bpo-44962 fix a race in WeakKeyDict, WeakValueDict and WeakSet when two threads attempt to commit the last pending removal

Lib/_weakrefset.py

@@ -51,10 +51,15 @@ def _remove(item, selfref=ref(self)):
             self.update(data)
 
     def _commit_removals(self):
-        l = self._pending_removals
+        pop = self._pending_removals.pop
         discard = self.data.discard
-        while l:
-            discard(l.pop())
+        while True:
+            try:
+                item = pop()
+            except IndexError:
+                return
+            else:
+                discard(item)
 
     def __iter__(self):
         with _IterationGuard(self):

Misc/NEWS.d/next/Core and Builtins/2021-08-23-19-55-08.bpo-44962.J00ftt.rst

@@ -0,0 +1 @@
+fix a race in WeakSet