Cannot use vizplugins together with subprocesses

[mbway]

I wanted to track CPU and memory usage of a program that uses subprocesses and found that using vizplugins crashes when the first subprocess closes.

For example:

from concurrent.futures import ProcessPoolExecutor

def multiply(a, b):
    return a * b

with ProcessPoolExecutor(max_workers=1) as e:
    f = e.submit(multiply, 1, 2)
    print(f.result())

Run with

viztracer --plugin "vizplugins --cpu_usage --memory_usage" -- example.py

Crashes with:

AssertionError: can only test a child process
  File "/usr/lib/python3.11/multiprocessing/util.py", line 300, in _run_finalizers
    finalizer()
  File "/usr/lib/python3.11/multiprocessing/util.py", line 224, in __call__
    res = self._callback(*self._args, **self._kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<venv>/lib/python3.11/site-packages/viztracer/viztracer.py", line 303, in exit_routine
    self.stop()
  File "<venv>/lib/python3.11/site-packages/viztracer/viztracer.py", line 186, in stop
    self._plugin_manager.event("post-stop")
  File "<venv>/lib/python3.11/site-packages/viztracer/vizplugin.py", line 122, in event
    self._send_message(plugin, "event", {"when": when})
  File "<venv>/lib/python3.11/site-packages/viztracer/vizplugin.py", line 110, in _send_message
    ret = plugin.message(m_type, payload)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<venv>/lib/python3.11/site-packages/vizplugins/psutil_monitor.py", line 27, in message
    return self.stop_recording()
           ^^^^^^^^^^^^^^^^^^^^^
  File "<venv>/lib/python3.11/site-packages/vizplugins/psutil_monitor.py", line 47, in stop_recording
    self.recordings.append(self.send_action("stop"))
                           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "<venv>/lib/python3.11/site-packages/vizplugins/psutil_monitor.py", line 75, in send_action
    if not self.cpu_process.is_alive():
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/multiprocessing/process.py", line 160, in is_alive
    assert self._parent_pid == os.getpid(), 'can only test a child process'
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: can only test a child process

I ended up creating a custom reporter that logs system-wide CPU and RAM usage to a VizCounter from a thread of the main process as a workaround.