wgengine/router: consolidate routes before reconfiguring router for mobile clients

This helps reduce memory pressure on tailnets with large numbers
of routes.

Updates tailscale/corp#19332

Signed-off-by: Percy Wegmann <percy@tailscale.com>

wgengine/router/consolidating_router_test.go

@@ -0,0 +1,68 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package router
+
+import (
+	"log"
+	"net/netip"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestConsolidateRoutes(t *testing.T) {
+	parseRoutes := func(routes ...string) []netip.Prefix {
+		parsed := make([]netip.Prefix, 0, len(routes))
+		for _, routeString := range routes {
+			route, err := netip.ParsePrefix(routeString)
+			if err != nil {
+				t.Fatal(err)
+			}
+			parsed = append(parsed, route)
+		}
+		return parsed
+	}
+
+	tests := []struct {
+		name string
+		cfg  *Config
+		want *Config
+	}{
+		{
+			"nil cfg",
+			nil,
+			nil,
+		},
+		{
+			"single route",
+			&Config{Routes: parseRoutes("10.0.0.0/32")},
+			&Config{Routes: parseRoutes("10.0.0.0/32")},
+		},
+		{
+			"two routes from different families",
+			&Config{Routes: parseRoutes("10.0.0.0/32", "2603:1030:c02::/47")},
+			&Config{Routes: parseRoutes("10.0.0.0/32", "2603:1030:c02::/47")},
+		},
+		{
+			"two disjoint routes",
+			&Config{Routes: parseRoutes("10.0.0.0/32", "10.0.2.0/32")},
+			&Config{Routes: parseRoutes("10.0.0.0/32", "10.0.2.0/32")},
+		},
+		{
+			"two overlapping routes",
+			&Config{Routes: parseRoutes("10.0.0.0/32", "10.0.0.0/31")},
+			&Config{Routes: parseRoutes("10.0.0.0/31")},
+		},
+	}
+
+	cr := &consolidatingRouter{logf: log.Printf}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := cr.consolidateRoutes(test.cfg)
+			if diff := cmp.Diff(got, test.want); diff != "" {
+				t.Errorf("wrong result; (-got+want):%v", diff)
+			}
+		})
+	}
+}