p2p/discover: fix Windows-specific issue for larger-than-buffer packets

On Windows, UDPConn.ReadFrom returns an error for packets larger
than the receive buffer. The error is not marked temporary, causing
our loop to exit when the first oversized packet arrived. The fix
is to treat this particular error as temporary.

Fixes: #1579, #2087
Updates: #2082

p2p/discover/udp_test.go

@@ -23,10 +23,8 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	logpkg "log"
 	"math/rand"
 	"net"
-	"os"
 	"path/filepath"
 	"reflect"
 	"runtime"
@@ -34,12 +32,62 @@ import (
 	"testing"
 	"time"
 
+	"github.com/davecgh/go-spew/spew"
 	"github.com/ethereum/go-ethereum/crypto"
-	"github.com/ethereum/go-ethereum/logger"
 )
 
 func init() {
-	logger.AddLogSystem(logger.NewStdLogSystem(os.Stdout, logpkg.LstdFlags, logger.ErrorLevel))
+	spew.Config.DisableMethods = true
+}
+
+// This test checks that isPacketTooBig correctly identifies
+// errors that result from receiving a UDP packet larger
+// than the supplied receive buffer.
+func TestIsPacketTooBig(t *testing.T) {
+	listener, err := net.ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer listener.Close()
+	sender, err := net.Dial("udp", listener.LocalAddr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer sender.Close()
+
+	sendN := 1800
+	recvN := 300
+	for i := 0; i < 20; i++ {
+		go func() {
+			buf := make([]byte, sendN)
+			for i := range buf {
+				buf[i] = byte(i)
+			}
+			sender.Write(buf)
+		}()
+
+		buf := make([]byte, recvN)
+		listener.SetDeadline(time.Now().Add(1 * time.Second))
+		n, _, err := listener.ReadFrom(buf)
+		if err != nil {
+			if nerr, ok := err.(net.Error); ok && nerr.Timeout() {
+				continue
+			}
+			if !isPacketTooBig(err) {
+				t.Fatal("unexpected read error:", spew.Sdump(err))
+			}
+			continue
+		}
+		if n != recvN {
+			t.Fatalf("short read: %d, want %d", n, recvN)
+		}
+		for i := range buf {
+			if buf[i] != byte(i) {
+				t.Fatalf("error in pattern")
+				break
+			}
+		}
+	}
 }
 
 // shared test variables