feat: write forked kernel to -forked.json and feed back info to client

Author: maartenbreddels

ipykernel/kernelapp.py

@@ -229,6 +229,8 @@ def init_connection_file(self):
     def init_sockets(self):
         # Create a context, a session, and the kernel sockets.
         self.log.info("Starting the kernel at pid: %i", os.getpid())
+        # Note: don't call .instance(), otherwise forking the kernel does not work
+        # since the forked process will then use the same context
         context = zmq.Context()
         # Uncomment this to try closing the context.
         # atexit.register(context.term)
@@ -510,6 +512,12 @@ def start(self):
             if not getattr(self.io_loop, '_fork_requested', False):
                 keep_running = False
             else:
+                app_template = IPKernelApp(connection_file='forked.json')
+                app_template.write_connection_file()
+                import json
+                with open(app_template.abs_connection_file) as f:
+                    conn = json.load(f)
+
                 pid = os.fork()
                 self.io_loop._fork_requested = False  # reset for parent AND child
                 if pid == 0:
@@ -569,13 +577,14 @@ def new_event_loop(self):
                     # NOTE: we actually start a new kernel, but once this works
                     # we can actually think about reusing the kernel object
                     self.kernel_class.clear_instance()
+                    self.initialize(argv=['-f', app_template.abs_connection_file, '--debug'])
                     self.start()
                     pass
                 else:
                     self.log.debug('Parent kernel will resume')
                     # keep a reference, since the will set this to None
                     post_fork_callback = self.io_loop._post_fork_callback
-                    self.io_loop.add_callback(lambda: post_fork_callback(pid))
+                    self.io_loop.add_callback(lambda: post_fork_callback(pid, conn))
                     self.io_loop._post_fork_callback = None
 
 

ipykernel/kernelbase.py

@@ -165,10 +165,10 @@ def fork(self, stream, ident, parent):
         # information up the callstack
         loop = ioloop.IOLoop.current()
         loop._fork_requested = True
-        def post_fork_callback(pid):
+        def post_fork_callback(pid, conn):
             # we might be able to pass back the port information/connection
             # info file here. This is just a proof of concept
-            reply_content = json_clean({'status': 'ok', 'fork_id': pid})
+            reply_content = json_clean({'status': 'ok', 'fork_id': pid, 'conn': conn})
             metadata = {}
             metadata = self.finish_metadata(parent, metadata, reply_content)