change: drop subnet-router full update, use policy change

Don't send a full update when a subnet router goes up or down; the gated
policy change already recomputes peers and is a smaller payload.

Updates #3293

hscontrol/types/change/change.go

@@ -456,29 +456,19 @@ func NodeRemoved(id types.NodeID) Change {
 	return PeersRemoved(id)
 }
 
-// NodeOnlineFor returns a [Change] for when a node comes online.
-// If the node is a subnet router, a full update is sent instead of a patch.
+// NodeOnlineFor returns the [Change] for a node coming online: a lightweight
+// [NodeOnline] peer patch. Subnet routers, relay targets, and via targets get
+// their full peer recompute from the gated [PolicyChange] that State.Connect
+// emits, so no full update is needed here.
 func NodeOnlineFor(node types.NodeView) Change {
-	if node.IsSubnetRouter() {
-		c := FullUpdate()
-		c.Reason = "subnet router online"
-
-		return c
-	}
-
 	return NodeOnline(node.ID())
 }
 
-// NodeOfflineFor returns a [Change] for when a node goes offline.
-// If the node is a subnet router, a full update is sent instead of a patch.
+// NodeOfflineFor returns the [Change] for a node going offline: a lightweight
+// [NodeOffline] peer patch. As with [NodeOnlineFor], subnet routers and other
+// recompute-forcing nodes rely on the gated [PolicyChange] from State.Disconnect
+// for the peer recompute, so no full update is needed here.
 func NodeOfflineFor(node types.NodeView) Change {
-	if node.IsSubnetRouter() {
-		c := FullUpdate()
-		c.Reason = "subnet router offline"
-
-		return c
-	}
-
 	return NodeOffline(node.ID())
 }
 

hscontrol/types/change/change_test.go

@@ -1,11 +1,13 @@
 package change
 
 import (
+	"net/netip"
 	"reflect"
 	"testing"
 
 	"github.com/juanfont/headscale/hscontrol/types"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"tailscale.com/tailcfg"
 )
 
@@ -612,3 +614,42 @@ func TestUniqueNodeIDs(t *testing.T) {
 		})
 	}
 }
+
+func TestNodeOnlineOfflineForSubnetRouter(t *testing.T) {
+	route := netip.MustParsePrefix("10.0.0.0/24")
+	router := types.Node{
+		ID:             1,
+		Hostinfo:       &tailcfg.Hostinfo{RoutableIPs: []netip.Prefix{route}},
+		ApprovedRoutes: []netip.Prefix{route},
+	}
+	view := router.View()
+	require.True(t, view.IsSubnetRouter(), "test node must be a subnet router")
+
+	tests := []struct {
+		name       string
+		got        Change
+		wantOnline bool
+	}{
+		{name: "online", got: NodeOnlineFor(view), wantOnline: true},
+		{name: "offline", got: NodeOfflineFor(view), wantOnline: false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// A subnet router's online/offline transition rides the lightweight
+			// peer patch, not a full update: the gated PolicyChange that
+			// State.Connect/Disconnect emit owns the netmap recompute.
+			assert.False(t, tt.got.IsFull(),
+				"subnet router online/offline must be a peer patch, not a full update")
+
+			require.NotEmpty(t, tt.got.PeerPatches,
+				"expected an online/offline peer patch")
+
+			patch := tt.got.PeerPatches[0]
+			assert.Equal(t, view.ID().NodeID(), patch.NodeID)
+
+			require.NotNil(t, patch.Online)
+			assert.Equal(t, tt.wantOnline, *patch.Online)
+		})
+	}
+}