ipn/ipnlocal: add netmap mutations to the ipn bus (#19120)

ipn/local: add netmap mutations to the ipn bus

updates tailscale/tailscale#1909

This adds a new new NotifyWatchOpt that allows watchers to
receive PeerChange events (derived from node mutations)
on the IPN bus in lieu of a complete netmap.  We'll continue
to send the full netmap for any map response that includes it,
but for  mutations, sending PeerChange events gives the client
the option to manage it's own models more selectively and cuts
way down on json serialization overhead.

On chatty tailnets, this will vastly reduce the amount of
chatter on the bus.

This change should be backwards compatible, it is
purely additive.  Clients that subscribe to NotifyNetmap will
get the full netmap for every delta.  New clients can
omit that and instead opt into NotifyPeerChanges.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

ipn/ipnlocal/bus_test.go

@@ -12,6 +12,7 @@ import (
 
 	"tailscale.com/drive"
 	"tailscale.com/ipn"
+	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
 	"tailscale.com/tstime"
 	"tailscale.com/types/logger"
@@ -29,6 +30,7 @@ func TestIsNotableNotify(t *testing.T) {
 		{"empty", &ipn.Notify{}, false},
 		{"version", &ipn.Notify{Version: "foo"}, false},
 		{"netmap", &ipn.Notify{NetMap: new(netmap.NetworkMap)}, false},
+		{"peerchanges", &ipn.Notify{PeerChanges: []*tailcfg.PeerChange{{}}}, false},
 		{"engine", &ipn.Notify{Engine: new(ipn.EngineStatus)}, false},
 	}
 
@@ -39,7 +41,7 @@ func TestIsNotableNotify(t *testing.T) {
 	for sf := range rt.Fields() {
 		n := &ipn.Notify{}
 		switch sf.Name {
-		case "_", "NetMap", "Engine", "Version":
+		case "_", "NetMap", "PeerChanges", "Engine", "Version":
 			// Already covered above or not applicable.
 			continue
 		case "DriveShares":
@@ -217,3 +219,103 @@ func TestRateLimitingBusSender(t *testing.T) {
 		st.s.Run(ctx, incoming)
 	})
 }
+
+func TestMergePeerChanges(t *testing.T) {
+	online := true
+	offline := false
+
+	t.Run("no_overlap_appends", func(t *testing.T) {
+		old := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 1},
+		}
+		new := []*tailcfg.PeerChange{
+			{NodeID: 2, DERPRegion: 2},
+		}
+		got := mergePeerChanges(old, new)
+		if len(got) != 2 {
+			t.Fatalf("len = %d; want 2", len(got))
+		}
+		if got[0].NodeID != 1 || got[1].NodeID != 2 {
+			t.Errorf("got NodeIDs %d, %d; want 1, 2", got[0].NodeID, got[1].NodeID)
+		}
+	})
+
+	t.Run("overlap_merges", func(t *testing.T) {
+		old := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 1, Online: &online},
+			{NodeID: 2, DERPRegion: 10},
+		}
+		new := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 5, Online: &offline},
+		}
+		got := mergePeerChanges(old, new)
+		if len(got) != 2 {
+			t.Fatalf("len = %d; want 2 (merged, not appended)", len(got))
+		}
+		if got[0].DERPRegion != 5 {
+			t.Errorf("DERPRegion = %d; want 5 (from new)", got[0].DERPRegion)
+		}
+		if *got[0].Online != false {
+			t.Errorf("Online = %v; want false (from new)", *got[0].Online)
+		}
+		// Node 2 should be untouched.
+		if got[1].NodeID != 2 || got[1].DERPRegion != 10 {
+			t.Errorf("node 2 was modified unexpectedly")
+		}
+	})
+
+	t.Run("partial_overlap_merges_and_appends", func(t *testing.T) {
+		old := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 1},
+		}
+		new := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 2},
+			{NodeID: 3, DERPRegion: 30},
+		}
+		got := mergePeerChanges(old, new)
+		if len(got) != 2 {
+			t.Fatalf("len = %d; want 2", len(got))
+		}
+		if got[0].NodeID != 1 || got[0].DERPRegion != 2 {
+			t.Errorf("node 1: DERPRegion = %d; want 2", got[0].DERPRegion)
+		}
+		if got[1].NodeID != 3 || got[1].DERPRegion != 30 {
+			t.Errorf("node 3: DERPRegion = %d; want 30", got[1].DERPRegion)
+		}
+	})
+
+	t.Run("preserves_old_fields_on_merge", func(t *testing.T) {
+		old := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 1, Online: &online, Cap: 10},
+		}
+		new := []*tailcfg.PeerChange{
+			{NodeID: 1, Online: &offline},
+		}
+		got := mergePeerChanges(old, new)
+		if len(got) != 1 {
+			t.Fatalf("len = %d; want 1", len(got))
+		}
+		if got[0].DERPRegion != 1 {
+			t.Errorf("DERPRegion = %d; want 1 (preserved from old)", got[0].DERPRegion)
+		}
+		if got[0].Cap != 10 {
+			t.Errorf("Cap = %d; want 10 (preserved from old)", got[0].Cap)
+		}
+		if *got[0].Online != false {
+			t.Errorf("Online = %v; want false (from new)", *got[0].Online)
+		}
+	})
+
+	t.Run("nil_old", func(t *testing.T) {
+		new := []*tailcfg.PeerChange{
+			{NodeID: 1, DERPRegion: 1},
+		}
+		got := mergePeerChanges(nil, new)
+		if len(got) != 1 {
+			t.Fatalf("len = %d; want 1", len(got))
+		}
+		if got[0].NodeID != 1 {
+			t.Errorf("NodeID = %d; want 1", got[0].NodeID)
+		}
+	})
+}