Fix bpo-30596: Add close() method to multiprocessing.Process (#2010)

* Fix bpo-30596: Add close() method to multiprocessing.Process

* Raise ValueError if close() is called before the Process is finished running

* Add docs

* Add NEWS blurb

Lib/multiprocessing/process.py

@@ -76,6 +76,7 @@ def __init__(self, group=None, target=None, name=None, args=(), kwargs={},
         self._config = _current_process._config.copy()
         self._parent_pid = os.getpid()
         self._popen = None
+        self._closed = False
         self._target = target
         self._args = tuple(args)
         self._kwargs = dict(kwargs)
@@ -85,6 +86,10 @@ def __init__(self, group=None, target=None, name=None, args=(), kwargs={},
             self.daemon = daemon
         _dangling.add(self)
 
+    def _check_closed(self):
+        if self._closed:
+            raise ValueError("process object is closed")
+
     def run(self):
         '''
         Method to be run in sub-process; can be overridden in sub-class
@@ -96,6 +101,7 @@ def start(self):
         '''
         Start child process
         '''
+        self._check_closed()
         assert self._popen is None, 'cannot start a process twice'
         assert self._parent_pid == os.getpid(), \
                'can only start a process object created by current process'
@@ -110,12 +116,14 @@ def terminate(self):
         '''
         Terminate process; sends SIGTERM signal or uses TerminateProcess()
         '''
+        self._check_closed()
         self._popen.terminate()
 
     def join(self, timeout=None):
         '''
         Wait until child process terminates
         '''
+        self._check_closed()
         assert self._parent_pid == os.getpid(), 'can only join a child process'
         assert self._popen is not None, 'can only join a started process'
         res = self._popen.wait(timeout)
@@ -126,6 +134,7 @@ def is_alive(self):
         '''
         Return whether process is alive
         '''
+        self._check_closed()
         if self is _current_process:
             return True
         assert self._parent_pid == os.getpid(), 'can only test a child process'
@@ -134,6 +143,23 @@ def is_alive(self):
         self._popen.poll()
         return self._popen.returncode is None
 
+    def close(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.
+        '''
+        if self._popen is not None:
+            if self._popen.poll() is None:
+                raise ValueError("Cannot close a process while it is still running. "
+                                 "You should first call join() or terminate().")
+            self._popen.close()
+            self._popen = None
+            del self._sentinel
+            _children.discard(self)
+        self._closed = True
+
     @property
     def name(self):
         return self._name
@@ -174,6 +200,7 @@ def exitcode(self):
         '''
         Return exit code of process or `None` if it has yet to stop
         '''
+        self._check_closed()
         if self._popen is None:
             return self._popen
         return self._popen.poll()
@@ -183,6 +210,7 @@ def ident(self):
         '''
         Return identifier (PID) of process or `None` if it has yet to start
         '''
+        self._check_closed()
         if self is _current_process:
             return os.getpid()
         else:
@@ -196,6 +224,7 @@ def sentinel(self):
         Return a file descriptor (Unix) or handle (Windows) suitable for
         waiting for process termination.
         '''
+        self._check_closed()
         try:
             return self._sentinel
         except AttributeError:
@@ -204,6 +233,8 @@ def sentinel(self):
     def __repr__(self):
         if self is _current_process:
             status = 'started'
+        elif self._closed:
+            status = 'closed'
         elif self._parent_pid != os.getpid():
             status = 'unknown'
         elif self._popen is None:
@@ -295,6 +326,7 @@ def __init__(self):
         self._name = 'MainProcess'
         self._parent_pid = None
         self._popen = None
+        self._closed = False
         self._config = {'authkey': AuthenticationString(os.urandom(32)),
                         'semprefix': '/mp'}
         # Note that some versions of FreeBSD only allow named
@@ -307,6 +339,9 @@ def __init__(self):
         # Everything in self._config will be inherited by descendant
         # processes.
 
+    def close(self):
+        pass
+
 
 _current_process = _MainProcess()
 _process_counter = itertools.count(1)