Fix: Publish the heartbeat port only after binding succeeds

ipykernel/heartbeat.py

@@ -13,6 +13,7 @@
 
 import errno
 import socket
+from concurrent.futures import Future
 from pathlib import Path
 from threading import Thread
 
@@ -46,6 +47,7 @@ def __init__(self, context, addr=None, *, curve_publickey=None, curve_secretkey=
             addr = ("tcp", localhost(), 0)
         Thread.__init__(self, name="Heartbeat")
         self.context = context
+        self._bind_future: Future[None] = Future()
         self.transport, self.ip, self.port = addr
         self.original_port = self.port
         self._curve_publickey = curve_publickey
@@ -104,6 +106,11 @@ def _bind_socket(self):
             else:
                 return
 
+    def start(self):
+        """Start the heartbeat thread and wait for its socket to bind."""
+        super().start()
+        self._bind_future.result()
+
     def run(self):
         """Run the heartbeat thread."""
         self.name = "Heartbeat"
@@ -119,6 +126,9 @@ def run(self):
             self.socket.close()
             raise
 
+        self.addr = (self.ip, self.port)
+        self._bind_future.set_result(None)
+
         while True:
             try:
                 zmq.device(zmq.QUEUE, self.socket, self.socket)

ipykernel/kernelapp.py

@@ -444,9 +444,10 @@ def init_heartbeat(self):
             curve_publickey=self.curve_publickey,
             curve_secretkey=self.curve_secretkey,
         )
+        # Binding may select a new port if the original candidate was taken.
+        self.heartbeat.start()
         self.hb_port = self.heartbeat.port
         self.log.debug("Heartbeat REP Channel on port: %i", self.hb_port)
-        self.heartbeat.start()
 
     def close(self):
         """Close zmq sockets in an orderly fashion"""