Add __rsgi_del__ support (#322) (#343)

docs/spec/RSGI.md

@@ -97,6 +97,30 @@ class App:
         # RSGI protocol handling
 ```
 
+#### `__rsgi_del__` method
+
+The del method provides a way for RSGI applications to perform cleanup operations during server shutdown.
+
+The signature of `__rsgi_del__` is defined as follows, with the `loop` argument being the Python `asyncio` event loop:
+
+```
+function __rsgi_del__(loop)
+```
+
+> **Note:** the event loop won't be running at the time the del function gets called
+
+Thus, an application exposing the RSGI del interface might look like the following:
+
+```python
+class App:
+    def __rsgi_del__(self, loop):
+        some_sync_cleanup_task()
+        loop.run_until_complete(some_async_cleanup_task())
+
+    async def __rsgi__(self, scope, protocol):
+        # RSGI protocol handling
+```
+
 ## Protocols
 
 ### HTTP protocol

granian/server.py

@@ -310,6 +310,9 @@ def _spawn_rsgi_worker(
         callback_init = (
             getattr(target, '__rsgi_init__') if hasattr(target, '__rsgi_init__') else lambda *args, **kwargs: None
         )
+        callback_del = (
+            getattr(target, '__rsgi_del__') if hasattr(target, '__rsgi_del__') else lambda *args, **kwargs: None
+        )
         callback = _rsgi_call_wrap(callback, log_access_fmt)
 
         shutdown_event = set_loop_signals(loop, [signal.SIGTERM, signal.SIGINT])
@@ -335,6 +338,7 @@ def _spawn_rsgi_worker(
             contextvars.copy_context(),
             shutdown_event,
         )
+        callback_del(loop)
 
     @staticmethod
     def _spawn_wsgi_worker(