use c.f.Future to wait across threads

Future allows us to propagate errors across threads,
instead of threading.Event, which only waits forever and can result in uninformative hangs if there are errors.

jupyter_client/threaded.py

@@ -5,7 +5,8 @@
 import atexit
 import time
 from concurrent.futures import Future
-from threading import Event, Thread
+from functools import partial
+from threading import Thread
 from typing import Any, Dict, List, Optional
 
 import zmq
@@ -54,17 +55,22 @@ def __init__(
         self.socket = socket
         self.session = session
         self.ioloop = loop
-        evt = Event()
+        f: Future = Future()
 
         def setup_stream():
-            assert self.socket is not None
-            self.stream = zmqstream.ZMQStream(self.socket, self.ioloop)
-            self.stream.on_recv(self._handle_recv)
-            evt.set()
+            try:
+                assert self.socket is not None
+                self.stream = zmqstream.ZMQStream(self.socket, self.ioloop)
+                self.stream.on_recv(self._handle_recv)
+            except Exception as e:
+                f.set_exception(e)
+            else:
+                f.set_result(None)
 
         assert self.ioloop is not None
         self.ioloop.add_callback(setup_stream)
-        evt.wait()
+        # don't wait forever, raise any errors
+        f.result(timeout=10)
 
     _is_alive = False
 
@@ -179,13 +185,31 @@ def flush(self, timeout: float = 1.0) -> None:
         """
         # We do the IOLoop callback process twice to ensure that the IOLoop
         # gets to perform at least one full poll.
-        stop_time = time.time() + timeout
+        stop_time = time.monotonic() + timeout
         assert self.ioloop is not None
+        if self.stream is None or self.stream.closed():
+            # don't bother scheduling flush on a thread if we're closed
+            _msg = "Attempt to flush closed stream"
+            raise OSError(_msg)
+
+        def flush(f):
+            try:
+                self._flush()
+            except Exception as e:
+                f.set_exception(e)
+            else:
+                f.set_result(None)
+
         for _ in range(2):
-            self._flushed = False
-            self.ioloop.add_callback(self._flush)
-            while not self._flushed and time.time() < stop_time:
-                time.sleep(0.01)
+            f: Future = Future()
+            self.ioloop.add_callback(partial(flush, f))
+            # wait for async flush, re-raise any errors
+            timeout = max(stop_time - time.monotonic(), 0)
+            try:
+                f.result(max(stop_time - time.monotonic(), 0))
+            except TimeoutError:
+                # flush with a timeout means stop waiting, not raise
+                return
 
     def _flush(self) -> None:
         """Callback for :method:`self.flush`."""
@@ -219,24 +243,32 @@ def start(self) -> None:
         Don't return until self.ioloop is defined,
         which is created in the thread
         """
-        self._start_event = Event()
+        self._start_future: Future = Future()
         Thread.start(self)
-        self._start_event.wait()
+        # wait for start, re-raise any errors
+        self._start_future.result(timeout=10)
 
     def run(self) -> None:
         """Run my loop, ignoring EINTR events in the poller"""
-        loop = asyncio.new_event_loop()
-        asyncio.set_event_loop(loop)
+        try:
+            loop = asyncio.new_event_loop()
+            asyncio.set_event_loop(loop)
+
+            async def assign_ioloop():
+                self.ioloop = IOLoop.current()
+
+            loop.run_until_complete(assign_ioloop())
+        except Exception as e:
+            self._start_future.set_exception(e)
+        else:
+            self._start_future.set_result(None)
+
         loop.run_until_complete(self._async_run())
 
     async def _async_run(self):
-        self.ioloop = IOLoop.current()
-        # signal that self.ioloop is defined
-        self._start_event.set()
-        while True:
+        """Run forever (until self._exiting is set)"""
+        while not self._exiting:
             await asyncio.sleep(1)
-            if self._exiting:
-                break
 
     def stop(self) -> None:
         """Stop the channel's event loop and join its thread.