net: platform-dependent default socket options

This CL revises existing platform-dependent default socket
options to make it possible to accomodate multiple multicast
datagram listeners on a single service port.

Also removes unnecessary SO_REUSEADDR, SO_REUSEPORT socket
options from unicast datagram sockets by default.

Fixes #1692.

R=devon.odell, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5538052

6 files changed, 103 insertions, 28 deletions

diff --git a/src/pkg/net/multicast_test.go b/src/pkg/net/multicast_test.go
index 96bac458da..183d5a8aba 100644
--- a/src/pkg/net/multicast_test.go
+++ b/src/pkg/net/multicast_test.go
@@ -95,6 +95,49 @@ func TestMulticastUDP(t *testing.T) {
 	}
 }
 
+func TestSimpleMulticastUDP(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		return
+	}
+	if !*multicast {
+		t.Logf("test disabled; use --multicast to enable")
+		return
+	}
+
+	for _, tt := range multicastUDPTests {
+		var ifi *Interface
+		if tt.ipv6 {
+			continue
+		}
+		tt.flags = FlagUp | FlagMulticast
+		ift, err := Interfaces()
+		if err != nil {
+			t.Fatalf("Interfaces failed: %v", err)
+		}
+		for _, x := range ift {
+			if x.Flags&tt.flags == tt.flags {
+				ifi = &x
+				break
+			}
+		}
+		if ifi == nil {
+			t.Logf("an appropriate multicast interface not found")
+			return
+		}
+		c, err := ListenUDP(tt.net, &UDPAddr{IP: tt.laddr})
+		if err != nil {
+			t.Fatalf("ListenUDP failed: %v", err)
+		}
+		defer c.Close()
+		if err := c.JoinGroup(ifi, tt.gaddr); err != nil {
+			t.Fatalf("JoinGroup failed: %v", err)
+		}
+		if err := c.LeaveGroup(ifi, tt.gaddr); err != nil {
+			t.Fatalf("LeaveGroup failed: %v", err)
+		}
+	}
+}
+
 func testIPv4MulticastSocketOptions(t *testing.T, fd *netFD, ifi *Interface) {
 	ifmc, err := ipv4MulticastInterface(fd)
 	if err != nil {
diff --git a/src/pkg/net/sock.go b/src/pkg/net/sock.go
index 7732d2e063..881c922a25 100644
--- a/src/pkg/net/sock.go
+++ b/src/pkg/net/sock.go
@@ -28,7 +28,7 @@ func socket(net string, f, p, t int, la, ra syscall.Sockaddr, toAddr func(syscal
 	syscall.CloseOnExec(s)
 	syscall.ForkLock.RUnlock()
 
-	setKernelSpecificSockopt(s, f)
+	setDefaultSockopts(s, f, p)
 
 	if la != nil {
 		e = syscall.Bind(s, la)
diff --git a/src/pkg/net/sockopt_bsd.go b/src/pkg/net/sockopt_bsd.go
index 370831fe5f..e99fb418cd 100644
--- a/src/pkg/net/sockopt_bsd.go
+++ b/src/pkg/net/sockopt_bsd.go
@@ -12,22 +12,33 @@ import (
 	"syscall"
 )
 
-func setKernelSpecificSockopt(s, f int) {
-	// Allow reuse of recently-used addresses.
-	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+func setDefaultSockopts(s, f, p int) {
+	switch f {
+	case syscall.AF_INET6:
+		// Allow both IP versions even if the OS default is otherwise.
+		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, 0)
+	}
 
-	// Allow reuse of recently-used ports.
-	// This option is supported only in descendants of 4.4BSD,
-	// to make an effective multicast application and an application
-	// that requires quick draw possible.
-	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEPORT, 1)
+	if f == syscall.AF_UNIX || p == syscall.IPPROTO_TCP {
+		// Allow reuse of recently-used addresses.
+		syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+
+		// Allow reuse of recently-used ports.
+		// This option is supported only in descendants of 4.4BSD,
+		// to make an effective multicast application and an application
+		// that requires quick draw possible.
+		syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEPORT, 1)
+	}
 
 	// Allow broadcast.
 	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1)
+}
 
-	if f == syscall.AF_INET6 {
-		// using ip, tcp, udp, etc.
-		// allow both protocols even if the OS default is otherwise.
-		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, 0)
-	}
+func setDefaultMulticastSockopts(fd *netFD) {
+	fd.incref()
+	defer fd.decref()
+	// Allow multicast UDP and raw IP datagram sockets to listen
+	// concurrently across multiple listeners.
+	syscall.SetsockoptInt(fd.sysfd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+	syscall.SetsockoptInt(fd.sysfd, syscall.SOL_SOCKET, syscall.SO_REUSEPORT, 1)
 }
diff --git a/src/pkg/net/sockopt_linux.go b/src/pkg/net/sockopt_linux.go
index e55c3c5ce8..51583844f1 100644
--- a/src/pkg/net/sockopt_linux.go
+++ b/src/pkg/net/sockopt_linux.go
@@ -10,16 +10,27 @@ import (
 	"syscall"
 )
 
-func setKernelSpecificSockopt(s, f int) {
-	// Allow reuse of recently-used addresses.
-	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+func setDefaultSockopts(s, f, p int) {
+	switch f {
+	case syscall.AF_INET6:
+		// Allow both IP versions even if the OS default is otherwise.
+		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, 0)
+	}
+
+	if f == syscall.AF_UNIX || p == syscall.IPPROTO_TCP {
+		// Allow reuse of recently-used addresses.
+		syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+	}
 
 	// Allow broadcast.
 	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1)
 
-	if f == syscall.AF_INET6 {
-		// using ip, tcp, udp, etc.
-		// allow both protocols even if the OS default is otherwise.
-		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, 0)
-	}
+}
+
+func setDefaultMulticastSockopts(fd *netFD) {
+	fd.incref()
+	defer fd.decref()
+	// Allow multicast UDP and raw IP datagram sockets to listen
+	// concurrently across multiple listeners.
+	syscall.SetsockoptInt(fd.sysfd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
 }
diff --git a/src/pkg/net/sockopt_windows.go b/src/pkg/net/sockopt_windows.go
index df15b8c4c8..485c14a2d3 100644
--- a/src/pkg/net/sockopt_windows.go
+++ b/src/pkg/net/sockopt_windows.go
@@ -10,7 +10,13 @@ import (
 	"syscall"
 )
 
-func setKernelSpecificSockopt(s syscall.Handle, f int) {
+func setDefaultSockopts(s syscall.Handle, f, p int) {
+	switch f {
+	case syscall.AF_INET6:
+		// Allow both IP versions even if the OS default is otherwise.
+		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, 0)
+	}
+
 	// Windows will reuse recently-used addresses by default.
 	// SO_REUSEADDR should not be used here, as it allows
 	// a socket to forcibly bind to a port in use by another socket.
@@ -21,9 +27,12 @@ func setKernelSpecificSockopt(s syscall.Handle, f int) {
 	// Allow broadcast.
 	syscall.SetsockoptInt(s, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1)
 
-	if f == syscall.AF_INET6 {
-		// using ip, tcp, udp, etc.
-		// allow both protocols even if the OS default is otherwise.
-		syscall.SetsockoptInt(s, syscall.IPPROTO_IPV6, syscall.IPV6_V6ONLY, 0)
-	}
+}
+
+func setDefaultMulticastSockopts(fd *netFD) {
+	fd.incref()
+	defer fd.decref()
+	// Allow multicast UDP and raw IP datagram sockets to listen
+	// concurrently across multiple listeners.
+	syscall.SetsockoptInt(fd.sysfd, syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
 }
diff --git a/src/pkg/net/udpsock_posix.go b/src/pkg/net/udpsock_posix.go
index 7bc4cb9f7e..d0bdb14755 100644
--- a/src/pkg/net/udpsock_posix.go
+++ b/src/pkg/net/udpsock_posix.go
@@ -251,6 +251,7 @@ func (c *UDPConn) JoinGroup(ifi *Interface, addr IP) error {
 	if !c.ok() {
 		return os.EINVAL
 	}
+	setDefaultMulticastSockopts(c.fd)
 	ip := addr.To4()
 	if ip != nil {
 		return joinIPv4GroupUDP(c, ifi, ip)