Detect asyncio when the loop is running without a Task

newsfragments/51.bugfix.rst

@@ -0,0 +1,4 @@
+``current_async_library`` now reports ``asyncio`` when an event loop is
+running but there is no current Task, such as in ``call_soon`` callbacks
+and Twisted's asyncioreactor. Detection uses ``asyncio.get_running_loop()``
+only; ``current_task()`` is a slower subset of the same check.

sniffio/_impl.py

@@ -75,14 +75,11 @@ async def generic_sleep(seconds):
     if "asyncio" in sys.modules:
         import asyncio
         try:
-            current_task = asyncio.current_task  # type: ignore[attr-defined]
-        except AttributeError:
-            current_task = asyncio.Task.current_task  # type: ignore[attr-defined]
-        try:
-            if current_task() is not None:
-                return "asyncio"
+            asyncio.get_running_loop()
         except RuntimeError:
             pass
+        else:
+            return "asyncio"
 
     # Sniff for curio (for now)
     if 'curio' in sys.modules:

sniffio/_tests/test_sniffio.py

@@ -58,6 +58,27 @@ async def this_is_asyncio():
         current_async_library()
 
 
+def test_asyncio_running_loop_without_task():
+    import asyncio
+
+    result = []
+
+    def sync_cb():
+        try:
+            result.append(current_async_library())
+        finally:
+            loop.stop()
+
+    loop = asyncio.new_event_loop()
+    try:
+        loop.call_soon(sync_cb)
+        loop.run_forever()
+    finally:
+        loop.close()
+
+    assert result == ["asyncio"]
+
+
 @pytest.mark.skipif(
     sys.version_info >= (3, 12),
     reason=