net/dnsfallback: cache most recent DERP map on disk (#5545)

This is especially helpful as we launch newer DERPs over time, and older clients have progressively out-of-date static DERP maps baked in. After this, as long as the client has successfully connected once, it'll cache the most recent DERP map it knows about. Resolves an in-code comment from @bradfitz Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
2022-09-05 14:36:30 -04:00
parent 060ecb010f
commit b8596f2a2f
4 changed files with 271 additions and 6 deletions
@@ -4,7 +4,15 @@

 package dnsfallback

-import "testing"
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"reflect"
+	"testing"
+
+	"tailscale.com/tailcfg"
+)

 func TestGetDERPMap(t *testing.T) {
 	dm := getDERPMap()
@@ -15,3 +23,151 @@ func TestGetDERPMap(t *testing.T) {
 		t.Fatal("no regions")
 	}
 }
+
+func TestCache(t *testing.T) {
+	cacheFile := filepath.Join(t.TempDir(), "cache.json")
+
+	// Write initial cache value
+	initialCache := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			99: {
+				RegionID:   99,
+				RegionCode: "test",
+				RegionName: "Testville",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:     "99a",
+					RegionID: 99,
+					HostName: "derp99a.tailscale.com",
+					IPv4:     "1.2.3.4",
+				}},
+			},
+
+			// Intentionally attempt to "overwrite" something
+			1: {
+				RegionID:   1,
+				RegionCode: "r1",
+				RegionName: "r1",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:     "1c",
+					RegionID: 1,
+					HostName: "derp1c.tailscale.com",
+					IPv4:     "127.0.0.1",
+					IPv6:     "::1",
+				}},
+			},
+		},
+	}
+	d, err := json.Marshal(initialCache)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(cacheFile, d, 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	// Clear any existing cached DERP map(s)
+	cachedDERPMap.Store(nil)
+
+	// Load the cache
+	SetCachePath(cacheFile)
+	if cm := cachedDERPMap.Load(); !reflect.DeepEqual(initialCache, cm) {
+		t.Fatalf("cached map was %+v; want %+v", cm, initialCache)
+	}
+
+	// Verify that our DERP map is merged with the cache.
+	dm := getDERPMap()
+	region, ok := dm.Regions[99]
+	if !ok {
+		t.Fatal("expected region 99")
+	}
+	if !reflect.DeepEqual(region, initialCache.Regions[99]) {
+		t.Fatalf("region 99: got %+v; want %+v", region, initialCache.Regions[99])
+	}
+
+	// Verify that our cache can't override a statically-baked-in DERP server.
+	n0 := dm.Regions[1].Nodes[0]
+	if n0.IPv4 == "127.0.0.1" || n0.IPv6 == "::1" {
+		t.Errorf("got %+v; expected no overwrite for node", n0)
+	}
+
+	// Also, make sure that the static DERP map still has the same first
+	// node as when this test was last written/updated; this ensures that
+	// we don't accidentally start allowing overwrites due to some of the
+	// test's assumptions changing out from underneath us as we update the
+	// JSON file of fallback servers.
+	if getStaticDERPMap().Regions[1].Nodes[0].HostName != "derp1c.tailscale.com" {
+		t.Errorf("DERP server has a different name; please update this test")
+	}
+}
+
+func TestCacheUnchanged(t *testing.T) {
+	cacheFile := filepath.Join(t.TempDir(), "cache.json")
+
+	// Write initial cache value
+	initialCache := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			99: {
+				RegionID:   99,
+				RegionCode: "test",
+				RegionName: "Testville",
+				Nodes: []*tailcfg.DERPNode{{
+					Name:     "99a",
+					RegionID: 99,
+					HostName: "derp99a.tailscale.com",
+					IPv4:     "1.2.3.4",
+				}},
+			},
+		},
+	}
+	d, err := json.Marshal(initialCache)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(cacheFile, d, 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	// Clear any existing cached DERP map(s)
+	cachedDERPMap.Store(nil)
+
+	// Load the cache
+	SetCachePath(cacheFile)
+	if cm := cachedDERPMap.Load(); !reflect.DeepEqual(initialCache, cm) {
+		t.Fatalf("cached map was %+v; want %+v", cm, initialCache)
+	}
+
+	// Remove the cache file on-disk, then re-set to the current value. If
+	// our equality comparison is working, we won't rewrite the file
+	// on-disk since the cached value won't have changed.
+	if err := os.Remove(cacheFile); err != nil {
+		t.Fatal(err)
+	}
+
+	UpdateCache(initialCache)
+	if _, err := os.Stat(cacheFile); !os.IsNotExist(err) {
+		t.Fatalf("got err=%v; expected to not find cache file", err)
+	}
+
+	// Now, update the cache with something slightly different and verify
+	// that we did re-write the file on-disk.
+	updatedCache := &tailcfg.DERPMap{
+		Regions: map[int]*tailcfg.DERPRegion{
+			99: {
+				RegionID:   99,
+				RegionCode: "test",
+				RegionName: "Testville",
+				Nodes:      []*tailcfg.DERPNode{ /* set below */ },
+			},
+		},
+	}
+	clonedNode := *initialCache.Regions[99].Nodes[0]
+	clonedNode.IPv4 = "1.2.3.5"
+	updatedCache.Regions[99].Nodes = append(updatedCache.Regions[99].Nodes, &clonedNode)
+
+	UpdateCache(updatedCache)
+	if st, err := os.Stat(cacheFile); err != nil {
+		t.Fatalf("could not stat cache file; err=%v", err)
+	} else if !st.Mode().IsRegular() || st.Size() == 0 {
+		t.Fatalf("didn't find non-empty regular file; mode=%v size=%d", st.Mode(), st.Size())
+	}
+}