add shutdown case

docker/controller.py

@@ -809,11 +809,13 @@ class DockerWatcher:
         limiter = rate_limit(60)
         log.debug("watcher started")
 
-        while not self._shutdown:
+        while not self._shutdown:   # pragma: nobranch
             next(limiter)
             try:
                 for event in self._client.events(filters={"event": "die"}):
-                    log.debug("event %r", event)
+                    if self._shutdown:
+                        return
+                    log.debug("docker event %r", event)
                     self._loop.call_soon_threadsafe(self._event, event)
             except Exception:
                 log.exception("docker watcher exception")
@@ -836,7 +838,7 @@ class DockerWatcher:
         if not self._shutdown:
             self._shutdown = True
             for fut in self._futures.values():
-                if not fut.done():
+                if not fut.done():  # pragma: nobranch
                     fut.set_exception(ShuttingDown())