replace allocTransfer with a new more generic newUSBTransfer

usb/endpoint.go

@@ -99,3 +99,36 @@ func interrupt_xfer(e *endpoint, buf []byte, timeout time.Duration) (int, error)
 	}
 	return int(cnt), nil
 }
+
+func (e *endpoint) newUSBTransfer(tt TransferType, buf []byte) (*usbTransfer, error) {
+	var isoPackets int
+	if tt == TRANSFER_TYPE_ISOCHRONOUS {
+		isoPackets = len(buf) / int(e.EndpointInfo.MaxIsoPacket)
+	}
+
+	xfer := C.libusb_alloc_transfer(C.int(isoPackets))
+	if xfer == nil {
+		return nil, fmt.Errorf("libusb_alloc_transfer(%d) failed", isoPackets)
+	}
+
+	done := make(chan struct{}, 1)
+	xfer.user_data = (unsafe.Pointer)(&done)
+
+	xfer.dev_handle = e.Device.handle
+	xfer.endpoint = C.uchar(e.Address)
+	xfer._type = C.uchar(tt)
+
+	xfer.buffer = (*C.uchar)((unsafe.Pointer)(&buf[0]))
+	xfer.length = C.int(len(buf))
+
+	if tt == TRANSFER_TYPE_ISOCHRONOUS {
+		xfer.num_iso_packets = C.int(isoPackets)
+		C.libusb_set_iso_packet_lengths(xfer, C.uint(e.EndpointInfo.MaxIsoPacket))
+	}
+
+	return &usbTransfer{
+		xfer: xfer,
+		done: done,
+		buf:  buf,
+	}, nil
+}