ipn/ipnlocal: update netmap cache after peer deltas are applied (#20111)

Add an UpdatePeers method to the cache. This allows us to support netmap peer deltas,
by allowing just the peers to be updated in an existing cache. As a safety check, reject
an update if there was no base netmap data to apply a change to.

Then, when processing peer mutations in the backend, capture any changes that should
be applied to the cache and update it, if one is enabled.

Updates #12542

Change-Id: I2f8790a8fdc5e85fce6700ba4821a8cb10dddffa
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

ipn/ipnlocal/netmapcache/netmapcache_test.go

@@ -307,6 +307,55 @@ func TestUpdateSelfOnly(t *testing.T) {
 	}
 }
 
+func TestUpdatePeers(t *testing.T) {
+	modNode1 := (&tailcfg.Node{
+		ID:       99001,
+		StableID: "n99001FAKE",
+		Name:     "test1altered.example.com.",
+	}).View() // a modified version of testNode1
+	newNode3 := (&tailcfg.Node{
+		ID:       99003,
+		StableID: "n99003FAKE",
+		Name:     "test3.example.com.",
+	}).View() // a new node hitherto unseen
+
+	update := []tailcfg.NodeView{newNode3, modNode1}
+	remove := []tailcfg.StableNodeID{testNode2.StableID()}
+
+	// Modify a shallow copy of the map so we can perform an update and verify
+	// that it round-trips through a Load after calling UpdatePeers.
+	updated := *testMap
+	updated.Peers = []tailcfg.NodeView{modNode1, newNode3} // N.B. sorted
+
+	s := make(testStore)
+	c := netmapcache.NewCache(s)
+
+	// Prior to storing anything, the cache should reject an update because it
+	// has no base netmap to apply changes to.
+	if err := c.UpdatePeers(t.Context(), update, remove); err == nil {
+		t.Error("UpdatePeers on empty cache unexpectedly succeeded")
+	} else {
+		t.Logf("UpdatePeers on empty cache: %v (OK)", err)
+	}
+
+	// Initialize the cache with the test map so we get a baseline.
+	if err := c.Store(t.Context(), testMap); err != nil {
+		t.Fatalf("Store initial netmap: %v", err)
+	}
+
+	if err := c.UpdatePeers(t.Context(), update, remove); err != nil {
+		t.Fatalf("UpdatePeers failed; %v", err)
+	}
+
+	got, err := c.Load(t.Context())
+	if err != nil {
+		t.Fatalf("Load netmap failed: %v", err)
+	}
+	if diff := diffNetMaps(got, &updated); diff != "" {
+		t.Fatalf("Updated map differs (-got, +want):\n%s", diff)
+	}
+}
+
 // skippedMapFields are the names of fields that should not be considered by
 // network map caching, and thus skipped when comparing test results.
 var skippedMapFields = []string{