Fix regression in CopiedFailure

src/twisted/newsfragments/12279.bugfix

@@ -0,0 +1 @@
+Fixed unreleased regression caused by PR 12109.

src/twisted/python/failure.py

@@ -429,6 +429,10 @@ def parents(self):
             self._parents = [self.type]
         return self._parents
 
+    @parents.setter
+    def parents(self, parents):
+        self._parents = parents
+
     def _extrapolate(self, otherFailure):
         """
         Extrapolate from one failure into another, copying its stack frames.

src/twisted/test/test_failure.py

@@ -568,6 +568,17 @@ def test_failurePicklingIncludesParents(self) -> None:
         f = failure.Failure(ComparableException("hello"))
         self.assertEqual(f.__getstate__()["parents"], f.parents)
 
+    def test_settableParents(self) -> None:
+        """
+        C{Failure.parents} can be set, both before and after pickling.
+
+        This is used by Perspective Broker.
+        """
+        original_failure = failure.Failure(ComparableException("hello"))
+        original_failure.parents = original_failure.parents[:]
+        failure2 = pickle.loads(pickle.dumps(original_failure))
+        failure2.parents = failure2.parents[:]
+
 
 class BrokenStr(Exception):
     """