wgengine/magicsock: rename opts.PacketListener to TestOnlyPacketListener.

The docstring said it was meant for use in tests, but it's specifically a
special codepath that is _only_ used in tests, so make the claim stronger.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -279,7 +279,7 @@ type Conn struct {
 	epFunc                 func([]tailcfg.Endpoint)
 	derpActiveFunc         func()
 	idleFunc               func() time.Duration // nil means unknown
-	packetListener   nettype.PacketListener
+	testOnlyPacketListener nettype.PacketListener
 	noteRecvActivity       func(tailcfg.DiscoKey) // or nil, see Options.NoteRecvActivity
 	simulatedNetwork       bool
 
@@ -524,9 +524,9 @@ type Options struct {
 	// it's been since a TUN packet was sent or received.
 	IdleFunc func() time.Duration
 
-	// PacketListener optionally specifies how to create PacketConns.
-	// It's meant for testing.
-	PacketListener nettype.PacketListener
+	// TestOnlyPacketListener optionally specifies how to create PacketConns.
+	// Only used by tests.
+	TestOnlyPacketListener nettype.PacketListener
 
 	// NoteRecvActivity, if provided, is a func for magicsock to
 	// call whenever it receives a packet from a a
@@ -603,7 +603,7 @@ func NewConn(opts Options) (*Conn, error) {
 	c.epFunc = opts.endpointsFunc()
 	c.derpActiveFunc = opts.derpActiveFunc()
 	c.idleFunc = opts.IdleFunc
-	c.packetListener = opts.PacketListener
+	c.testOnlyPacketListener = opts.TestOnlyPacketListener
 	c.noteRecvActivity = opts.NoteRecvActivity
 	c.simulatedNetwork = opts.SimulatedNetwork
 	c.portMapper = portmapper.NewClient(logger.WithPrefix(c.logf, "portmapper: "), c.onPortMapChanged)
@@ -2688,8 +2688,8 @@ func (c *Conn) listenPacket(network string, host netaddr.IP, port uint16) (net.P
 		s = host.String()
 	}
 	addr := net.JoinHostPort(s, fmt.Sprint(port))
-	if c.packetListener != nil {
-		return c.packetListener.ListenPacket(ctx, network, addr)
+	if c.testOnlyPacketListener != nil {
+		return c.testOnlyPacketListener.ListenPacket(ctx, network, addr)
 	}
 	return netns.Listener().ListenPacket(ctx, network, addr)
 }

wgengine/magicsock/magicsock_test.go

@@ -146,7 +146,7 @@ func newMagicStack(t testing.TB, logf logger.Logf, l nettype.PacketListener, der
 	epCh := make(chan []tailcfg.Endpoint, 100) // arbitrary
 	conn, err := NewConn(Options{
 		Logf:                   logf,
-		PacketListener: l,
+		TestOnlyPacketListener: l,
 		EndpointsFunc: func(eps []tailcfg.Endpoint) {
 			epCh <- eps
 		},