Improve shutdown process in Multiprocess mode

Explicitly clos listening sockets in both parent and child processes early in
the shutdown process.  That way, no new connections can hit workers after
shutdown is initiated by the parent process.

While here, speedup shutdown by splitting process shutdown into terminate and
join loop, so all processes are shutdown in parallel.

uvicorn/server.py

@@ -265,10 +265,10 @@ async def shutdown(self, sockets: Optional[List[socket.socket]] = None) -> None:
         logger.info("Shutting down")
 
         # Stop accepting new connections.
-        for server in self.servers:
-            server.close()
         for sock in sockets or []:
             sock.close()
+        for server in self.servers:
+            server.close()
         for server in self.servers:
             await server.wait_closed()
 

uvicorn/supervisors/multiprocess.py

@@ -63,8 +63,13 @@ def startup(self) -> None:
             self.processes.append(process)
 
     def shutdown(self) -> None:
+        for socket in self.sockets:
+            socket.close()
+
         for process in self.processes:
             process.terminate()
+
+        for process in self.processes:
             process.join()
 
         message = "Stopping parent process [{}]".format(str(self.pid))