Remove unnecessary use of SO_REUSEPORT

Fixes #856.

fs/go.mod

@@ -5,7 +5,7 @@ go 1.20
 require (
 	github.com/anacrolix/envpprof v1.2.1
 	github.com/anacrolix/fuse v0.2.0
-	github.com/anacrolix/log v0.14.0
+	github.com/anacrolix/log v0.14.1
 	github.com/anacrolix/missinggo/v2 v2.7.2-0.20230527121029-a582b4f397b9
 	github.com/anacrolix/tagflag v1.3.0
 	github.com/anacrolix/torrent v1.47.1-0.20221102120345-c63f7e1bd720

fs/go.sum

@@ -46,6 +46,8 @@ github.com/anacrolix/log v0.10.1-0.20220123034749-3920702c17f8/go.mod h1:GmnE2c0
 github.com/anacrolix/log v0.13.1/go.mod h1:D4+CvN8SnruK6zIFS/xPoRJmtvtnxs+CSfDQ+BFxZ68=
 github.com/anacrolix/log v0.14.0 h1:mYhTSemILe/Z8tIxbGdTIWWpPspI8W/fhZHpoFbDaL0=
 github.com/anacrolix/log v0.14.0/go.mod h1:1OmJESOtxQGNMlUO5rcv96Vpp9mfMqXXbe2RdinFLdY=
+github.com/anacrolix/log v0.14.1 h1:j2FcIpYZ5FbANetUcm5JNu+zUBGADSp/VbjhUPrAY0k=
+github.com/anacrolix/log v0.14.1/go.mod h1:1OmJESOtxQGNMlUO5rcv96Vpp9mfMqXXbe2RdinFLdY=
 github.com/anacrolix/lsan v0.0.0-20211126052245-807000409a62 h1:P04VG6Td13FHMgS5ZBcJX23NPC/fiC4cp9bXwYujdYM=
 github.com/anacrolix/lsan v0.0.0-20211126052245-807000409a62/go.mod h1:66cFKPCO7Sl4vbFnAaSq7e4OXtdMhRSBagJGWgmpJbM=
 github.com/anacrolix/missinggo v0.0.0-20180725070939-60ef2fbf63df/go.mod h1:kwGiTUTZ0+p4vAz3VbAI5a30t2YbvemcmspjKwrAz5s=

socket.go

@@ -54,38 +54,50 @@ var tcpListenConfig = net.ListenConfig{
 
 func listenTcp(network, address string) (s socket, err error) {
 	l, err := tcpListenConfig.Listen(context.Background(), network, address)
-	return tcpSocket{
+	// Dialling TCP from a local port limits us to a single outgoing TCP connection to each remote
+	// client. Instead, this should be a last resort if we need to use holepunching, and only then to
+	// connect to other clients that actually try to holepunch TCP.
+	const dialFromListenPort = false
+	netDialer := net.Dialer{
+		// We don't want fallback, as we explicitly manage the IPv4/IPv6 distinction ourselves,
+		// although it's probably not triggered as I think the network is already constrained to
+		// tcp4 or tcp6 at this point.
+		FallbackDelay: -1,
+		// BitTorrent connections manage their own keepalives.
+		KeepAlive: tcpListenConfig.KeepAlive,
+		Control: func(network, address string, c syscall.RawConn) (err error) {
+			controlErr := c.Control(func(fd uintptr) {
+				err = setSockNoLinger(fd)
+				if err != nil {
+					// Failing to disable linger is undesirable, but not fatal.
+					log.Levelf(log.Debug, "error setting linger socket option on tcp socket: %v", err)
+					err = nil
+				}
+				// This is no longer required I think, see
+				// https://github.com/anacrolix/torrent/discussions/856. I added this originally to
+				// allow dialling out from the client's listen port, but that doesn't really work. I
+				// think Linux older than ~2013 doesn't support SO_REUSEPORT.
+				if dialFromListenPort {
+					err = setReusePortSockOpts(fd)
+				}
+			})
+			if err == nil {
+				err = controlErr
+			}
+			return
+		},
+	}
+	if dialFromListenPort {
+		netDialer.LocalAddr = l.Addr()
+	}
+	s = tcpSocket{
 		Listener: l,
 		NetworkDialer: NetworkDialer{
 			Network: network,
-			Dialer: &net.Dialer{
-				// Dialling TCP from a local port limits us to a single outgoing TCP connection to
-				// each remote client. Instead this should be a last resort if we need to use holepunching, and only then to connect to other clients that actually try to holepunch TCP.
-				//LocalAddr: l.Addr(),
-
-				// We don't want fallback, as we explicitly manage the IPv4/IPv6 distinction
-				// ourselves, although it's probably not triggered as I think the network is already
-				// constrained to tcp4 or tcp6 at this point.
-				FallbackDelay: -1,
-				// BitTorrent connections manage their own keep-alives.
-				KeepAlive: tcpListenConfig.KeepAlive,
-				Control: func(network, address string, c syscall.RawConn) (err error) {
-					controlErr := c.Control(func(fd uintptr) {
-						err = setSockNoLinger(fd)
-						if err != nil {
-							// Failing to disable linger is undesirable, but not fatal.
-							log.Printf("error setting linger socket option on tcp socket: %v", err)
-						}
-						err = setReusePortSockOpts(fd)
-					})
-					if err == nil {
-						err = controlErr
-					}
-					return
-				},
-			},
+			Dialer:  &netDialer,
 		},
-	}, err
+	}
+	return
 }
 
 type tcpSocket struct {