Backport PR #2916: allow disabling offline message buffering (#2986)

notebook/services/kernels/kernelmanager.py

@@ -83,6 +83,17 @@ def _update_root_dir(self, proposal):
         Only effective if cull_idle_timeout is not 0."""
     )
 
+    buffer_offline_messages = Bool(True, config=True,
+        help="""Whether messages from kernels whose frontends have disconnected should be buffered in-memory.
+
+        When True (default), messages are buffered and replayed on reconnect,
+        avoiding lost messages due to interrupted connectivity.
+
+        Disable if long-running kernels will produce too much output while
+        no frontends are connected.
+        """
+    )
+
     _kernel_buffers = Any()
     @default('_kernel_buffers')
     def _default_kernel_buffers(self):
@@ -105,7 +116,7 @@ def cwd_for_path(self, path):
         while not os.path.isdir(os_path) and os_path != self.root_dir:
             os_path = os.path.dirname(os_path)
         return os_path
-    
+
     @gen.coroutine
     def start_kernel(self, kernel_id=None, path=None, **kwargs):
         """Start a kernel for a session and return its kernel_id.
@@ -148,7 +159,7 @@ def start_kernel(self, kernel_id=None, path=None, **kwargs):
 
         # py2-compat
         raise gen.Return(kernel_id)
-    
+
     def start_buffering(self, kernel_id, session_key, channels):
         """Start buffering messages for a kernel
 
@@ -163,6 +174,12 @@ def start_buffering(self, kernel_id, session_key, channels):
         channels: dict({'channel': ZMQStream})
             The zmq channels whose messages should be buffered.
         """
+
+        if not self.buffer_offline_messages:
+            for channel, stream in channels.items():
+                stream.close()
+            return
+
         self.log.info("Starting buffering for %s", session_key)
         self._check_kernel_id(kernel_id)
         # clear previous buffering state
@@ -182,7 +199,6 @@ def buffer_msg(channel, msg_parts):
         for channel, stream in channels.items():
             stream.on_recv(partial(buffer_msg, channel))
 
-
     def get_buffer(self, kernel_id, session_key):
         """Get the buffer for a given kernel