[3.6] bpo-24484: Avoid race condition in multiprocessing cleanup (GH-2159) (#2166)

* bpo-24484: Avoid race condition in multiprocessing cleanup

The finalizer registry can be mutated while inspected by multiprocessing
at process exit.

* Use test.support.start_threads()

* Add Misc/NEWS.
(cherry picked from commit 1eb6c0074d)

Lib/multiprocessing/util.py

@@ -241,20 +241,28 @@ def _run_finalizers(minpriority=None):
         return
 
     if minpriority is None:
-        f = lambda p : p[0][0] is not None
+        f = lambda p : p[0] is not None
     else:
-        f = lambda p : p[0][0] is not None and p[0][0] >= minpriority
+        f = lambda p : p[0] is not None and p[0] >= minpriority
 
-    items = [x for x in list(_finalizer_registry.items()) if f(x)]
-    items.sort(reverse=True)
+    # Careful: _finalizer_registry may be mutated while this function
+    # is running (either by a GC run or by another thread).
 
-    for key, finalizer in items:
-        sub_debug('calling %s', finalizer)
-        try:
-            finalizer()
-        except Exception:
-            import traceback
-            traceback.print_exc()
+    # list(_finalizer_registry) should be atomic, while
+    # list(_finalizer_registry.items()) is not.
+    keys = [key for key in list(_finalizer_registry) if f(key)]
+    keys.sort(reverse=True)
+
+    for key in keys:
+        finalizer = _finalizer_registry.get(key)
+        # key may have been removed from the registry
+        if finalizer is not None:
+            sub_debug('calling %s', finalizer)
+            try:
+                finalizer()
+            except Exception:
+                import traceback
+                traceback.print_exc()
 
     if minpriority is None:
         _finalizer_registry.clear()