net: make LocalAddr on multicast return group address

Fixes #3067.

R=golang-dev, rsc, rsc
CC=golang-dev
https://golang.org/cl/5656098

src/pkg/net/multicast_test.go

@@ -81,6 +81,9 @@ func TestListenMulticastUDP(t *testing.T) {
 		if !found {
 			t.Fatalf("%q not found in RIB", tt.gaddr.String())
 		}
+		if c.LocalAddr().String() != tt.gaddr.String() {
+			t.Fatalf("LocalAddr returns %q, expected %q", c.LocalAddr().String(), tt.gaddr.String())
+		}
 	}
 }
 
@@ -114,6 +117,9 @@ func TestSimpleListenMulticastUDP(t *testing.T) {
 		if err != nil {
 			t.Fatalf("ListenMulticastUDP failed: %v", err)
 		}
+		if c.LocalAddr().String() != tt.gaddr.String() {
+			t.Fatalf("LocalAddr returns %q, expected %q", c.LocalAddr().String(), tt.gaddr.String())
+		}
 		c.Close()
 	}
 }

src/pkg/net/sock.go

@@ -33,13 +33,14 @@ func socket(net string, f, t, p int, la, ra syscall.Sockaddr, toAddr func(syscal
 		return nil, err
 	}
 
+	var bla syscall.Sockaddr
 	if la != nil {
-		la, err = listenerSockaddr(s, f, la, toAddr)
+		bla, err = listenerSockaddr(s, f, la, toAddr)
 		if err != nil {
 			closesocket(s)
 			return nil, err
 		}
-		err = syscall.Bind(s, la)
+		err = syscall.Bind(s, bla)
 		if err != nil {
 			closesocket(s)
 			return nil, err
@@ -61,7 +62,12 @@ func socket(net string, f, t, p int, la, ra syscall.Sockaddr, toAddr func(syscal
 	}
 
 	sa, _ := syscall.Getsockname(s)
-	laddr := toAddr(sa)
+	var laddr Addr
+	if la != nil && bla != la {
+		laddr = toAddr(la)
+	} else {
+		laddr = toAddr(sa)
+	}
 	sa, _ = syscall.Getpeername(s)
 	raddr := toAddr(sa)
 

src/pkg/net/udpsock_posix.go

@@ -262,7 +262,7 @@ func ListenMulticastUDP(net string, ifi *Interface, gaddr *UDPAddr) (*UDPConn, e
 		return nil, UnknownNetworkError(net)
 	}
 	if gaddr == nil || gaddr.IP == nil {
-		return nil, &OpError{"listenmulticastudp", "udp", nil, errMissingAddress}
+		return nil, &OpError{"listenmulticast", net, nil, errMissingAddress}
 	}
 	fd, err := internetSocket(net, gaddr.toAddr(), nil, syscall.SOCK_DGRAM, 0, "listen", sockaddrToUDP)
 	if err != nil {