net: make LocalAddr on multicast UDPConn return a listening address

The package go.net/ipv4 allows to exist a single UDP listener
that join multiple different group addresses. That means that
LocalAddr on multicast UDPConn returns a first joined group
address is not desirable.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6822108

src/pkg/net/multicast_posix_test.go

@@ -118,16 +118,29 @@ func TestSimpleMulticastListener(t *testing.T) {
 
 func checkMulticastListener(t *testing.T, err error, c *UDPConn, gaddr *UDPAddr) {
 	if !multicastRIBContains(t, gaddr.IP) {
-		t.Fatalf("%q not found in RIB", gaddr.String())
+		t.Errorf("%q not found in RIB", gaddr.String())
+		return
+	}
+	la := c.LocalAddr()
+	if la == nil {
+		t.Error("LocalAddr failed")
+		return
 	}
-	if c.LocalAddr().String() != gaddr.String() {
-		t.Fatalf("LocalAddr returns %q, expected %q", c.LocalAddr().String(), gaddr.String())
+	if a, ok := la.(*UDPAddr); !ok || a.Port == 0 {
+		t.Errorf("got %v; expected a proper address with non-zero port number", la)
+		return
 	}
 }
 
 func checkSimpleMulticastListener(t *testing.T, err error, c *UDPConn, gaddr *UDPAddr) {
-	if c.LocalAddr().String() != gaddr.String() {
-		t.Fatalf("LocalAddr returns %q, expected %q", c.LocalAddr().String(), gaddr.String())
+	la := c.LocalAddr()
+	if la == nil {
+		t.Error("LocalAddr failed")
+		return
+	}
+	if a, ok := la.(*UDPAddr); !ok || a.Port == 0 {
+		t.Errorf("got %v; expected a proper address with non-zero port number", la)
+		return
 	}
 }
 

src/pkg/net/sock_posix.go

@@ -33,13 +33,19 @@ func socket(net string, f, t, p int, ipv6only bool, ulsa, ursa syscall.Sockaddr,
 		return nil, err
 	}
 
-	var blsa syscall.Sockaddr
 	if ulsa != nil {
-		if blsa, err = listenerSockaddr(s, f, ulsa, toAddr); err != nil {
+		// We provide a socket that listens to a wildcard
+		// address with reusable UDP port when the given ulsa
+		// is an appropriate UDP multicast address prefix.
+		// This makes it possible for a single UDP listener
+		// to join multiple different group addresses, for
+		// multiple UDP listeners that listen on the same UDP
+		// port to join the same group address.
+		if ulsa, err = listenerSockaddr(s, f, ulsa, toAddr); err != nil {
 			closesocket(s)
 			return nil, err
 		}
-		if err = syscall.Bind(s, blsa); err != nil {
+		if err = syscall.Bind(s, ulsa); err != nil {
 			closesocket(s)
 			return nil, err
 		}
@@ -64,12 +70,7 @@ func socket(net string, f, t, p int, ipv6only bool, ulsa, ursa syscall.Sockaddr,
 	}
 
 	lsa, _ := syscall.Getsockname(s)
-	var laddr Addr
-	if ulsa != nil && blsa != ulsa {
-		laddr = toAddr(ulsa)
-	} else {
-		laddr = toAddr(lsa)
-	}
+	laddr := toAddr(lsa)
 	rsa, _ := syscall.Getpeername(s)
 	raddr := toAddr(rsa)
 	fd.setAddr(laddr, raddr)