state: return a node's primary routes in stable order

They came from map iteration, so an unchanged router rendered its self
node differently on every call.

Updates #3502

hscontrol/state/node_store.go

@@ -986,7 +986,7 @@ func (s *NodeStore) PrimaryRouteFor(prefix netip.Prefix) (types.NodeID, bool) {
 }
 
 // PrimaryRoutesForNode returns the prefixes for which id is the current
-// primary advertiser.
+// primary advertiser, sorted so an unchanged node renders identically.
 func (s *NodeStore) PrimaryRoutesForNode(id types.NodeID) []netip.Prefix {
 	snap := s.data.Load()
 	if !snap.isPrimaryRoute[id] {
@@ -1001,6 +1001,8 @@ func (s *NodeStore) PrimaryRoutesForNode(id types.NodeID) []netip.Prefix {
 		}
 	}
 
+	slices.SortFunc(out, netip.Prefix.Compare)
+
 	return out
 }
 

hscontrol/state/node_store_test.go

@@ -1496,6 +1496,28 @@ func TestHealthOnlyWriteReusesPeerMap(t *testing.T) {
 		peersCalls.Load())
 }
 
+// TestPrimaryRoutesForNodeSorted checks a router's primaries come back in
+// a stable order: they render into its self node, which the mapper resends
+// whenever it differs from what the node holds (issue #3502).
+func TestPrimaryRoutesForNodeSorted(t *testing.T) {
+	node := createTestNode(1, 1, "user1", "router")
+
+	prefixes := make([]netip.Prefix, 0, 8)
+	for i := range 8 {
+		prefixes = append(prefixes, netip.PrefixFrom(netip.AddrFrom4([4]byte{10, byte(i), 0, 0}), 16))
+	}
+
+	node.Hostinfo = &tailcfg.Hostinfo{Hostname: "router", RoutableIPs: prefixes}
+	node.ApprovedRoutes = prefixes
+	node.IsOnline = new(true)
+
+	store := NewNodeStore(types.Nodes{&node}, allowAllPeersFunc, TestBatchSize, TestBatchTimeout)
+
+	for range 20 {
+		require.Equal(t, prefixes, store.PrimaryRoutesForNode(1))
+	}
+}
+
 func BenchmarkSnapshotPayloadDense(b *testing.B) {
 	const nodeCount = 500