govmm: Optionally start QMP with a pre-configured connection

When QEMU is launched daemonized, we have the guarantee that the
QMP socket is available. In order to launch a non-daemonized QEMU,
the QMP connection should be created before QEMU is started in order
to avoid a race. Introduce a variant of QMPStart() that can use such
an existing connection.

Signed-off-by: Greg Kurz <groug@kaod.org>

src/runtime/pkg/govmm/qemu/qmp.go

@@ -702,6 +702,16 @@ func QMPStart(ctx context.Context, socket string, cfg QMPConfig, disconnectedCh
 		return nil, nil, err
 	}
 
+	return QMPStartWithConn(ctx, conn, cfg, disconnectedCh)
+}
+
+// Same as QMPStart but with a pre-established connection
+func QMPStartWithConn(ctx context.Context, conn net.Conn, cfg QMPConfig, disconnectedCh chan struct{}) (*QMP, *QMPVersion, error) {
+	if conn == nil {
+		close(disconnectedCh)
+		return nil, nil, fmt.Errorf("invalid connection")
+	}
+
 	connectedCh := make(chan *QMPVersion)
 
 	q := startQMPLoop(conn, cfg, connectedCh, disconnectedCh)

src/runtime/pkg/govmm/qemu/qmp_test.go

@@ -273,6 +273,22 @@ func TestQMPStartBadPath(t *testing.T) {
 	<-disconnectedCh
 }
 
+// Checks that a call to QMPStartWithConn with a nil connection exits gracefully.
+//
+// We call QMPStartWithConn with a nil connection.
+//
+// An error should be returned and the disconnected channel should be closed.
+func TestQMPStartWithConnNil(t *testing.T) {
+	cfg := QMPConfig{Logger: qmpTestLogger{}}
+	disconnectedCh := make(chan struct{})
+	q, _, err := QMPStartWithConn(context.Background(), nil, cfg, disconnectedCh)
+	if err == nil {
+		t.Errorf("Expected error")
+		q.Shutdown()
+	}
+	<-disconnectedCh
+}
+
 // Checks that the qmp_capabilities command is correctly sent.
 //
 // We start a QMPLoop, send the qmp_capabilities command and stop the