wgengine/magicsock: fix data race from undocumented wireguard-go requirement

Endpoints need to be Stringers apparently.

Fixes tailscale/corp#422

wgengine/magicsock/magicsock.go

@@ -2750,6 +2750,13 @@ func (de *discoEndpoint) initFakeUDPAddr() {
 	de.fakeWGAddrStd = de.fakeWGAddr.UDPAddr()
 }
 
+// String exists purely so wireguard-go internals can log.Printf("%v")
+// its internal conn.Endpoints and we don't end up with data races
+// from fmt (via log) reading mutex fields and such.
+func (de *discoEndpoint) String() string {
+	return fmt.Sprintf("magicsock.discoEndpoint{%v, %v}", de.publicKey.ShortString(), de.discoShort)
+}
+
 func (de *discoEndpoint) Addrs() []wgcfg.Endpoint {
 	// This has to be the same string that was passed to
 	// CreateEndpoint, otherwise Reconfig will end up recreating

wgengine/magicsock/magicsock_test.go

@@ -10,6 +10,7 @@ import (
 	"crypto/tls"
 	"encoding/binary"
 	"fmt"
+	"io/ioutil"
 	"net"
 	"net/http"
 	"net/http/httptest"
@@ -880,3 +881,22 @@ func TestDiscoMessage(t *testing.T) {
 		t.Error("failed to open it")
 	}
 }
+
+// tests that having a discoEndpoint.String prevents wireguard-go's
+// log.Printf("%v") of its conn.Endpoint values from using reflect to
+// walk into read mutex while they're being used and then causing data
+// races.
+func TestDiscoStringLogRace(t *testing.T) {
+	de := new(discoEndpoint)
+	var wg sync.WaitGroup
+	wg.Add(2)
+	go func() {
+		defer wg.Done()
+		fmt.Fprintf(ioutil.Discard, "%v", de)
+	}()
+	go func() {
+		defer wg.Done()
+		de.mu.Lock()
+	}()
+	wg.Wait()
+}