Fix issue pytest-dev#112: 'Dynamically calling a fixture causes a run…

…time error' by failing over to a ThreadPoolExecutor in cases where the expected event loop is already running. Notably this happens when calling 'request.getfixturevalue(argname)' because it dynamically calls a fixture that needs to be setup on an event loop that's already running pytest async tests

pytest_asyncio/plugin.py

@@ -1,5 +1,6 @@
 """pytest-asyncio implementation."""
 import asyncio
+import concurrent.futures
 import contextlib
 import functools
 import inspect
@@ -94,7 +95,10 @@ async def async_finalizer():
 
             request.addfinalizer(finalizer)
 
-            return loop.run_until_complete(setup())
+            pool = concurrent.futures.ThreadPoolExecutor(1)
+            loop = asyncio.new_event_loop()
+            pool.submit(asyncio.set_event_loop, loop).result()
+            return pool.submit(loop.run_until_complete, setup()).result()
 
         fixturedef.func = wrapper