bpo-35424: emit ResourceWarning at multiprocessing.Pool destruction (GH-10974)

multiprocessing.Pool destructor now emits ResourceWarning
if the pool is still running.

Lib/multiprocessing/pool.py

@@ -13,13 +13,14 @@ __all__ = ['Pool', 'ThreadPool']
 # Imports
 #
 
-import threading
-import queue
-import itertools
 import collections
+import itertools
 import os
+import queue
+import threading
 import time
 import traceback
+import warnings
 
 # If threading is available then ThreadPool should be provided.  Therefore
 # we avoid top-level imports which are liable to fail on some systems.
@@ -30,6 +31,7 @@ from . import get_context, TimeoutError
 # Constants representing the state of a pool
 #
 
+INIT = "INIT"
 RUN = "RUN"
 CLOSE = "CLOSE"
 TERMINATE = "TERMINATE"
@@ -154,11 +156,15 @@ class Pool(object):
 
     def __init__(self, processes=None, initializer=None, initargs=(),
                  maxtasksperchild=None, context=None):
+        # Attributes initialized early to make sure that they exist in
+        # __del__() if __init__() raises an exception
+        self._pool = []
+        self._state = INIT
+
         self._ctx = context or get_context()
         self._setup_queues()
         self._taskqueue = queue.SimpleQueue()
         self._cache = {}
-        self._state = RUN
         self._maxtasksperchild = maxtasksperchild
         self._initializer = initializer
         self._initargs = initargs
@@ -172,7 +178,6 @@ class Pool(object):
             raise TypeError('initializer must be a callable')
 
         self._processes = processes
-        self._pool = []
         try:
             self._repopulate_pool()
         except Exception:
@@ -216,6 +221,14 @@ class Pool(object):
                   self._result_handler, self._cache),
             exitpriority=15
             )
+        self._state = RUN
+
+    # Copy globals as function locals to make sure that they are available
+    # during Python shutdown when the Pool is destroyed.
+    def __del__(self, _warn=warnings.warn, RUN=RUN):
+        if self._state == RUN:
+            _warn(f"unclosed running multiprocessing pool {self!r}",
+                  ResourceWarning, source=self)
 
     def __repr__(self):
         cls = self.__class__

Lib/test/_test_multiprocessing.py

@@ -2577,6 +2577,22 @@ class _TestPool(BaseTestCase):
                 pass
         pool.join()
 
+    def test_resource_warning(self):
+        if self.TYPE == 'manager':
+            self.skipTest("test not applicable to manager")
+
+        pool = self.Pool(1)
+        pool.terminate()
+        pool.join()
+
+        # force state to RUN to emit ResourceWarning in __del__()
+        pool._state = multiprocessing.pool.RUN
+
+        with support.check_warnings(('unclosed running multiprocessing pool',
+                                     ResourceWarning)):
+            pool = None
+            support.gc_collect()
+
 
 def raising():
     raise KeyError("key")

Misc/NEWS.d/next/Library/2018-12-06-02-02-28.bpo-35424.gXxOJU.rst

+:class:`multiprocessing.Pool` destructor now emits :exc:`ResourceWarning`
+if the pool is still running.