gh-123089: Make weakref.WeakSet safe against concurrent mutations while it is being iterated (#123279)

* Make `weakref.WeakSet` safe against concurrent mutations while it is being iterated.

`_IterationGuard` is no longer used for `WeakSet`, it now relies on copying the underlying set which is an atomic operation while iterating so that it can be modified by other threads.

2 files changed, 11 insertions, 43 deletions

diff --git a/Lib/_weakrefset.py b/Lib/_weakrefset.py
index 489eec7..2071755 100644
--- a/Lib/_weakrefset.py
+++ b/Lib/_weakrefset.py
@@ -36,41 +36,26 @@ class _IterationGuard:
 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.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):
-        pop = self._pending_removals.pop
-        discard = self.data.discard
-        while True:
-            try:
-                item = pop()
-            except IndexError:
-                return
-            discard(item)
-
     def __iter__(self):
-        with _IterationGuard(self):
-            for itemref in self.data:
-                item = itemref()
-                if item is not None:
-                    # Caveat: the iterator will keep a strong reference to
-                    # `item` until it is resumed or closed.
-                    yield item
+        for itemref in self.data.copy():
+            item = itemref()
+            if item is not None:
+                # Caveat: the iterator will keep a strong reference to
+                # `item` until it is resumed or closed.
+                yield item
 
     def __len__(self):
-        return len(self.data) - len(self._pending_removals)
+        return len(self.data)
 
     def __contains__(self, item):
         try:
@@ -83,21 +68,15 @@ class WeakSet:
         return self.__class__, (list(self),), self.__getstate__()
 
     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()
@@ -108,18 +87,12 @@ 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()
         for element in other:
             self.add(element)
 
@@ -136,8 +109,6 @@ class WeakSet:
     def difference_update(self, other):
         self.__isub__(other)
     def __isub__(self, other):
-        if self._pending_removals:
-            self._commit_removals()
         if self is other:
             self.data.clear()
         else:
@@ -151,8 +122,6 @@ class WeakSet:
     def intersection_update(self, other):
         self.__iand__(other)
     def __iand__(self, other):
-        if self._pending_removals:
-            self._commit_removals()
         self.data.intersection_update(ref(item) for item in other)
         return self
 
@@ -184,8 +153,6 @@ class WeakSet:
     def symmetric_difference_update(self, other):
         self.__ixor__(other)
     def __ixor__(self, other):
-        if self._pending_removals:
-            self._commit_removals()
         if self is other:
             self.data.clear()
         else:
diff --git a/Misc/NEWS.d/next/Library/2024-08-27-12-38-42.gh-issue-123089.vA7iFR.rst b/Misc/NEWS.d/next/Library/2024-08-27-12-38-42.gh-issue-123089.vA7iFR.rst
new file mode 100644
index 0000000..74cbdd5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-08-27-12-38-42.gh-issue-123089.vA7iFR.rst
@@ -0,0 +1 @@
+Make :class:`weakref.WeakSet` safe against concurrent mutations while it is being iterated. Patch by Kumar Aditya.