net: return nil UDPAddr from ReadFromUDP

In cases where the socket operation has no underlying address,
golang.org/cl/291509 unintentionally changed ReadFromUDP from return a
nil *UDPAddr to a non-nil (but zero value) *UDPAddr.

This may break callers that assume "no address" is always addr == nil,
so change it back to remain nil.

Fixes #46238

Change-Id: I8531e8fa16b853ed7560088eabda0b9e3e53f5be
Reviewed-on: https://go-review.googlesource.com/c/go/+/320909
Trust: Michael Pratt <mpratt@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/net/udpsock_posix.go

@@ -50,6 +50,9 @@ func (c *UDPConn) readFrom(b []byte, addr *UDPAddr) (int, *UDPAddr, error) {
 		*addr = UDPAddr{IP: sa.Addr[0:], Port: sa.Port}
 	case *syscall.SockaddrInet6:
 		*addr = UDPAddr{IP: sa.Addr[0:], Port: sa.Port, Zone: zoneCache.name(int(sa.ZoneId))}
+	default:
+		// No sockaddr, so don't return UDPAddr.
+		addr = nil
 	}
 	return n, addr, err
 }

src/net/udpsock_test.go

@@ -8,7 +8,9 @@
 package net
 
 import (
+	"errors"
 	"internal/testenv"
+	"os"
 	"reflect"
 	"runtime"
 	"testing"
@@ -446,6 +448,33 @@ func TestUDPReadSizeError(t *testing.T) {
 	}
 }
 
+// TestUDPReadTimeout verifies that ReadFromUDP with timeout returns an error
+// without data or an address.
+func TestUDPReadTimeout(t *testing.T) {
+	la, err := ResolveUDPAddr("udp4", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	c, err := ListenUDP("udp4", la)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c.Close()
+
+	c.SetDeadline(time.Now())
+	b := make([]byte, 1)
+	n, addr, err := c.ReadFromUDP(b)
+	if !errors.Is(err, os.ErrDeadlineExceeded) {
+		t.Errorf("ReadFromUDP got err %v want os.ErrDeadlineExceeded", err)
+	}
+	if n != 0 {
+		t.Errorf("ReadFromUDP got n %d want 0", n)
+	}
+	if addr != nil {
+		t.Errorf("ReadFromUDP got addr %+#v want nil", addr)
+	}
+}
+
 func BenchmarkWriteToReadFromUDP(b *testing.B) {
 	conn, err := ListenUDP("udp4", &UDPAddr{IP: IPv4(127, 0, 0, 1)})
 	if err != nil {