Issue #7105: Make WeakKeyDictionary and WeakValueDictionary robust against

the destruction of weakref'ed objects while iterating.

Doc/library/weakref.rst

@@ -159,7 +159,7 @@ than needed.
 
 .. method:: WeakKeyDictionary.keyrefs()
 
-   Return an :term:`iterator` that yields the weak references to the keys.
+   Return an iterable of the weak references to the keys.
 
 
 .. class:: WeakValueDictionary([dict])
@@ -182,7 +182,7 @@ These method have the same issues as the and :meth:`keyrefs` method of
 
 .. method:: WeakValueDictionary.valuerefs()
 
-   Return an :term:`iterator` that yields the weak references to the values.
+   Return an iterable of the weak references to the values.
 
 
 .. class:: WeakSet([elements])

Lib/_weakrefset.py

@@ -6,18 +6,57 @@ from _weakref import ref
 
 __all__ = ['WeakSet']
 
+
+class _IterationGuard:
+    # This context manager registers itself in the current iterators of the
+    # weak container, such as to delay all removals until the context manager
+    # exits.
+    # This technique should be relatively thread-safe (since sets are).
+
+    def __init__(self, weakcontainer):
+        # Don't create cycles
+        self.weakcontainer = ref(weakcontainer)
+
+    def __enter__(self):
+        w = self.weakcontainer()
+        if w is not None:
+            w._iterating.add(self)
+        return self
+
+    def __exit__(self, e, t, b):
+        w = self.weakcontainer()
+        if w is not None:
+            s = w._iterating
+            s.remove(self)
+            if not s:
+                w._commit_removals()
+
+
 class WeakSet:
     def __init__(self, data=None):
         self.data = set()
         def _remove(item, selfref=ref(self)):
             self = selfref()
             if self is not None:
+                if self._iterating:
+                    self._pending_removals.append(item)
+                else:
                     self.data.discard(item)
         self._remove = _remove
+        # A list of keys to be removed
+        self._pending_removals = []
+        self._iterating = set()
         if data is not None:
             self.update(data)
 
+    def _commit_removals(self):
+        l = self._pending_removals
+        discard = self.data.discard
+        while l:
+            discard(l.pop())
+
     def __iter__(self):
+        with _IterationGuard(self):
             for itemref in self.data:
                 item = itemref()
                 if item is not None:
@@ -34,15 +73,21 @@ class WeakSet:
                 getattr(self, '__dict__', None))
 
     def add(self, item):
+        if self._pending_removals:
+            self._commit_removals()
         self.data.add(ref(item, self._remove))
 
     def clear(self):
+        if self._pending_removals:
+            self._commit_removals()
         self.data.clear()
 
     def copy(self):
         return self.__class__(self)
 
     def pop(self):
+        if self._pending_removals:
+            self._commit_removals()
         while True:
             try:
                 itemref = self.data.pop()
@@ -53,17 +98,24 @@ class WeakSet:
                 return item
 
     def remove(self, item):
+        if self._pending_removals:
+            self._commit_removals()
         self.data.remove(ref(item))
 
     def discard(self, item):
+        if self._pending_removals:
+            self._commit_removals()
         self.data.discard(ref(item))
 
     def update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         if isinstance(other, self.__class__):
             self.data.update(other.data)
         else:
             for element in other:
                 self.add(element)
+
     def __ior__(self, other):
         self.update(other)
         return self
@@ -82,11 +134,15 @@ class WeakSet:
     __sub__ = difference
 
     def difference_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         if self is other:
             self.data.clear()
         else:
             self.data.difference_update(ref(item) for item in other)
     def __isub__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         if self is other:
             self.data.clear()
         else:
@@ -98,8 +154,12 @@ class WeakSet:
     __and__ = intersection
 
     def intersection_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         self.data.intersection_update(ref(item) for item in other)
     def __iand__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         self.data.intersection_update(ref(item) for item in other)
         return self
 
@@ -127,11 +187,15 @@ class WeakSet:
     __xor__ = symmetric_difference
 
     def symmetric_difference_update(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         if self is other:
             self.data.clear()
         else:
             self.data.symmetric_difference_update(ref(item) for item in other)
     def __ixor__(self, other):
+        if self._pending_removals:
+            self._commit_removals()
         if self is other:
             self.data.clear()
         else:

Lib/test/test_weakref.py

@@ -4,6 +4,8 @@ import unittest
 import collections
 import weakref
 import operator
+import contextlib
+import copy
 
 from test import support
 
@@ -788,6 +790,10 @@ class Object:
         self.arg = arg
     def __repr__(self):
         return "<Object %r>" % self.arg
+    def __eq__(self, other):
+        if isinstance(other, Object):
+            return self.arg == other.arg
+        return NotImplemented
     def __lt__(self, other):
         if isinstance(other, Object):
             return self.arg < other.arg
@@ -935,6 +941,87 @@ class MappingTestCase(TestBase):
         self.assertFalse(values,
                      "itervalues() did not touch all values")
 
+    def check_weak_destroy_while_iterating(self, dict, objects, iter_name):
+        n = len(dict)
+        it = iter(getattr(dict, iter_name)())
+        next(it)             # Trigger internal iteration
+        # Destroy an object
+        del objects[-1]
+        gc.collect()    # just in case
+        # We have removed either the first consumed object, or another one
+        self.assertIn(len(list(it)), [len(objects), len(objects) - 1])
+        del it
+        # The removal has been committed
+        self.assertEqual(len(dict), n - 1)
+
+    def check_weak_destroy_and_mutate_while_iterating(self, dict, testcontext):
+        # Check that we can explicitly mutate the weak dict without
+        # interfering with delayed removal.
+        # `testcontext` should create an iterator, destroy one of the
+        # weakref'ed objects and then return a new key/value pair corresponding
+        # to the destroyed object.
+        with testcontext() as (k, v):
+            self.assertFalse(k in dict)
+        with testcontext() as (k, v):
+            self.assertRaises(KeyError, dict.__delitem__, k)
+        self.assertFalse(k in dict)
+        with testcontext() as (k, v):
+            self.assertRaises(KeyError, dict.pop, k)
+        self.assertFalse(k in dict)
+        with testcontext() as (k, v):
+            dict[k] = v
+        self.assertEqual(dict[k], v)
+        ddict = copy.copy(dict)
+        with testcontext() as (k, v):
+            dict.update(ddict)
+        self.assertEqual(dict, ddict)
+        with testcontext() as (k, v):
+            dict.clear()
+        self.assertEqual(len(dict), 0)
+
+    def test_weak_keys_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        dict, objects = self.make_weak_keyed_dict()
+        self.check_weak_destroy_while_iterating(dict, objects, 'keys')
+        self.check_weak_destroy_while_iterating(dict, objects, 'items')
+        self.check_weak_destroy_while_iterating(dict, objects, 'values')
+        self.check_weak_destroy_while_iterating(dict, objects, 'keyrefs')
+        dict, objects = self.make_weak_keyed_dict()
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(dict.items())
+                next(it)
+                # Schedule a key/value for removal and recreate it
+                v = objects.pop().arg
+                gc.collect()      # just in case
+                yield Object(v), v
+            finally:
+                it = None           # should commit all removals
+        self.check_weak_destroy_and_mutate_while_iterating(dict, testcontext)
+
+    def test_weak_values_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        dict, objects = self.make_weak_valued_dict()
+        self.check_weak_destroy_while_iterating(dict, objects, 'keys')
+        self.check_weak_destroy_while_iterating(dict, objects, 'items')
+        self.check_weak_destroy_while_iterating(dict, objects, 'values')
+        self.check_weak_destroy_while_iterating(dict, objects, 'itervaluerefs')
+        self.check_weak_destroy_while_iterating(dict, objects, 'valuerefs')
+        dict, objects = self.make_weak_valued_dict()
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(dict.items())
+                next(it)
+                # Schedule a key/value for removal and recreate it
+                k = objects.pop().arg
+                gc.collect()      # just in case
+                yield k, Object(k)
+            finally:
+                it = None           # should commit all removals
+        self.check_weak_destroy_and_mutate_while_iterating(dict, testcontext)
+
     def test_make_weak_keyed_dict_from_dict(self):
         o = Object(3)
         dict = weakref.WeakKeyDictionary({o:364})

Lib/test/test_weakset.py

@@ -10,6 +10,8 @@ import sys
 import warnings
 import collections
 from collections import UserString as ustr
+import gc
+import contextlib
 
 
 class Foo:
@@ -307,6 +309,54 @@ class TestWeakSet(unittest.TestCase):
         self.assertFalse(self.s == WeakSet([Foo]))
         self.assertFalse(self.s == 1)
 
+    def test_weak_destroy_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        # Create new items to be sure no-one else holds a reference
+        items = [ustr(c) for c in ('a', 'b', 'c')]
+        s = WeakSet(items)
+        it = iter(s)
+        next(it)             # Trigger internal iteration
+        # Destroy an item
+        del items[-1]
+        gc.collect()    # just in case
+        # We have removed either the first consumed items, or another one
+        self.assertIn(len(list(it)), [len(items), len(items) - 1])
+        del it
+        # The removal has been committed
+        self.assertEqual(len(s), len(items))
+
+    def test_weak_destroy_and_mutate_while_iterating(self):
+        # Issue #7105: iterators shouldn't crash when a key is implicitly removed
+        items = [ustr(c) for c in string.ascii_letters]
+        s = WeakSet(items)
+        @contextlib.contextmanager
+        def testcontext():
+            try:
+                it = iter(s)
+                next(it)
+                # Schedule an item for removal and recreate it
+                u = ustr(str(items.pop()))
+                gc.collect()      # just in case
+                yield u
+            finally:
+                it = None           # should commit all removals
+
+        with testcontext() as u:
+            self.assertFalse(u in s)
+        with testcontext() as u:
+            self.assertRaises(KeyError, s.remove, u)
+        self.assertFalse(u in s)
+        with testcontext() as u:
+            s.add(u)
+        self.assertTrue(u in s)
+        t = s.copy()
+        with testcontext() as u:
+            s.update(t)
+        self.assertEqual(len(s), len(t))
+        with testcontext() as u:
+            s.clear()
+        self.assertEqual(len(s), 0)
+
 
 def test_main(verbose=None):
     support.run_unittest(TestWeakSet)

Lib/weakref.py

@@ -18,7 +18,7 @@ from _weakref import (
      ProxyType,
      ReferenceType)
 
-from _weakrefset import WeakSet
+from _weakrefset import WeakSet, _IterationGuard
 
 import collections  # Import after _weakref to avoid circular import.
 
@@ -46,11 +46,25 @@ class WeakValueDictionary(collections.MutableMapping):
         def remove(wr, selfref=ref(self)):
             self = selfref()
             if self is not None:
+                if self._iterating:
+                    self._pending_removals.append(wr.key)
+                else:
                     del self.data[wr.key]
         self._remove = remove
+        # A list of keys to be removed
+        self._pending_removals = []
+        self._iterating = set()
         self.data = d = {}
         self.update(*args, **kw)
 
+    def _commit_removals(self):
+        l = self._pending_removals
+        d = self.data
+        # We shouldn't encounter any KeyError, because this method should
+        # always be called *before* mutating the dict.
+        while l:
+            del d[l.pop()]
+
     def __getitem__(self, key):
         o = self.data[key]()
         if o is None:
@@ -59,6 +73,8 @@ class WeakValueDictionary(collections.MutableMapping):
             return o
 
     def __delitem__(self, key):
+        if self._pending_removals:
+            self._commit_removals()
         del self.data[key]
 
     def __len__(self):
@@ -75,6 +91,8 @@ class WeakValueDictionary(collections.MutableMapping):
         return "<WeakValueDictionary at %s>" % id(self)
 
     def __setitem__(self, key, value):
+        if self._pending_removals:
+            self._commit_removals()
         self.data[key] = KeyedRef(value, self._remove, key)
 
     def copy(self):
@@ -110,24 +128,19 @@ class WeakValueDictionary(collections.MutableMapping):
                 return o
 
     def items(self):
-        L = []
-        for key, wr in self.data.items():
-            o = wr()
-            if o is not None:
-                L.append((key, o))
-        return L
-
-    def items(self):
-        for wr in self.data.values():
-            value = wr()
-            if value is not None:
-                yield wr.key, value
+        with _IterationGuard(self):
+            for k, wr in self.data.items():
+                v = wr()
+                if v is not None:
+                    yield k, v
 
     def keys(self):
-        return iter(self.data.keys())
+        with _IterationGuard(self):
+            for k, wr in self.data.items():
+                if wr() is not None:
+                    yield k
 
-    def __iter__(self):
-        return iter(self.data.keys())
+    __iter__ = keys
 
     def itervaluerefs(self):
         """Return an iterator that yields the weak references to the values.
@@ -139,15 +152,20 @@ class WeakValueDictionary(collections.MutableMapping):
         keep the values around longer than needed.
 
         """
-        return self.data.values()
+        with _IterationGuard(self):
+            for wr in self.data.values():
+                yield wr
 
     def values(self):
+        with _IterationGuard(self):
             for wr in self.data.values():
                 obj = wr()
                 if obj is not None:
                     yield obj
 
     def popitem(self):
+        if self._pending_removals:
+            self._commit_removals()
         while 1:
             key, wr = self.data.popitem()
             o = wr()
@@ -155,6 +173,8 @@ class WeakValueDictionary(collections.MutableMapping):
                 return key, o
 
     def pop(self, key, *args):
+        if self._pending_removals:
+            self._commit_removals()
         try:
             o = self.data.pop(key)()
         except KeyError:
@@ -170,12 +190,16 @@ class WeakValueDictionary(collections.MutableMapping):
         try:
             wr = self.data[key]
         except KeyError:
+            if self._pending_removals:
+                self._commit_removals()
             self.data[key] = KeyedRef(default, self._remove, key)
             return default
         else:
             return wr()
 
     def update(self, dict=None, **kwargs):
+        if self._pending_removals:
+            self._commit_removals()
         d = self.data
         if dict is not None:
             if not hasattr(dict, "items"):
@@ -195,7 +219,7 @@ class WeakValueDictionary(collections.MutableMapping):
         keep the values around longer than needed.
 
         """
-        return self.data.values()
+        return list(self.data.values())
 
 
 class KeyedRef(ref):
@@ -235,9 +259,29 @@ class WeakKeyDictionary(collections.MutableMapping):
         def remove(k, selfref=ref(self)):
             self = selfref()
             if self is not None:
+                if self._iterating:
+                    self._pending_removals.append(k)
+                else:
                     del self.data[k]
         self._remove = remove
-        if dict is not None: self.update(dict)
+        # A list of dead weakrefs (keys to be removed)
+        self._pending_removals = []
+        self._iterating = set()
+        if dict is not None:
+            self.update(dict)
+
+    def _commit_removals(self):
+        # NOTE: We don't need to call this method before mutating the dict,
+        # because a dead weakref never compares equal to a live weakref,
+        # even if they happened to refer to equal objects.
+        # However, it means keys may already have been removed.
+        l = self._pending_removals
+        d = self.data
+        while l:
+            try:
+                del d[l.pop()]
+            except KeyError:
+                pass
 
     def __delitem__(self, key):
         del self.data[ref(key)]
@@ -284,34 +328,26 @@ class WeakKeyDictionary(collections.MutableMapping):
         return wr in self.data
 
     def items(self):
+        with _IterationGuard(self):
             for wr, value in self.data.items():
                 key = wr()
                 if key is not None:
                     yield key, value
 
-    def keyrefs(self):
-        """Return an iterator that yields the weak references to the keys.
-
-        The references are not guaranteed to be 'live' at the time
-        they are used, so the result of calling the references needs
-        to be checked before being used.  This can be used to avoid
-        creating references that will cause the garbage collector to
-        keep the keys around longer than needed.
-
-        """
-        return self.data.keys()
-
     def keys(self):
-        for wr in self.data.keys():
+        with _IterationGuard(self):
+            for wr in self.data:
                 obj = wr()
                 if obj is not None:
                     yield obj
 
-    def __iter__(self):
-        return iter(self.keys())
+    __iter__ = keys
 
     def values(self):
-        return iter(self.data.values())
+        with _IterationGuard(self):
+            for wr, value in self.data.items():
+                if wr() is not None:
+                    yield value
 
     def keyrefs(self):
         """Return a list of weak references to the keys.
@@ -323,7 +359,7 @@ class WeakKeyDictionary(collections.MutableMapping):
         keep the keys around longer than needed.
 
         """
-        return self.data.keys()
+        return list(self.data)
 
     def popitem(self):
         while 1:

Misc/NEWS

@@ -194,6 +194,9 @@ C-API
 Library
 -------
 
+- Issue #7105: Make WeakKeyDictionary and WeakValueDictionary robust against
+  the destruction of weakref'ed objects while iterating.
+
 - Issue #7455: Fix possible crash in cPickle on invalid input.  Patch by
   Victor Stinner.