avoid starting IOPub background thread after it's been stopped (#1137)

ipykernel/iostream.py

@@ -57,6 +57,7 @@ def __init__(self, socket, pipe=False):
             piped from subprocesses.
         """
         self.socket = socket
+        self._stopped = False
         self.background_socket = BackgroundSocket(self)
         self._master_pid = os.getpid()
         self._pipe_flag = pipe
@@ -83,13 +84,21 @@ def _start_event_gc():
             self._event_pipe_gc_task = asyncio.ensure_future(self._run_event_pipe_gc())
 
         self.io_loop.run_sync(_start_event_gc)
-        self.io_loop.start()
+
+        if not self._stopped:
+            # avoid race if stop called before start thread gets here
+            # probably only comes up in tests
+            self.io_loop.start()
+
         if self._event_pipe_gc_task is not None:
             # cancel gc task to avoid pending task warnings
             async def _cancel():
                 self._event_pipe_gc_task.cancel()  # type:ignore
 
-            self.io_loop.run_sync(_cancel)
+            try:
+                self.io_loop.run_sync(_cancel)
+            except TimeoutError:
+                pass
         self.io_loop.close(all_fds=True)
 
     def _setup_event_pipe(self):
@@ -219,10 +228,16 @@ def start(self):
 
     def stop(self):
         """Stop the IOPub thread"""
+        self._stopped = True
         if not self.thread.is_alive():
             return
         self.io_loop.add_callback(self.io_loop.stop)
-        self.thread.join()
+
+        self.thread.join(timeout=30)
+        if self.thread.is_alive():
+            # avoid infinite hang if stop fails
+            msg = "IOPub thread did not terminate in 30 seconds"
+            raise TimeoutError(msg)
         # close *all* event pipes, created in any thread
         # event pipes can only be used from other threads while self.thread.is_alive()
         # so after thread.join, this should be safe