usb-transfer: Work around libgusb cancellation issue

We have plenty of code paths where a transfer may be cancelled before it
is submitted. Unfortunately, libgusb up to and including version 0.3.6
are not handling that case correctly (due to libusb ignoring
cancellation on transfers that are not yet submitted).

Work around this, but do so in a somewhat lazy fashion that is not
entirely race free.

Closes: #306

libfprint/fpi-usb-transfer.c

@@ -354,6 +354,24 @@ transfer_finish_cb (GObject *source_object, GAsyncResult *res, gpointer user_dat
   fpi_usb_transfer_unref (transfer);
 }
 
+static gboolean
+transfer_cancel_cb (FpiUsbTransfer *transfer)
+{
+  GError *error;
+  FpiUsbTransferCallback callback;
+
+  error = g_error_new_literal (G_IO_ERROR,
+                               G_IO_ERROR_CANCELLED,
+                               "Transfer was cancelled before being started");
+  callback = transfer->callback;
+  transfer->callback = NULL;
+  transfer->actual_length = -1;
+  callback (transfer, transfer->device, transfer->user_data, error);
+
+  fpi_usb_transfer_unref (transfer);
+
+  return G_SOURCE_REMOVE;
+}
 
 /**
  * fpi_usb_transfer_submit:
@@ -387,6 +405,18 @@ fpi_usb_transfer_submit (FpiUsbTransfer        *transfer,
 
   log_transfer (transfer, TRUE, NULL);
 
+  /* Work around libgusb cancellation issue, see
+   *   https://github.com/hughsie/libgusb/pull/42
+   * should be fixed with libgusb 0.3.7.
+   * Note that this is not race free, we rely on libfprint and API users
+   * not cancelling from a different thread here.
+   */
+  if (cancellable && g_cancellable_is_cancelled (cancellable))
+    {
+      g_idle_add ((GSourceFunc) transfer_cancel_cb, transfer);
+      return;
+    }
+
   switch (transfer->type)
     {
     case FP_TRANSFER_BULK: