bpo-34172: multiprocessing.Pool leaks resources after being deleted

Due to a circular reference inside the Pool implementation, there
was a resource leak if the object was not closed/terminated.
This patch removes the circular reference, and now the code acts
like the documentation.

Author: tzickel

Lib/multiprocessing/pool.py

@@ -149,8 +149,9 @@ class Pool(object):
     '''
     _wrap_exception = True
 
-    def Process(self, *args, **kwds):
-        return self._ctx.Process(*args, **kwds)
+    @staticmethod
+    def Process(ctx, *args, **kwds):
+        return ctx.Process(*args, **kwds)
 
     def __init__(self, processes=None, initializer=None, initargs=(),
                  maxtasksperchild=None, context=None):
@@ -177,13 +178,15 @@ def __init__(self, processes=None, initializer=None, initargs=(),
 
         self._worker_handler = threading.Thread(
             target=Pool._handle_workers,
-            args=(self, )
+            args=(self._cache, self._taskqueue, self._ctx, self.Process,
+                  self._processes, self._pool, self._inqueue, self._outqueue,
+                  self._initializer, self._initargs, self._maxtasksperchild,
+                  self._wrap_exception)
             )
         self._worker_handler.daemon = True
         self._worker_handler._state = RUN
         self._worker_handler.start()
 
-
         self._task_handler = threading.Thread(
             target=Pool._handle_tasks,
             args=(self._taskqueue, self._quick_put, self._outqueue,
@@ -209,43 +212,61 @@ def __init__(self, processes=None, initializer=None, initargs=(),
             exitpriority=15
             )
 
-    def _join_exited_workers(self):
+    @staticmethod
+    def _join_exited_workers(pool):
         """Cleanup after any worker processes which have exited due to reaching
         their specified lifetime.  Returns True if any workers were cleaned up.
         """
         cleaned = False
-        for i in reversed(range(len(self._pool))):
-            worker = self._pool[i]
+        for i in reversed(range(len(pool))):
+            worker = pool[i]
             if worker.exitcode is not None:
                 # worker exited
                 util.debug('cleaning up worker %d' % i)
                 worker.join()
                 cleaned = True
-                del self._pool[i]
+                del pool[i]
         return cleaned
 
     def _repopulate_pool(self):
+        return self._repopulate_pool_static(self._ctx, self.Process,
+                                            self._processes,
+                                            self._pool, self._inqueue,
+                                            self._outqueue, self._initializer,
+                                            self._initargs,
+                                            self._maxtasksperchild,
+                                            self._wrap_exception)
+
+    @staticmethod
+    def _repopulate_pool_static(ctx, Process, processes, pool, inqueue,
+                                outqueue, initializer, initargs,
+                                maxtasksperchild, wrap_exception):
         """Bring the number of pool processes up to the specified number,
         for use after reaping workers which have exited.
         """
-        for i in range(self._processes - len(self._pool)):
-            w = self.Process(target=worker,
-                             args=(self._inqueue, self._outqueue,
-                                   self._initializer,
-                                   self._initargs, self._maxtasksperchild,
-                                   self._wrap_exception)
-                            )
-            self._pool.append(w)
+        for i in range(processes - len(pool)):
+            w = Process(ctx, target=worker,
+                        args=(inqueue, outqueue,
+                              initializer,
+                              initargs, maxtasksperchild,
+                              wrap_exception)
+                       )
+            pool.append(w)
             w.name = w.name.replace('Process', 'PoolWorker')
             w.daemon = True
             w.start()
             util.debug('added worker')
 
-    def _maintain_pool(self):
+    @classmethod
+    def _maintain_pool(cls, ctx, Process, processes, pool, inqueue, outqueue,
+                       initializer, initargs, maxtasksperchild,
+                       wrap_exception):
         """Clean up any exited workers and start replacements for them.
         """
-        if self._join_exited_workers():
-            self._repopulate_pool()
+        if cls._join_exited_workers(pool):
+            cls._repopulate_pool_static(ctx, Process, processes, pool, inqueue,
+                                        outqueue, initializer, initargs,
+                                        maxtasksperchild, wrap_exception)
 
     def _setup_queues(self):
         self._inqueue = self._ctx.SimpleQueue()
@@ -402,17 +423,21 @@ def _map_async(self, func, iterable, mapper, chunksize=None, callback=None,
         )
         return result
 
-    @staticmethod
-    def _handle_workers(pool):
+    @classmethod
+    def _handle_workers(cls, cache, taskqueue, ctx, Process, processes, pool,
+                        inqueue, outqueue, initializer, initargs,
+                        maxtasksperchild, wrap_exception):
         thread = threading.current_thread()
 
         # Keep maintaining workers until the cache gets drained, unless the pool
         # is terminated.
-        while thread._state == RUN or (pool._cache and thread._state != TERMINATE):
-            pool._maintain_pool()
+        while thread._state == RUN or (cache and thread._state != TERMINATE):
+            cls._maintain_pool(ctx, Process, processes, pool, inqueue,
+                               outqueue, initializer, initargs,
+                               maxtasksperchild, wrap_exception)
             time.sleep(0.1)
         # send sentinel to stop workers
-        pool._taskqueue.put(None)
+        taskqueue.put(None)
         util.debug('worker handler exiting')
 
     @staticmethod
@@ -794,7 +819,7 @@ class ThreadPool(Pool):
     _wrap_exception = False
 
     @staticmethod
-    def Process(*args, **kwds):
+    def Process(ctx, *args, **kwds):
         from .dummy import Process
         return Process(*args, **kwds)
 

Lib/test/_test_multiprocessing.py

@@ -2527,6 +2527,12 @@ def test_release_task_refs(self):
         # they were released too.
         self.assertEqual(CountedObject.n_instances, 0)
 
+    def test_del_pool(self):
+        p = self.Pool(1)
+        wr = weakref.ref(p)
+        del p
+        gc.collect()
+        self.assertIsNone(wr())
 
 def raising():
     raise KeyError("key")

Misc/NEWS.d/next/Library/2018-07-26-10-31-52.bpo-34172.8ovLNi.rst

@@ -0,0 +1,3 @@
+Fixed a circular reference issue inside the multiprocessing.Pool
+implementation that caused a resource leak when not closing/terminating the
+pool before deleting it.