selectedByManagement) > 0 {
+		return minNetID(info.selectedByManagement)
+	}
+	return ""
 }
 
-func (m *DefaultManager) getRoutesToDeselect(allIDs []route.NetID) []route.NetID {
-	var routesToDeselect []route.NetID
-	for _, netID := range allIDs {
-		if !m.routeSelector.HasUserSelectionForRoute(netID) {
-			routesToDeselect = append(routesToDeselect, netID)
+// enforceSingleExitNode makes preferred the only selected exit node: every other
+// available exit node is deselected and preferred (if any) is selected, without
+// disturbing non-exit route selections. The whole reconciliation runs under a
+// single RouteSelector lock (SetExclusiveExitNode) so a concurrent deselect-all
+// cannot interleave and get undone; a global deselect-all is left untouched so
+// the user's "all off" stays in effect.
+func (m *DefaultManager) enforceSingleExitNode(preferred route.NetID, allIDs []route.NetID) {
+	m.routeSelector.SetExclusiveExitNode(preferred, allIDs)
+}
+
+func (m *DefaultManager) logExitNodeUpdate(info exitNodeInfo, preferred route.NetID) {
+	log.Debugf("Exit node selection: %d available, preferred=%q (%d user-selected, %d user-deselected, %d management-selected)",
+		len(info.allIDs), preferred, len(info.userSelected), len(info.userDeselected), len(info.selectedByManagement))
+}
+
+// minNetID returns the lexicographically smallest NetID, for a deterministic
+// default pick that stays stable across restarts.
+func minNetID(ids []route.NetID) route.NetID {
+	if len(ids) == 0 {
+		return ""
+	}
+	best := ids[0]
+	for _, id := range ids[1:] {
+		if id < best {
+			best = id
 		}
 	}
-	return routesToDeselect
-}
-
-func (m *DefaultManager) deselectExitNodes(routesToDeselect []route.NetID) {
-	if len(routesToDeselect) == 0 {
-		return
-	}
-
-	err := m.routeSelector.DeselectRoutes(routesToDeselect, routesToDeselect)
-	if err != nil {
-		log.Warnf("Failed to deselect exit nodes: %v", err)
-	}
-}
-
-func (m *DefaultManager) selectExitNodesByManagement(selectedByManagement []route.NetID, allIDs []route.NetID) {
-	if len(selectedByManagement) == 0 {
-		return
-	}
-
-	err := m.routeSelector.SelectRoutes(selectedByManagement, true, allIDs)
-	if err != nil {
-		log.Warnf("Failed to select exit nodes: %v", err)
-	}
-}
-
-func (m *DefaultManager) logExitNodeUpdate(info exitNodeInfo) {
-	log.Debugf("Updated route selector: %d exit nodes available, %d selected by management, %d user-selected, %d user-deselected",
-		len(info.allIDs), len(info.selectedByManagement), len(info.userSelected), len(info.userDeselected))
+	return best
 }

client/internal/routeselector/routeselector.go

@@ -115,7 +115,38 @@ func (rs *RouteSelector) DeselectAllRoutes() {
 	clear(rs.selectedRoutes)
 }
 
-// IsDeselectAll reports whether the user has explicitly deselected all routes.
+// SetExclusiveExitNode atomically makes preferred the only selected exit node
+// among exitIDs: every other ID in exitIDs is deselected and preferred (when
+// non-empty) is selected, all under a single lock. Holding the lock across the
+// whole reconciliation prevents a concurrent DeselectAllRoutes from interleaving
+// between the deselect and select steps and being silently undone. A global
+// deselect-all is left untouched so the user's "all off" stays in effect;
+// non-exit routes are never referenced, so their selection is preserved.
+func (rs *RouteSelector) SetExclusiveExitNode(preferred route.NetID, exitIDs []route.NetID) {
+	rs.mu.Lock()
+	defer rs.mu.Unlock()
+
+	if rs.deselectAll {
+		return
+	}
+
+	for _, id := range exitIDs {
+		if id == preferred {
+			continue
+		}
+		rs.deselectedRoutes[id] = struct{}{}
+		delete(rs.selectedRoutes, id)
+	}
+
+	if preferred != "" {
+		delete(rs.deselectedRoutes, preferred)
+		rs.selectedRoutes[preferred] = struct{}{}
+	}
+}
+
+// IsDeselectAll reports whether the global "deselect all" flag is set, i.e. the
+// user explicitly disabled every route. Callers enforcing per-route invariants
+// (e.g. single exit node) should leave the selection untouched when it is.
 func (rs *RouteSelector) IsDeselectAll() bool {
 	rs.mu.RLock()
 	defer rs.mu.RUnlock()