Preserve descendant subtrees when reprioritizing

Keep the moved descendant attached to its own children during cycle avoidance, as required by RFC 7540 section 5.3.3. Cover direct and indirect descendants with exclusive and nonexclusive dependencies, including child-list and queue consistency.

Generated with OpenAI Codex and independently reviewed by a native AI agent. No human review or certification is claimed.

Author: codex

src/priority/priority.py

@@ -428,7 +428,7 @@ def reprioritize(
         # continue.
         if cycle:
             new_parent.parent.remove_child(  # type: ignore[union-attr]
-                new_parent,
+                new_parent, strip_children=False
             )
             current_stream.parent.add_child(  # type: ignore[union-attr]
                 new_parent,

test/test_priority.py

@@ -252,6 +252,53 @@ def test_can_reprioritize_a_stream(
         actual_result = [next(t) for _ in range(len(result))]
         assert actual_result == result
 
+    @pytest.mark.parametrize("exclusive", [True, False])
+    @pytest.mark.parametrize("descendant_parent", [1, 5])
+    def test_reprioritize_preserves_descendant_children(
+        self, exclusive, descendant_parent
+    ):
+        """
+        Moving a descendant to avoid a cycle keeps its subtree intact.
+        """
+        p = priority.PriorityTree()
+        # RFC 7540 Figure 5: A=1, B=3, C=5, D=7, E=9, F=11.
+        for stream_id, parent in [
+            (1, 0),
+            (3, 1),
+            (5, 1),
+            (7, descendant_parent),
+            (9, 5),
+            (11, 7),
+        ]:
+            p.insert_stream(stream_id, depends_on=parent)
+
+        p.reprioritize(1, depends_on=7, exclusive=exclusive)
+
+        assert {
+            stream_id: stream.parent.stream_id
+            for stream_id, stream in p._streams.items()
+            if stream_id != 0
+        } == {1: 7, 3: 1, 5: 1, 7: 0, 9: 5, 11: 1 if exclusive else 7}
+
+        for stream in p._streams.values():
+            expected_children = sorted(
+                child.stream_id
+                for child in p._streams.values()
+                if child.parent is stream
+            )
+            assert (
+                sorted(child.stream_id for child in stream.children)
+                == expected_children
+            )
+            assert (
+                sorted(child.stream_id for _, child in stream.child_queue)
+                == expected_children
+            )
+
+        p.block(7)
+        expected = [1] * 10 if exclusive else [1, 11] * 5
+        assert [next(p) for _ in range(10)] == expected
+
     def test_priority_tree_raises_error_inserting_duplicate(self):
         """
         Attempting to insert a stream that is already in the tree raises a