[diffusion] fix: don't self-kill diffusion worker when PID 1 is the real parent (#31361)

Co-authored-by: Mick <mickjagger19@icloud.com>

python/sglang/multimodal_gen/test/unit/test_utils_parent_death.py

@@ -0,0 +1,55 @@
+import signal
+import unittest
+from unittest.mock import Mock, patch
+
+from sglang.multimodal_gen import utils
+
+
+class TestKillItselfWhenParentDied(unittest.TestCase):
+    def _run_linux_case(self, parent_pids):
+        libc = Mock()
+        libc.prctl.return_value = 0
+
+        with (
+            patch.object(utils.sys, "platform", "linux"),
+            patch.object(utils.ctypes, "CDLL", return_value=libc) as cdll,
+            patch.object(utils.os, "getppid", side_effect=parent_pids) as getppid,
+            patch.object(utils.os, "getpid", return_value=12345),
+            patch.object(utils.os, "kill") as kill,
+        ):
+            utils.kill_itself_when_parent_died()
+
+        return libc, cdll, getppid, kill
+
+    def test_stable_pid_1_parent_does_not_self_kill(self):
+        libc, cdll, getppid, kill = self._run_linux_case([1, 1])
+
+        cdll.assert_called_once_with("libc.so.6", use_errno=True)
+        libc.prctl.assert_called_once_with(1, signal.SIGKILL)
+        self.assertEqual(getppid.call_count, 2)
+        kill.assert_not_called()
+
+    def test_reparent_to_init_self_kills(self):
+        _, _, _, kill = self._run_linux_case([1000, 1])
+
+        kill.assert_called_once_with(12345, signal.SIGKILL)
+
+    def test_reparent_to_subreaper_self_kills(self):
+        _, _, _, kill = self._run_linux_case([1000, 42])
+
+        kill.assert_called_once_with(12345, signal.SIGKILL)
+
+    def test_non_linux_noop(self):
+        with (
+            patch.object(utils.sys, "platform", "darwin"),
+            patch.object(utils.ctypes, "CDLL") as cdll,
+            patch.object(utils.os, "kill") as kill,
+        ):
+            utils.kill_itself_when_parent_died()
+
+        cdll.assert_not_called()
+        kill.assert_not_called()
+
+
+if __name__ == "__main__":
+    unittest.main()

python/sglang/multimodal_gen/utils.py

@@ -533,11 +533,17 @@ def kill_itself_when_parent_died() -> None:
 
     # keep GPU workers tied to the CLI process even if the parent is SIGKILLed
     PR_SET_PDEATHSIG = 1
+    # Capture parent before arming PDEATHSIG: if the parent already died in the
+    # fork->prctl window, PDEATHSIG won't fire, so detect the reparent explicitly.
+    parent_pid = os.getppid()
     libc = ctypes.CDLL("libc.so.6", use_errno=True)
     if libc.prctl(PR_SET_PDEATHSIG, signal.SIGKILL) != 0:
         err = ctypes.get_errno()
         raise OSError(err, os.strerror(err))
-    if os.getppid() == 1:
+    # getppid() changing means we were reparented (parent gone). Comparing to the
+    # captured pid instead of "== 1" avoids self-killing when PID 1 is the real
+    # parent (e.g. running as a container's init process).
+    if os.getppid() != parent_pid:
         os.kill(os.getpid(), signal.SIGKILL)