derp: unify server's clientSet interface into concrete type

73280595a8 for #2751 added a "clientSet" interface to
distinguish the two cases of a client being singly connected (the
common case) vs tolerating multiple connections from the client at
once. At the time (three years ago) it was kinda an experiment
and we didn't know whether it'd stop the reconnect floods we saw
from certain clients. It did.

So this promotes it to a be first-class thing a bit, removing the
interface. The old tests from 73280595a were invaluable in ensuring
correctness while writing this change (they failed a bunch).

But the real motivation for this change is that it'll permit a future
optimization to add flow tracking for stats & performance where we
don't contend on Server.mu for each packet sent via DERP. Instead,
each client can track its active flows and hold on to a *clientSet and
ask the clientSet per packet what the active client is via one atomic
load rather than a mutex. And if the atomic load returns nil, we'll
know we need to ask the server to see if they died and reconnected and
got a new clientSet. But that's all coming later.

Updates #3560

Change-Id: I9ccda3e5381226563b5ec171ceeacf5c210e1faf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_test.go

@@ -731,7 +731,7 @@ func pubAll(b byte) (ret key.NodePublic) {
 
 func TestForwarderRegistration(t *testing.T) {
 	s := &Server{
-		clients:     make(map[key.NodePublic]clientSet),
+		clients:     make(map[key.NodePublic]*clientSet),
 		clientsMesh: map[key.NodePublic]PacketForwarder{},
 	}
 	want := func(want map[key.NodePublic]PacketForwarder) {
@@ -746,6 +746,11 @@ func TestForwarderRegistration(t *testing.T) {
 			t.Errorf("counter = %v; want %v", got, want)
 		}
 	}
+	singleClient := func(c *sclient) *clientSet {
+		cs := &clientSet{}
+		cs.activeClient.Store(c)
+		return cs
+	}
 
 	u1 := pubAll(1)
 	u2 := pubAll(2)
@@ -808,7 +813,7 @@ func TestForwarderRegistration(t *testing.T) {
 		key:  u1,
 		logf: logger.Discard,
 	}
-	s.clients[u1] = singleClient{u1c}
+	s.clients[u1] = singleClient(u1c)
 	s.RemovePacketForwarder(u1, testFwd(100))
 	want(map[key.NodePublic]PacketForwarder{
 		u1: nil,
@@ -828,7 +833,7 @@ func TestForwarderRegistration(t *testing.T) {
 	// Now pretend u1 was already connected locally (so clientsMesh[u1] is nil), and then we heard
 	// that they're also connected to a peer of ours. That shouldn't transition the forwarder
 	// from nil to the new one, not a multiForwarder.
-	s.clients[u1] = singleClient{u1c}
+	s.clients[u1] = singleClient(u1c)
 	s.clientsMesh[u1] = nil
 	want(map[key.NodePublic]PacketForwarder{
 		u1: nil,
@@ -860,7 +865,7 @@ func TestMultiForwarder(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 
 	s := &Server{
-		clients:     make(map[key.NodePublic]clientSet),
+		clients:     make(map[key.NodePublic]*clientSet),
 		clientsMesh: map[key.NodePublic]PacketForwarder{},
 	}
 	u := pubAll(1)
@@ -1078,43 +1083,48 @@ func TestServerDupClients(t *testing.T) {
 	}
 	wantSingleClient := func(t *testing.T, want *sclient) {
 		t.Helper()
-		switch s := s.clients[want.key].(type) {
-		case singleClient:
-			if s.c != want {
-				t.Error("wrong single client")
-				return
-			}
-			if want.isDup.Load() {
+		got, ok := s.clients[want.key]
+		if !ok {
+			t.Error("no clients for key")
+			return
+		}
+		if got.dup != nil {
+			t.Errorf("unexpected dup set for single client")
+		}
+		cur := got.activeClient.Load()
+		if cur != want {
+			t.Errorf("active client = %q; want %q", clientName[cur], clientName[want])
+		}
+		if cur != nil {
+			if cur.isDup.Load() {
 				t.Errorf("unexpected isDup on singleClient")
 			}
-			if want.isDisabled.Load() {
+			if cur.isDisabled.Load() {
 				t.Errorf("unexpected isDisabled on singleClient")
 			}
-		case nil:
-			t.Error("no clients for key")
-		case *dupClientSet:
-			t.Error("unexpected multiple clients for key")
 		}
 	}
 	wantNoClient := func(t *testing.T) {
 		t.Helper()
-		switch s := s.clients[clientPub].(type) {
-		case nil:
-			// Good.
+		_, ok := s.clients[clientPub]
+		if !ok {
+			// Good
 			return
-		default:
-			t.Errorf("got %T; want empty", s)
 		}
+		t.Errorf("got client; want empty")
 	}
 	wantDupSet := func(t *testing.T) *dupClientSet {
 		t.Helper()
-		switch s := s.clients[clientPub].(type) {
-		case *dupClientSet:
-			return s
-		default:
-			t.Fatalf("wanted dup set; got %T", s)
+		cs, ok := s.clients[clientPub]
+		if !ok {
+			t.Fatal("no set for key; want dup set")
 			return nil
 		}
+		if cs.dup != nil {
+			return cs.dup
+		}
+		t.Fatalf("no dup set for key; want dup set")
+		return nil
 	}
 	wantActive := func(t *testing.T, want *sclient) {
 		t.Helper()
@@ -1123,7 +1133,7 @@ func TestServerDupClients(t *testing.T) {
 			t.Error("no set for key")
 			return
 		}
-		got := set.ActiveClient()
+		got := set.activeClient.Load()
 		if got != want {
 			t.Errorf("active client = %q; want %q", clientName[got], clientName[want])
 		}