qubesu2f/tools/u2fdump: fix SIGINT handling

CancelledError thrown into loop.run_in_executor() gets raised back only
on next iteration, so when ioctl was blocked there, it didn't really
cancel the coroutine. Now this is done with loop's reader, with a catch
that selectors don't generally work with ioctl(), it is only by chance
that this particular character device has also read() defined which
works almost exactly like the ioctl in question.

qubesu2f/tools/u2fdump.py

@@ -165,14 +165,42 @@ class USBMon:
     '''Async iterator, which yields each packet as it is received.'''
     packet_class = USBMonPacket
 
-    def __init__(self, fd, bufsize=const.HID_FRAME_SIZE):
+    def __init__(self, fd, bufsize=const.HID_FRAME_SIZE, *, loop=None):
         self.fd = fd
         self.bufsize = bufsize
+        self.loop = loop or asyncio.get_event_loop()
+
+        self.queue = asyncio.Queue(32)
+        self.loop.add_reader(self.fd, self._reader)
 
     def __aiter__(self):
         return self
 
+    def cancel(self):
+        while True:
+            try:
+                self.queue.get_nowait()
+                self.queue.task_done()
+            except asyncio.QueueEmpty:
+                break
+        self.queue = None
+
     async def __anext__(self):
+        if self.queue is None:
+            raise StopAsyncIteration()
+
+        try:
+            packet = await self.queue.get()
+            self.queue.task_done()
+
+        except asyncio.CancelledError:
+            self.loop.remove_reader(self.fd)
+            self.cancel()
+            raise StopAsyncIteration()
+
+        return packet
+
+    def _reader(self):
         hdr = _USBMonPacket()
         data = ctypes.create_string_buffer(self.bufsize)
 
@@ -182,11 +210,12 @@ async def __anext__(self):
         arg.data = ctypes.cast(data, ctypes.c_void_p)
         arg.alloc = ctypes.sizeof(data)
 
-        # XXX this doesn't really work with .cancel()
-        await asyncio.get_event_loop().run_in_executor(None,
-            fcntl.ioctl, self.fd, MON_IOCX_GETX, arg)
+        fcntl.ioctl(self.fd, MON_IOCX_GETX, arg)
 
-        return self.packet_class(hdr, data)
+        try:
+            self.queue.put_nowait(self.packet_class(hdr, data))
+        except asyncio.QueueFull:
+            sys.stderr.write('warning: queue full, dropping packet\n')
 
 
 async def u2fmon(bus=0, device=-1):