plugin: Set unused_tcp_port_factory scope to 'session'

Factories in pytest usually have a scope greater than 'function' to let
one use the same factory within bigger scopes. Let us allow
unused_tcp_port_factory to be used throughout the same session scope.
This will let other session-scoped factories depend on
unused_tcp_port_factory without getting a "ScopeMismatch" error.

pytest_asyncio/plugin.py

@@ -198,7 +198,7 @@ def unused_tcp_port():
     return _unused_tcp_port()
 
 
-@pytest.fixture
+@pytest.fixture(scope='session')
 def unused_tcp_port_factory():
     """A factory function, producing different unused TCP ports."""
     produced = set()

tests/conftest.py

@@ -26,3 +26,10 @@ async def just_a_sleep():
     event_loop.run_until_complete(just_a_sleep())
 
     assert counter == 2
+
+
+@pytest.fixture(scope='session', name='factory_involving_factories')
+def factory_involving_factories_fixture(unused_tcp_port_factory):
+    def factory():
+        return unused_tcp_port_factory()
+    return factory

tests/test_dependent_fixtures.py

@@ -6,3 +6,8 @@
 async def test_dependent_fixture(dependent_fixture):
     """Test a dependent fixture."""
     await asyncio.sleep(0.1)
+
+
+@pytest.mark.asyncio
+async def test_factory_involving_factories(factory_involving_factories):
+    factory_involving_factories()