Raise an exception if the loop is the wrong type on Windows

Fixes: #86
Closes: #105

Author: saghul

README.rst

@@ -55,6 +55,21 @@ The API is pretty simple, three functions are provided in the ``DNSResolver`` cl
   ``ARES_ECANCELLED`` errno.
 
 
+Note for Windows users
+======================
+
+This library requires the asyncio loop to be a `SelectorEventLoop`, which is not the default on Windows since
+Python 3.8.
+
+The default can be changed as follows (do this very early in your application):
+
+.. code:: python
+
+    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
+
+This may have other implications for the rest of your codebase, so make sure to test thoroughly.
+
+
 Running the test suite
 ======================
 

aiodns/__init__.py

@@ -3,10 +3,10 @@
 import functools
 import pycares
 import socket
+import sys
 
 from typing import (
     Any,
-    List,
     Optional,
     Set,
     Sequence
@@ -50,6 +50,10 @@ def __init__(self, nameservers: Optional[Sequence[str]] = None,
                  **kwargs: Any) -> None:
         self.loop = loop or asyncio.get_event_loop()
         assert self.loop is not None
+        if sys.platform == 'win32':
+            if not isinstance(self.loop, asyncio.SelectorEventLoop):
+                raise RuntimeError(
+                    'aiodns needs a SelectorEventLoop on Windows. See more: https://github.com/saghul/aiodns/issues/86')
         kwargs.pop('sock_state_cb', None)
         self._channel = pycares.Channel(sock_state_cb=self._sock_state_cb, **kwargs)
         if nameservers: