From aca54f716f1a51ec778f8f7813e62e8745dce69c Mon Sep 17 00:00:00 2001
From: Sylvain Corlay <sylvain.corlay@gmail.com>
Date: Sat, 12 Dec 2020 10:52:15 +0100
Subject: [PATCH] Nudge kernel with info request until we receive IOPub
 messages

---
 jupyter_server/services/kernels/handlers.py | 90 ++++++++++++++++++---
 1 file changed, 81 insertions(+), 9 deletions(-)

diff --git a/jupyter_server/services/kernels/handlers.py b/jupyter_server/services/kernels/handlers.py
index e581c30471..ada458da95 100644
--- a/jupyter_server/services/kernels/handlers.py
+++ b/jupyter_server/services/kernels/handlers.py
@@ -127,6 +127,65 @@ def create_stream(self):
             self.channels[channel] = stream = meth(self.kernel_id, identity=identity)
             stream.channel = channel
 
+    def nudge(self):
+        shell_channel = self.channels['shell']
+        iopub_channel = self.channels['iopub']
+
+        future = Future()
+        info_future = Future()
+        iopub_future = Future()
+
+        def finish():
+            """Common cleanup"""
+            loop.remove_timeout(timeout)
+            loop.remove_timeout(nudge_handle)
+            iopub_channel.stop_on_recv()
+            shell_channel.stop_on_recv()
+
+        def on_shell_reply(msg):
+            if not info_future.done():
+                self.log.debug("Nudge: shell info reply received: %s", self.kernel_id)
+                shell_channel.stop_on_recv()
+                self.log.debug("Nudge: resolving shell future")
+                info_future.set_result(msg)
+                if iopub_future.done():
+                    finish()
+                    self.log.debug("Nudge: resolving main future in shell handler")
+                    future.set_result(info_future.result())
+
+        def on_iopub(msg):
+            if not iopub_future.done():
+                self.log.debug("Nudge: first IOPub received: %s", self.kernel_id)
+                iopub_channel.stop_on_recv()
+                self.log.debug("Nudge: resolving iopub future")
+                iopub_future.set_result(None)
+                if info_future.done():
+                    finish()
+                    self.log.debug("Nudge: resolving main future in iopub handler")
+                    future.set_result(info_future.result())
+
+        def on_timeout():
+            self.log.warning("Nudge: Timeout waiting for kernel_info_reply: %s", self.kernel_id)
+            finish()
+            if not future.done():
+                future.set_exception(TimeoutError("Timeout waiting for nudge"))
+
+        iopub_channel.on_recv(on_iopub)
+        shell_channel.on_recv(on_shell_reply)
+        loop = IOLoop.current()
+
+        # Nudge the kernel with kernel info requests until we get an IOPub message
+        def nudge():
+            self.log.debug("Nudge")
+            if not future.done():
+                self.log.debug("nudging")
+                self.session.send(shell_channel, "kernel_info_request")
+                nudge_handle = loop.call_later(0.5, nudge)
+        nudge_handle = loop.call_later(0, nudge)
+
+        timeout = loop.add_timeout(loop.time() + self.kernel_info_timeout, on_timeout)
+        return future
+
     def request_kernel_info(self):
         """send a request for kernel_info"""
         km = self.kernel_manager
@@ -249,7 +308,7 @@ async def _register_session(self):
             await stale_handler.close()
         self._open_sessions[self.session_key] = self
 
-    def open(self, kernel_id):
+    async def open(self, kernel_id):
         super(ZMQChannelsHandler, self).open()
         km = self.kernel_manager
         km.notify_connect(kernel_id)
@@ -259,15 +318,22 @@ def open(self, kernel_id):
         if buffer_info and buffer_info['session_key'] == self.session_key:
             self.log.info("Restoring connection for %s", self.session_key)
             self.channels = buffer_info['channels']
-            replay_buffer = buffer_info['buffer']
-            if replay_buffer:
-                self.log.info("Replaying %s buffered messages", len(replay_buffer))
-                for channel, msg_list in replay_buffer:
-                    stream = self.channels[channel]
-                    self._on_zmq_reply(stream, msg_list)
+
+            connected = self.nudge()
+
+            def replay(value):
+                replay_buffer = buffer_info['buffer']
+                if replay_buffer:
+                    self.log.info("Replaying %s buffered messages", len(replay_buffer))
+                    for channel, msg_list in replay_buffer:
+                        stream = self.channels[channel]
+                        self._on_zmq_reply(stream, msg_list)
+
+            connected.add_done_callback(replay)
         else:
             try:
                 self.create_stream()
+                connected = self.nudge()
             except web.HTTPError as e:
                 self.log.error("Error opening stream: %s", e)
                 # WebSockets don't response to traditional error codes so we
@@ -281,8 +347,14 @@ def open(self, kernel_id):
         km.add_restart_callback(self.kernel_id, self.on_kernel_restarted)
         km.add_restart_callback(self.kernel_id, self.on_restart_failed, 'dead')
 
-        for channel, stream in self.channels.items():
-            stream.on_recv_stream(self._on_zmq_reply)
+        def subscribe(value):
+            for channel, stream in self.channels.items():
+                stream.on_recv_stream(self._on_zmq_reply)
+
+        connected.add_done_callback(subscribe)
+
+        return connected
+
 
     def on_message(self, msg):
         if not self.channels: