Issue python#20006: Fix sporadic failures in test_weakset.

Author: pitrou

Lib/_weakrefset.py

@@ -60,6 +60,8 @@ def __iter__(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
 
     def __len__(self):

Lib/test/test_weakset.py

@@ -370,9 +370,14 @@ def test_weak_destroy_and_mutate_while_iterating(self):
         def testcontext():
             try:
                 it = iter(s)
-                next(it)
+                # Start iterator
+                yielded = ustr(str(next(it)))
                 # Schedule an item for removal and recreate it
                 u = ustr(str(items.pop()))
+                if yielded == u:
+                    # The iterator still has a reference to the removed item,
+                    # advance it (issue #20006).
+                    next(it)
                 gc.collect()      # just in case
                 yield u
             finally: