Simplify bot loop cleanup routine.

Attempted solution for #2040

discord/client.py

@@ -55,6 +55,46 @@ from .appinfo import AppInfo
 
 log = logging.getLogger(__name__)
 
+class _ProperCleanup(Exception):
+    pass
+
+def _raise_proper_cleanup():
+    raise _ProperCleanup
+
+def _cancel_tasks(loop, tasks):
+    if not tasks:
+        return
+
+    log.info('Cleaning up after %d tasks.', len(tasks))
+    gathered = asyncio.gather(*tasks, loop=loop, return_exceptions=True)
+    gathered.cancel()
+    gathered.add_done_callback(lambda fut: loop.stop())
+
+    while not gathered.done():
+        loop.run_forever()
+
+    for task in tasks:
+        if task.cancelled():
+            continue
+        if task.exception() is not None:
+            loop.call_exception_handler({
+                'message': 'Unhandled exception during Client.run shutdown.',
+                'exception': task.exception(),
+                'task': task
+            })
+
+def _cleanup_loop(loop):
+    try:
+        task_retriever = asyncio.Task.all_tasks
+    except AttributeError:
+        # future proofing for 3.9 I guess
+        task_retriever = asyncio.all_tasks
+
+    all_tasks = {t for t in task_retriever(loop=loop) if not t.done()}
+    _cancel_tasks(loop, all_tasks)
+    if sys.version_info >= (3, 6):
+        loop.run_until_complete(loop.shutdown_asyncgens())
+
 class Client:
     r"""Represents a client connection that connects to Discord.
     This class is used to interact with the Discord WebSocket and API.
@@ -463,43 +503,28 @@ class Client:
     def _do_cleanup(self):
         log.info('Cleaning up event loop.')
         loop = self.loop
-        if loop.is_closed():
-            return # we're already cleaning up
 
         task = asyncio.ensure_future(self.close(), loop=loop)
 
-        def _silence_gathered(fut):
+        def stop_loop(fut):
             try:
                 fut.result()
-            except Exception:
+            except asyncio.CancelledError:
                 pass
+            except Exception as e:
+                loop.call_exception_handler({
+                    'message': 'Unexpected exception during Client.close',
+                    'exception': e
+                })
             finally:
                 loop.stop()
 
-        def when_future_is_done(fut):
-            pending = asyncio.Task.all_tasks(loop=loop)
-            if pending:
-                log.info('Cleaning up after %s tasks', len(pending))
-                gathered = asyncio.gather(*pending, loop=loop)
-                gathered.cancel()
-                gathered.add_done_callback(_silence_gathered)
-            else:
-                loop.stop()
-
-        task.add_done_callback(when_future_is_done)
-        if not loop.is_running():
-            loop.run_forever()
-        else:
-            # on Linux, we're still running because we got triggered via
-            # the signal handler rather than the natural KeyboardInterrupt
-            # Since that's the case, we're going to return control after
-            # registering the task for the event loop to handle later
-            return None
-
+        task.add_done_callback(stop_loop)
         try:
-            return task.result() # suppress unused task warning
-        except Exception:
-            return None
+            loop.run_forever()
+        finally:
+            _cleanup_loop(loop)
+            loop.close()
 
     def run(self, *args, **kwargs):
         """A blocking call that abstracts away the `event loop`_
@@ -528,29 +553,15 @@ class Client:
         is_windows = sys.platform == 'win32'
         loop = self.loop
         if not is_windows:
-            loop.add_signal_handler(signal.SIGINT, self._do_cleanup)
-            loop.add_signal_handler(signal.SIGTERM, self._do_cleanup)
-
-        task = asyncio.ensure_future(self.start(*args, **kwargs), loop=loop)
-
-        def stop_loop_on_finish(fut):
-            loop.stop()
-
-        task.add_done_callback(stop_loop_on_finish)
+            loop.add_signal_handler(signal.SIGINT, _raise_proper_cleanup)
+            loop.add_signal_handler(signal.SIGTERM, _raise_proper_cleanup)
 
         try:
-            loop.run_forever()
-        except KeyboardInterrupt:
+            loop.run_until_complete(self.start(*args, **kwargs))
+        except (_ProperCleanup, KeyboardInterrupt):
             log.info('Received signal to terminate bot and event loop.')
         finally:
-            task.remove_done_callback(stop_loop_on_finish)
-            if is_windows:
-                self._do_cleanup()
-
-            loop.close()
-            if task.cancelled() or not task.done():
-                return None
-            return task.result()
+            self._do_cleanup()
 
     # properties