fpi-ssm: Add a usb transfer callback with data as weak pointer

Allow to pass a double-pointer to be nullified as the transfer data in order
to mark it as NULL when the transfer is done.

This is useful if we're keeping the transfer around in order to check that
no one is currently running.

libfprint/fpi-ssm.c

@@ -413,3 +413,32 @@ fpi_ssm_usb_transfer_cb (FpiUsbTransfer *transfer, FpDevice *device,
   else
     fpi_ssm_next_state (transfer->ssm);
 }
+
+/**
+ * fpi_ssm_usb_transfer_with_weak_pointer_cb:
+ * @transfer: a #FpiUsbTransfer
+ * @device: a #FpDevice
+ * @weak_ptr: A #gpointer pointer to nullify. You can pass a pointer to any
+ *            #gpointer to nullify when the callback is completed. I.e a
+ *            pointer to the current #FpiUsbTransfer.
+ * @error: The #GError or %NULL
+ *
+ * Can be used in as a #FpiUsbTransfer callback handler to automatically
+ * advance or fail a statemachine on transfer completion.
+ * Passing a #gpointer* as @weak_ptr permits to nullify it once we're done
+ * with the transfer.
+ *
+ * Make sure to set the #FpiSsm on the transfer.
+ */
+void
+fpi_ssm_usb_transfer_with_weak_pointer_cb (FpiUsbTransfer *transfer,
+                                           FpDevice *device, gpointer weak_ptr,
+                                           GError *error)
+{
+  g_return_if_fail (transfer->ssm);
+
+  if (weak_ptr)
+    g_nullify_pointer ((gpointer *) weak_ptr);
+
+  fpi_ssm_usb_transfer_cb (transfer, device, weak_ptr, error);
+}

libfprint/fpi-ssm.h

@@ -91,5 +91,9 @@ void fpi_ssm_next_state_timeout_cb (FpDevice *dev,
                                     void     *data);
 void fpi_ssm_usb_transfer_cb (FpiUsbTransfer *transfer,
                               FpDevice       *device,
-                              gpointer        user_data,
+                              gpointer        user_date,
                               GError         *error);
+void fpi_ssm_usb_transfer_with_weak_pointer_cb (FpiUsbTransfer *transfer,
+                                                FpDevice       *device,
+                                                gpointer        weak_ptr,
+                                                GError         *error);