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 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

@@ -51,6 +51,25 @@ def pytest_pycollect_makeitem(collector, name, obj):
 @pytest.hookimpl(hookwrapper=True)
 def pytest_fixture_setup(fixturedef, request):
     """Adjust the event loop policy when an event loop is produced."""
+
+    def run_until_complete(loop, coro):
+        """
+        Fail over to a ThreadPoolExecutor in case the event loop is already
+        running. Particularly, this occurs when calling a dynamic fixture
+        using `request.getfixturevalue(argname)`
+        """
+
+        try:
+            return loop.run_until_complete(coro)
+        except RuntimeError:
+            with concurrent.futures.ThreadPoolExecutor(1) as pool:
+                loop = asyncio.new_event_loop()
+                try:
+                    pool.submit(asyncio.set_event_loop, loop).result()
+                    return pool.submit(loop.run_until_complete, coro).result()
+                finally:
+                    loop.close()
+
     if isasyncgenfunction(fixturedef.func):
         # This is an async generator function. Wrap it accordingly.
         f = fixturedef.func
@@ -94,7 +113,7 @@ async def async_finalizer():
 
             request.addfinalizer(finalizer)
 
-            return loop.run_until_complete(setup())
+            return run_until_complete(loop, setup())
 
         fixturedef.func = wrapper
 
@@ -116,7 +135,7 @@ async def setup():
                 res = await f(*args, **kwargs)
                 return res
 
-            return loop.run_until_complete(setup())
+            return run_until_complete(loop, setup())
 
         fixturedef.func = wrapper