derp/derphttp: don't block in LocalAddr method

The derphttp.Client mutex is held during connects (for up to 10
seconds) so this LocalAddr method (blocking on said mutex) could also
block for up to 10 seconds, causing a pileup upstream in
magicsock/wgengine and ultimately a watchdog timeout resulting in a
crash.

Updates #11519

Change-Id: Idd1d94ee00966be1b901f6899d8b9492f18add0f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/derphttp_client.go

@@ -86,6 +86,7 @@ type Client struct {
 	addrFamSelAtomic syncs.AtomicValue[AddressFamilySelector]
 
 	mu           sync.Mutex
+	atomicState  syncs.AtomicValue[ConnectedState] // hold mu to write
 	started      bool                              // true upon first connect, never transitions to false
 	preferred    bool
 	canAckPings  bool
@@ -99,6 +100,14 @@ type Client struct {
 	clock        tstime.Clock
 }
 
+// ConnectedState describes the state of a derphttp Client.
+type ConnectedState struct {
+	Connected  bool
+	Connecting bool
+	Closed     bool
+	LocalAddr  netip.AddrPort // if Connected
+}
+
 func (c *Client) String() string {
 	return fmt.Sprintf("<derphttp_client.Client %s url=%s>", c.ServerPublicKey().ShortString(), c.url)
 }
@@ -307,6 +316,12 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 	if c.client != nil {
 		return c.client, c.connGen, nil
 	}
+	c.atomicState.Store(ConnectedState{Connecting: true})
+	defer func() {
+		if err != nil {
+			c.atomicState.Store(ConnectedState{Connecting: false})
+		}
+	}()
 
 	// timeout is the fallback maximum time (if ctx doesn't limit
 	// it further) to do all of: DNS + TCP + TLS + HTTP Upgrade +
@@ -524,6 +539,12 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 	c.netConn = tcpConn
 	c.tlsState = tlsState
 	c.connGen++
+
+	localAddr, _ := c.client.LocalAddr()
+	c.atomicState.Store(ConnectedState{
+		Connected: true,
+		LocalAddr: localAddr,
+	})
 	return c.client, c.connGen, nil
 }
 
@@ -906,16 +927,15 @@ func (c *Client) SendPing(data [8]byte) error {
 // LocalAddr reports c's local TCP address, without any implicit
 // connect or reconnect.
 func (c *Client) LocalAddr() (netip.AddrPort, error) {
-	c.mu.Lock()
-	closed, client := c.closed, c.client
-	c.mu.Unlock()
-	if closed {
+	st := c.atomicState.Load()
+	if st.Closed {
 		return netip.AddrPort{}, ErrClientClosed
 	}
-	if client == nil {
+	la := st.LocalAddr
+	if !st.Connected && !la.IsValid() {
 		return netip.AddrPort{}, errors.New("client not connected")
 	}
-	return client.LocalAddr()
+	return la, nil
 }
 
 func (c *Client) ForwardPacket(from, to key.NodePublic, b []byte) error {
@@ -1049,6 +1069,7 @@ func (c *Client) Close() error {
 	if c.netConn != nil {
 		c.netConn.Close()
 	}
+	c.atomicState.Store(ConnectedState{Closed: true})
 	return nil
 }
 

derp/derphttp/derphttp_test.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"context"
 	"crypto/tls"
+	"fmt"
 	"net"
 	"net/http"
 	"net/netip"
@@ -447,3 +448,16 @@ func TestRunWatchConnectionLoopServeConnect(t *testing.T) {
 	}
 	watcher.RunWatchConnectionLoop(ctx, key.NodePublic{}, t.Logf, noopAdd, noopRemove)
 }
+
+// verify that the LocalAddr method doesn't acquire the mutex.
+// See https://github.com/tailscale/tailscale/issues/11519
+func TestLocalAddrNoMutex(t *testing.T) {
+	var c Client
+	c.mu.Lock()
+	defer c.mu.Unlock() // not needed in test but for symmetry
+
+	_, err := c.LocalAddr()
+	if got, want := fmt.Sprint(err), "client not connected"; got != want {
+		t.Errorf("got error %q; want %q", got, want)
+	}
+}