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/magicsock_test.go

@@ -4235,3 +4235,73 @@ func Test_lazyEndpoint_FromPeer(t *testing.T) {
 		})
 	}
 }
+
+func TestRotateDiscoKey(t *testing.T) {
+	c := newConn(t.Logf)
+
+	oldPrivate, oldPublic := c.discoAtomic.Pair()
+	oldShort := c.discoAtomic.Short()
+
+	if oldPublic != oldPrivate.Public() {
+		t.Fatalf("old public key doesn't match old private key")
+	}
+	if oldShort != oldPublic.ShortString() {
+		t.Fatalf("old short string doesn't match old public key")
+	}
+
+	testDiscoKey := key.NewDisco().Public()
+	c.mu.Lock()
+	c.discoInfo[testDiscoKey] = &discoInfo{
+		discoKey:   testDiscoKey,
+		discoShort: testDiscoKey.ShortString(),
+	}
+	if len(c.discoInfo) != 1 {
+		t.Fatalf("expected 1 discoInfo entry, got %d", len(c.discoInfo))
+	}
+	c.mu.Unlock()
+
+	c.RotateDiscoKey()
+
+	newPrivate, newPublic := c.discoAtomic.Pair()
+	newShort := c.discoAtomic.Short()
+
+	if newPublic.Compare(oldPublic) == 0 {
+		t.Fatalf("disco key didn't change after rotation")
+	}
+	if newShort == oldShort {
+		t.Fatalf("short string didn't change after rotation")
+	}
+
+	if newPublic != newPrivate.Public() {
+		t.Fatalf("new public key doesn't match new private key")
+	}
+	if newShort != newPublic.ShortString() {
+		t.Fatalf("new short string doesn't match new public key")
+	}
+
+	c.mu.Lock()
+	if len(c.discoInfo) != 0 {
+		t.Fatalf("expected discoInfo to be cleared, got %d entries", len(c.discoInfo))
+	}
+	c.mu.Unlock()
+}
+
+func TestRotateDiscoKeyMultipleTimes(t *testing.T) {
+	c := newConn(t.Logf)
+
+	keys := make([]key.DiscoPublic, 0, 5)
+	keys = append(keys, c.discoAtomic.Public())
+
+	for i := 0; i < 4; i++ {
+		c.RotateDiscoKey()
+		newKey := c.discoAtomic.Public()
+
+		for j, oldKey := range keys {
+			if newKey.Compare(oldKey) == 0 {
+				t.Fatalf("rotation %d produced same key as rotation %d", i+1, j)
+			}
+		}
+
+		keys = append(keys, newKey)
+	}
+}