Bug report

I have a project that uses threading.Thread and multiprocessing.Process for handling concurrent tasks. While under certain circumstances, there is the possibility that when I try new aProcess() instance and call theProcess.start() function, an error would occur:
AttributeError: 'NoneType' object has no attribute 'poll'

I tried debugging my project and found that under themultiprocessing.process module a global variable_children is used for managing all child processes, the variable is not thread-safe and the error would occur occasionally. I made a simple test code with minor modification on themultiprocessing.process

test.py

frommultiprocessingimportProcessfromthreadingimportThreaddefhelper():time.sleep(0.1)return1defclose_process(p_:Process):p.terminate()p_.close()if__name__=="__main__":process_list= []for_inrange(1):p=Process(target=helper)p.start()time.sleep(0.2)process_list.append(p)for_inrange(1):t=Thread(target=close_process,args=(process_list[_],))t.start()new_p=Process(target=helper)time.sleep(0.2)new_p.start()

multiprocessing.process.py class BaseProcess

defclose(self):'''        Close the Process object.        This method releases resources held by the Process object.  It is        an error to call this method if the child process is still running.        '''ifself._popenisnotNone:ifself._popen.poll()isNone:raiseValueError("Cannot close a process while it is still running. ""You should first call join() or terminate().")self._popen.close()self._popen=Noneimporttimetime.sleep(5)delself._sentinel_children.discard(self)self._closed=True

I simply addtime.sleep() forBaseProcess.close() class and now every time I run the test.py, the error would occur.

Your environment

• CPython versions tested on: python 3.7
• Operating system and architecture: windows 10 or linux el7

Linked PRs

• gh-104536: Improvemultiprocessing.process._cleanup logic #104537
• [3.11] gh-104536: Improvemultiprocessing.process._cleanup logic (GH-104537) #104737