derp: add Client.LocalAddr method

So magicsock can later ask a DERP connection whether its source IP
would've changed if it reconnected.

Updates #3619

Change-Id: Ibc8810340c511d6786b60c78c1a61c09f5800e40
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derp_client.go

@@ -12,10 +12,12 @@
 	"fmt"
 	"io"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"go4.org/mem"
 	"golang.org/x/time/rate"
+	"inet.af/netaddr"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
 )
@@ -37,8 +39,8 @@ type Client struct {
 	rate *rate.Limiter // if non-nil, rate limiter to use
 
 	// Owned by Recv:
-	peeked  int   // bytes to discard on next Recv
-	readErr error // sticky read error
+	peeked  int          // bytes to discard on next Recv
+	readErr atomic.Value // of error; sticky (set by Recv)
 }
 
 // ClientOpt is an option passed to NewClient.
@@ -441,13 +443,14 @@ func (c *Client) Recv() (m ReceivedMessage, err error) {
 }
 
 func (c *Client) recvTimeout(timeout time.Duration) (m ReceivedMessage, err error) {
-	if c.readErr != nil {
-		return nil, c.readErr
+	readErr, _ := c.readErr.Load().(error)
+	if readErr != nil {
+		return nil, readErr
 	}
 	defer func() {
 		if err != nil {
 			err = fmt.Errorf("derp.Recv: %w", err)
-			c.readErr = err
+			c.readErr.Store(err)
 		}
 	}()
 
@@ -587,3 +590,22 @@ func (c *Client) setSendRateLimiter(sm ServerInfoMessage) {
 			sm.TokenBucketBytesBurst)
 	}
 }
+
+// LocalAddr returns the TCP connection's local address.
+//
+// If the client is broken in some previously detectable way, it
+// returns an error.
+func (c *Client) LocalAddr() (netaddr.IPPort, error) {
+	readErr, _ := c.readErr.Load().(error)
+	if readErr != nil {
+		return netaddr.IPPort{}, readErr
+	}
+	if c.nc == nil {
+		return netaddr.IPPort{}, errors.New("nil conn")
+	}
+	a := c.nc.LocalAddr()
+	if a == nil {
+		return netaddr.IPPort{}, errors.New("nil addr")
+	}
+	return netaddr.ParseIPPort(a.String())
+}

derp/derp_server.go

@@ -23,6 +23,7 @@
 	"math"
 	"math/big"
 	"math/rand"
+	"net"
 	"net/http"
 	"os"
 	"os/exec"
@@ -283,9 +284,8 @@ type PacketForwarder interface {
 // It is a defined type so that non-net connections can be used.
 type Conn interface {
 	io.WriteCloser
-
+	LocalAddr() net.Addr
 	// The *Deadline methods follow the semantics of net.Conn.
-
 	SetDeadline(time.Time) error
 	SetReadDeadline(time.Time) error
 	SetWriteDeadline(time.Time) error