From 3fdb0630e04ad454f071510ba186c5e2f7b6a711 Mon Sep 17 00:00:00 2001
From: Min RK <benjaminrk@gmail.com>
Date: Fri, 18 Dec 2020 11:30:57 +0100
Subject: [PATCH] artificially drop iopub messages

for testing https://github.com/jupyter/jupyter_client/issues/593
---
 ipykernel/iostream.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/ipykernel/iostream.py b/ipykernel/iostream.py
index 4d1e6c95d..476b8eaa6 100644
--- a/ipykernel/iostream.py
+++ b/ipykernel/iostream.py
@@ -32,6 +32,10 @@
 MASTER = 0
 CHILD = 1
 
+# drop the first this-many iopub messages,
+# to mock iopub subscription-propagation delays
+IOPUB_DROP_COUNT = 30 
+
 #-----------------------------------------------------------------------------
 # IO classes
 #-----------------------------------------------------------------------------
@@ -69,6 +73,7 @@ def __init__(self, socket, pipe=False):
         self._setup_event_pipe()
         self.thread = threading.Thread(target=self._thread_main)
         self.thread.daemon = True
+        self.counter = 0
 
     def _thread_main(self):
         """The inner loop that's actually run in a thread"""
@@ -214,6 +219,11 @@ def send_multipart(self, *args, **kwargs):
     def _really_send(self, msg, *args, **kwargs):
         """The callback that actually sends messages"""
         mp_mode = self._check_mp_mode()
+        if self.counter < IOPUB_DROP_COUNT:
+            sys.__stderr__.write("Not sending %r" % msg)
+            sys.__stderr__.flush()
+            self.counter += 1
+            return
 
         if mp_mode != CHILD:
             # we are master, do a regular send