set linger to 0 for both inbound and outbound connections

This causes us to send RST packets instead of FIN packets when closing
connections and means connections immediately enter the "reset" state instead of
entering the TIME-WAIT state.

Importantly, this means we can immediately reuse the 5-tuple and reconnect.

tcp.go

@@ -2,6 +2,7 @@ package tcp
 
 import (
 	"context"
+	"net"
 	"time"
 
 	logging "github.com/ipfs/go-log"
@@ -20,6 +21,29 @@ var DefaultConnectTimeout = 5 * time.Second
 
 var log = logging.Logger("tcp-tpt")
 
+// try to set linger on the connection, if possible.
+func tryLinger(conn net.Conn, sec int) {
+	if lingerConn, ok := conn.(interface {
+		SetLinger(int) error
+	}); ok {
+		_ = lingerConn.SetLinger(sec)
+	}
+}
+
+type lingerListener struct {
+	manet.Listener
+	sec int
+}
+
+func (ll *lingerListener) Accept() (manet.Conn, error) {
+	c, err := ll.Listener.Accept()
+	if err != nil {
+		return nil, err
+	}
+	tryLinger(c, ll.sec)
+	return c, nil
+}
+
 // TcpTransport is the TCP transport.
 type TcpTransport struct {
 	// Connection upgrader for upgrading insecure stream connections to
@@ -73,6 +97,10 @@ func (t *TcpTransport) Dial(ctx context.Context, raddr ma.Multiaddr, p peer.ID)
 	if err != nil {
 		return nil, err
 	}
+	// Set linger to 0 so we never get stuck in the TIME-WAIT state. When
+	// linger is 0, connections are _reset_ instead of closed with a FIN.
+	// This means we can immediately reuse the 5-tuple and reconnect.
+	tryLinger(conn, 0)
 	return t.Upgrader.UpgradeOutbound(ctx, t, conn, p)
 }
 
@@ -94,6 +122,7 @@ func (t *TcpTransport) Listen(laddr ma.Multiaddr) (tpt.Listener, error) {
 	if err != nil {
 		return nil, err
 	}
+	list = &lingerListener{list, 0}
 	return t.Upgrader.UpgradeListener(t, list), nil
 }