[Server] Listen and serve on a unix socket

Allow to use a unix socket as a listener.
To specify an endpoint type we use an optional configuration
field 'net', as there's no way to distinguish a relative
socket path from a hostname.

Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>

docs/listener/listener.go

@@ -0,0 +1,74 @@
+package listener
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"time"
+)
+
+// tcpKeepAliveListener sets TCP keep-alive timeouts on accepted
+// connections. It's used by ListenAndServe and ListenAndServeTLS so
+// dead TCP connections (e.g. closing laptop mid-download) eventually
+// go away.
+// it is a plain copy-paste from net/http/server.go
+type tcpKeepAliveListener struct {
+	*net.TCPListener
+}
+
+func (ln tcpKeepAliveListener) Accept() (c net.Conn, err error) {
+	tc, err := ln.AcceptTCP()
+	if err != nil {
+		return
+	}
+	tc.SetKeepAlive(true)
+	tc.SetKeepAlivePeriod(3 * time.Minute)
+	return tc, nil
+}
+
+// NewListener announces on laddr and net. Accepted values of the net are
+// 'unix' and 'tcp'
+func NewListener(net, laddr string) (net.Listener, error) {
+	switch net {
+	case "unix":
+		return newUnixListener(laddr)
+	case "tcp", "": // an empty net means tcp
+		return newTCPListener(laddr)
+	default:
+		return nil, fmt.Errorf("unknown address type %s", net)
+	}
+}
+
+func newUnixListener(laddr string) (net.Listener, error) {
+	fi, err := os.Stat(laddr)
+	if err == nil {
+		// the file exists.
+		// try to remove it if it's a socket
+		if !isSocket(fi.Mode()) {
+			return nil, fmt.Errorf("file %s exists and is not a socket", laddr)
+		}
+
+		if err := os.Remove(laddr); err != nil {
+			return nil, err
+		}
+	} else if !os.IsNotExist(err) {
+		// we can't do stat on the file.
+		// it means we can not remove it
+		return nil, err
+	}
+
+	return net.Listen("unix", laddr)
+}
+
+func isSocket(m os.FileMode) bool {
+	return m&os.ModeSocket != 0
+}
+
+func newTCPListener(laddr string) (net.Listener, error) {
+	ln, err := net.Listen("tcp", laddr)
+	if err != nil {
+		return nil, err
+	}
+
+	return tcpKeepAliveListener{ln.(*net.TCPListener)}, nil
+}