python: kill the sidecar when its parent dies

PR_SET_PDEATHSIG on the spawned lightpanda mcp process (Linux) so a
SIGKILLed or crashed interpreter cannot leak a browser process — a
2h44m orphan surfaced during the v6 benchmark campaign. Regression
test kills a parent and asserts the sidecar exits.

bindings/python/lightpanda/client.py

@@ -29,6 +29,19 @@ _SPAWN_ATTEMPTS = 3
 _READY_TIMEOUT = 15.0
 
 
+def _die_with_parent():
+    """Linux: ask the kernel to SIGTERM the sidecar when the parent dies,
+    so a SIGKILLed interpreter can't leak a browser process."""
+    try:
+        import ctypes
+        import signal as _signal
+
+        PR_SET_PDEATHSIG = 1
+        ctypes.CDLL(None, use_errno=True).prctl(PR_SET_PDEATHSIG, _signal.SIGTERM)
+    except Exception:
+        pass
+
+
 def find_binary(explicit: str | os.PathLike | None = None) -> Path:
     """Locate the lightpanda binary: explicit arg, $LIGHTPANDA_BIN, the
     bundled package copy, then PATH."""
@@ -91,6 +104,7 @@ class Client:
                 stdout=subprocess.DEVNULL,
                 stderr=stderr,
                 env=child_env,
+                preexec_fn=_die_with_parent if sys.platform == "linux" else None,
             )
             try:
                 self._wait_ready(proc, port)

bindings/python/tests/test_browser.py

@@ -1,3 +1,10 @@
+import os
+import signal
+import subprocess
+import sys
+import time
+from pathlib import Path
+
 import pytest
 
 from lightpanda import ToolError, run_script
@@ -64,6 +71,40 @@ def test_extra_args_passthrough(binary, fixture_url, tmp_path):
     assert cache_dir.exists()
 
 
+@pytest.mark.skipif(sys.platform != "linux", reason="PDEATHSIG is Linux-only")
+def test_sidecar_dies_with_killed_parent(binary, tmp_path):
+    child_src = tmp_path / "spawn_and_hang.py"
+    child_src.write_text(
+        "import os, sys, time\n"
+        "from lightpanda import Browser\n"
+        "b = Browser(binary=sys.argv[1])\n"
+        "print(b._client._proc.pid, flush=True)\n"
+        "time.sleep(60)\n"
+    )
+    env = dict(os.environ, PYTHONPATH=str(Path(__file__).parent.parent))
+    child = subprocess.Popen(
+        [sys.executable, str(child_src), binary],
+        stdout=subprocess.PIPE, text=True, env=env,
+    )
+    sidecar_pid = int(child.stdout.readline())
+    assert _alive(sidecar_pid)
+
+    os.kill(child.pid, signal.SIGKILL)
+    child.wait()
+    deadline = time.monotonic() + 5
+    while time.monotonic() < deadline and _alive(sidecar_pid):
+        time.sleep(0.1)
+    assert not _alive(sidecar_pid), "sidecar survived its parent's SIGKILL"
+
+
+def _alive(pid):
+    try:
+        os.kill(pid, 0)
+        return True
+    except ProcessLookupError:
+        return False
+
+
 def test_run_script(binary, fixture_url, tmp_path):
     script = tmp_path / "visit.js"
     script.write_text('const page = new Page();\nawait page.goto("$LP_TEST_URL");\n')