Agent: Kill hung child processes on exit

Plugins that hang can prevent the agent from closing. On agent shutdown,
we'll forcefully kill all child processes that have not shutdown on
their own.

Issue #3557

monkey/infection_monkey/main.py

@@ -18,6 +18,8 @@
 from pathlib import Path
 from typing import Sequence, Tuple, Union
 
+import psutil
+
 # dummy import for pyinstaller
 # noinspection PyUnresolvedReferences
 from common.common_consts import AGENT_OTP_ENVIRONMENT_VARIABLE
@@ -179,6 +181,22 @@ def _run_agent(
         logger.exception(
             "Exception thrown from monkey's cleanup function: More info: {}".format(err)
         )
+    finally:
+        _kill_hung_child_processes(logger)
+
+
+def _kill_hung_child_processes(logger: logging.Logger):
+    for p in psutil.Process().children(recursive=True):
+        if "multiprocessing.resource_tracker" in p.cmdline()[2]:
+            # This process will clean itself up, but no other processes should be running at
+            # this time.
+            continue
+
+        logger.warning(
+            "Killing hung child process: "
+            f"pid={p.pid}, name={p.name()}, status={p.status()}, cmdline={p.cmdline()}"
+        )
+        p.kill()
 
 
 if "__main__" == __name__: