Move listenfd to utility package

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)

systemd/listendfd.go

@@ -0,0 +1,41 @@
+package systemd
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"strconv"
+
+	"github.com/coreos/go-systemd/activation"
+)
+
+// ListenFD returns the specified socket activated files as a slice of
+// net.Listeners or all of the activated files if "*" is given.
+func ListenFD(addr string) ([]net.Listener, error) {
+	files := activation.Files(false)
+	if files == nil || len(files) == 0 {
+		return nil, errors.New("No sockets found")
+	}
+
+	fdNum, _ := strconv.Atoi(addr)
+	fdOffset := fdNum - 3
+	if (addr != "*") && (len(files) < int(fdOffset)+1) {
+		return nil, errors.New("Too few socket activated files passed in")
+	}
+
+	// socket activation
+	listeners := make([]net.Listener, len(files))
+	for i, f := range files {
+		var err error
+		listeners[i], err = net.FileListener(f)
+		if err != nil {
+			return nil, fmt.Errorf("Error setting up FileListener for fd %d: %s", f.Fd(), err.Error())
+		}
+	}
+
+	if addr == "*" {
+		return listeners, nil
+	}
+
+	return []net.Listener{listeners[fdOffset]}, nil
+}