Fix Object.Call blocking

When used with the FlagNoReplyExpected flag
Object.Call blocks forever
because the returned Call.Done is nil

Returning a Call.Done with a closed channel
(with one reference to a call in it)
means reads to Call.Done won't block.

conn_test.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"io/ioutil"
 	"testing"
+	"time"
 )
 
 func TestSessionBus(t *testing.T) {
@@ -44,6 +45,23 @@ func TestSend(t *testing.T) {
 	}
 }
 
+func TestFlagNoReplyExpectedSend(t *testing.T) {
+	bus, err := SessionBus()
+	if err != nil {
+		t.Fatal(err)
+	}
+	done := make(chan struct{})
+	go func() {
+		bus.BusObject().Call("org.freedesktop.DBus.ListNames", FlagNoReplyExpected)
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(1 * time.Second):
+		t.Error("Failed to announce that the call was done")
+	}
+}
+
 func TestRemoveSignal(t *testing.T) {
 	bus, err := NewConn(nil)
 	if err != nil {

object.go

@@ -43,7 +43,8 @@ func (o *Object) AddMatchSignal(iface, member string) *Call {
 // will be allocated. Otherwise, ch has to be buffered or Go will panic.
 //
 // If the flags include FlagNoReplyExpected, ch is ignored and a Call structure
-// is returned of which only the Err member is valid.
+// is returned with any error in Err and a closed channel in Done containing
+// the returned Call as it's one entry.
 //
 // If the method parameter contains a dot ('.'), the part before the last dot
 // specifies the interface on which the method is called.
@@ -97,11 +98,21 @@ func (o *Object) Go(method string, flags Flags, ch chan *Call, args ...interface
 	}
 	o.conn.outLck.RLock()
 	defer o.conn.outLck.RUnlock()
+	done := make(chan *Call, 1)
+	call := &Call{
+		Err:  nil,
+		Done: done,
+	}
+	defer func() {
+		call.Done <- call
+		close(done)
+	}()
 	if o.conn.closed {
-		return &Call{Err: ErrClosed}
+		call.Err = ErrClosed
+		return call
 	}
 	o.conn.out <- msg
-	return &Call{Err: nil}
+	return call
 }
 
 // GetProperty calls org.freedesktop.DBus.Properties.GetProperty on the given