add comments to explain signal handling under jupyterhub (#1452)

Author: oliver-sanders

jupyter_server/serverapp.py

@@ -2410,7 +2410,14 @@ def init_signal(self) -> None:
             signal.signal(signal.SIGINFO, self._signal_info)
 
     def _handle_sigint(self, sig: t.Any, frame: t.Any) -> None:
-        """SIGINT handler spawns confirmation dialog"""
+        """SIGINT handler spawns confirmation dialog
+
+        Note:
+            JupyterHub replaces this method with _signal_stop
+            in order to bypass the interactive prompt.
+            https://github.com/jupyterhub/jupyterhub/pull/4864
+
+        """
         # register more forceful signal handler for ^C^C case
         signal.signal(signal.SIGINT, self._signal_stop)
         # request confirmation dialog in bg thread, to avoid
@@ -2468,7 +2475,13 @@ def _confirm_exit(self) -> None:
         self.io_loop.add_callback_from_signal(self._restore_sigint_handler)
 
     def _signal_stop(self, sig: t.Any, frame: t.Any) -> None:
-        """Handle a stop signal."""
+        """Handle a stop signal.
+
+        Note:
+            JupyterHub configures this method to be called for SIGINT.
+            https://github.com/jupyterhub/jupyterhub/pull/4864
+
+        """
         self.log.critical(_i18n("received signal %s, stopping"), sig)
         self.stop(from_signal=True)