Detect parent change in more cases on unix (#1271)

ipykernel/kernelapp.py

@@ -220,7 +220,7 @@ def init_poller(self):
             # PID 1 (init) is special and will never go away,
             # only be reassigned.
             # Parent polling doesn't work if ppid == 1 to start with.
-            self.poller = ParentPollerUnix()
+            self.poller = ParentPollerUnix(parent_pid=self.parent_handle)
 
     def _try_bind_socket(self, s, port):
         iface = f"{self.transport}://{self.ip}"

ipykernel/parentpoller.py

@@ -22,19 +22,41 @@ class ParentPollerUnix(Thread):
     when the parent process no longer exists.
     """
 
-    def __init__(self):
-        """Initialize the poller."""
+    def __init__(self, parent_pid=0):
+        """Initialize the poller.
+
+        Parameters
+        ----------
+        parent_handle : int, optional
+            If provided, the program will terminate immediately when
+            process parent is no longer this original parent.
+        """
         super().__init__()
+        self.parent_pid = parent_pid
         self.daemon = True
 
     def run(self):
         """Run the poller."""
         # We cannot use os.waitpid because it works only for child processes.
         from errno import EINTR
 
+        # before start, check if the passed-in parent pid is valid
+        original_ppid = os.getppid()
+        if original_ppid != self.parent_pid:
+            self.parent_pid = 0
+
+        get_logger().debug(
+            "%s: poll for parent change with original parent pid=%d",
+            type(self).__name__,
+            self.parent_pid,
+        )
+
         while True:
             try:
-                if os.getppid() == 1:
+                ppid = os.getppid()
+                parent_is_init = not self.parent_pid and ppid == 1
+                parent_has_changed = self.parent_pid and ppid != self.parent_pid
+                if parent_is_init or parent_has_changed:
                     get_logger().warning("Parent appears to have exited, shutting down.")
                     os._exit(1)
                 time.sleep(1.0)

tests/test_parentpoller.py

@@ -9,7 +9,7 @@
 
 
 @pytest.mark.skipif(os.name == "nt", reason="only works on posix")
-def test_parent_poller_unix():
+def test_parent_poller_unix_to_pid1():
     poller = ParentPollerUnix()
     with mock.patch("os.getppid", lambda: 1):  # noqa: PT008
 
@@ -27,6 +27,22 @@ def mock_getppid():
         poller.run()
 
 
+@pytest.mark.skipif(os.name == "nt", reason="only works on posix")
+def test_parent_poller_unix_reparent_not_pid1():
+    parent_pid = 221
+    parent_pids = iter([parent_pid, parent_pid - 1])
+
+    poller = ParentPollerUnix(parent_pid=parent_pid)
+
+    with mock.patch("os.getppid", lambda: next(parent_pids)):  # noqa: PT008
+
+        def exit_mock(*args):
+            sys.exit(1)
+
+        with mock.patch("os._exit", exit_mock), pytest.raises(SystemExit):
+            poller.run()
+
+
 @pytest.mark.skipif(os.name != "nt", reason="only works on windows")
 def test_parent_poller_windows():
     poller = ParentPollerWindows(interrupt_handle=1)