types/key,wgengine/magicsock,control/controlclient,ipn: add debug disco key rotation

Adds the ability to rotate discovery keys on running clients, needed for
testing upcoming disco key distribution changes.

Introduces key.DiscoKey, an atomic container for a disco private key,
public key, and the public key's ShortString, replacing the prior
separate atomic fields.

magicsock.Conn has a new RotateDiscoKey method, and access to this is
provided via localapi and a CLI debug command.

Note that this implementation is primarily for testing as it stands, and
regular use should likely introduce an additional mechanism that allows
the old key to be used for some time, to provide a seamless key rotation
rather than one that invalidates all sessions.

Updates tailscale/corp#34037

Signed-off-by: James Tucker <james@tailscale.com>

wgengine/magicsock/endpoint_test.go

@@ -146,15 +146,22 @@ func TestProbeUDPLifetimeConfig_Valid(t *testing.T) {
 }
 
 func Test_endpoint_maybeProbeUDPLifetimeLocked(t *testing.T) {
+	var lowerPriv, higherPriv key.DiscoPrivate
 	var lower, higher key.DiscoPublic
-	a := key.NewDisco().Public()
-	b := key.NewDisco().Public()
+	privA := key.NewDisco()
+	privB := key.NewDisco()
+	a := privA.Public()
+	b := privB.Public()
 	if a.String() < b.String() {
 		lower = a
 		higher = b
+		lowerPriv = privA
+		higherPriv = privB
 	} else {
 		lower = b
 		higher = a
+		lowerPriv = privB
+		higherPriv = privA
 	}
 	addr := addrQuality{epAddr: epAddr{ap: netip.MustParseAddrPort("1.1.1.1:1")}}
 	newProbeUDPLifetime := func() *probeUDPLifetime {
@@ -281,10 +288,18 @@ func Test_endpoint_maybeProbeUDPLifetimeLocked(t *testing.T) {
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
+			c := &Conn{}
+			if tt.localDisco.IsZero() {
+				c.discoAtomic.Set(key.NewDisco())
+			} else if tt.localDisco.Compare(lower) == 0 {
+				c.discoAtomic.Set(lowerPriv)
+			} else if tt.localDisco.Compare(higher) == 0 {
+				c.discoAtomic.Set(higherPriv)
+			} else {
+				t.Fatalf("unexpected localDisco value")
+			}
 			de := &endpoint{
-				c: &Conn{
-					discoPublic: tt.localDisco,
-				},
+				c:        c,
 				bestAddr: tt.bestAddr,
 			}
 			if tt.remoteDisco != nil {