usb1: Make libusb_device_p a c_void_p.

Fixes a pypy3 segfault in USBDevice's finalizer with examples/hotplug.py .

The pointers in the device_p array obtained from libusb_get_device_list are
visibly not copied to the ctypes objects created while iterating the list.
Once the list is freed by libusb_free_device_list, those ctypes pointers
objects are reading the memory address from a freed memory location. With
c_void_p, the iteration produces regular python integers, which forces the
interpreter to get its own copy, which then survives list free.
Unfortunately, this removes a ctypes typecheck on every function taking
a libusb_device_p parameter.

Author: vpelletier

usb1/__init__.py

@@ -2255,14 +2255,9 @@ def getDeviceIterator(self, skip_on_error=False):
         try:
             for device_p in device_p_p[:device_list_len]:
                 try:
-                    # Instanciate our own libusb_device_p object so we can free
-                    # libusb-provided device list. Is this a bug in ctypes that
-                    # it doesn't copy pointer value (=pointed memory address) ?
-                    # At least, it's not so convenient and forces using such
-                    # weird code.
                     device = USBDevice(
                         context=self,
-                        device_p=libusb_device_p(device_p.contents),
+                        device_p=device_p,
                         registerFinalizer=self.__registerFinalizer,
                         unregisterFinalizer=self.__unregisterFinalizer,
                         can_load_configuration=True,

usb1/_libusb1.py

@@ -971,9 +971,10 @@ class libusb_control_setup(Structure):
 # 1, and libusb_free_device_list() can optionally decrease the reference count
 # on all devices in the list. libusb_open() adds another reference which is
 # later destroyed by libusb_close().
-class libusb_device(Structure):
-    pass
-libusb_device_p = POINTER(libusb_device)
+#class libusb_device(Structure):
+#    pass
+#libusb_device_p = POINTER(libusb_device)
+libusb_device_p = c_void_p # Any pointer is fine
 libusb_device_p_p = POINTER(libusb_device_p)
 libusb_device_p_p_p = POINTER(libusb_device_p_p)