net: add multicast UDP support for plan9

The new implementation adds listening support on a multicast
address with plan9 network interfaces.

Fixes #17218

Change-Id: I2c75515e72e120acb71610cd077fddfbf9cf4e29
Reviewed-on: https://go-review.googlesource.com/29964
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/udpsock.go

@@ -15,7 +15,7 @@ import (
 // BUG(mikio): On Windows, the File method of UDPConn is not
 // implemented.
 
-// BUG(mikio): On NaCl and Plan 9, the ListenMulticastUDP function is
+// BUG(mikio): On NaCl the ListenMulticastUDP function is
 // not implemented.
 
 // UDPAddr represents the address of a UDP end point.

src/net/udpsock_plan9.go

@@ -109,5 +109,41 @@ func listenUDP(ctx context.Context, network string, laddr *UDPAddr) (*UDPConn, e
 }
 
 func listenMulticastUDP(ctx context.Context, network string, ifi *Interface, gaddr *UDPAddr) (*UDPConn, error) {
-	return nil, syscall.EPLAN9
+	l, err := listenPlan9(ctx, network, gaddr)
+	if err != nil {
+		return nil, err
+	}
+	_, err = l.ctl.WriteString("headers")
+	if err != nil {
+		return nil, err
+	}
+	var addrs []Addr
+	if ifi != nil {
+		addrs, err = ifi.Addrs()
+		if err != nil {
+			return nil, err
+		}
+	} else {
+		addrs, err = InterfaceAddrs()
+		if err != nil {
+			return nil, err
+		}
+	}
+	for _, addr := range addrs {
+		if ipnet, ok := addr.(*IPNet); ok {
+			_, err = l.ctl.WriteString("addmulti " + ipnet.IP.String() + " " + gaddr.IP.String())
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
+	l.data, err = os.OpenFile(l.dir+"/data", os.O_RDWR, 0)
+	if err != nil {
+		return nil, err
+	}
+	fd, err := l.netFD()
+	if err != nil {
+		return nil, err
+	}
+	return newUDPConn(fd), nil
 }

src/net/udpsock_plan9_test.go

@@ -0,0 +1,69 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package net
+
+import (
+	"internal/testenv"
+	"runtime"
+	"testing"
+)
+
+func TestListenMulticastUDP(t *testing.T) {
+	testenv.MustHaveExternalNetwork(t)
+
+	ifcs, err := Interfaces()
+	if err != nil {
+		t.Skip(err.Error())
+	}
+	if len(ifcs) == 0 {
+		t.Skip("no network interfaces found")
+	}
+
+	var mifc *Interface
+	for _, ifc := range ifcs {
+		if ifc.Flags&FlagUp|FlagMulticast != FlagUp|FlagMulticast {
+			continue
+		}
+		mifc = &ifc
+		break
+	}
+
+	if mifc == nil {
+		t.Skipf("no multicast interfaces found")
+	}
+
+	c1, err := ListenMulticastUDP("udp4", mifc, &UDPAddr{IP: ParseIP("224.0.0.254")})
+	if err != nil {
+		t.Fatalf("multicast not working on %s", runtime.GOOS)
+	}
+	c1addr := c1.LocalAddr().(*UDPAddr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c1.Close()
+
+	c2, err := ListenUDP("udp4", &UDPAddr{IP: IPv4zero, Port: 0})
+	c2addr := c2.LocalAddr().(*UDPAddr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer c2.Close()
+
+	n, err := c2.WriteToUDP([]byte("data"), c1addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 4 {
+		t.Fatalf("got %d; want 4", n)
+	}
+
+	n, err = c1.WriteToUDP([]byte("data"), c2addr)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 4 {
+		t.Fatalf("got %d; want 4", n)
+	}
+}