[bug]: Long node chains into an If branch raise RecursionError #9821

Is there an existing issue for this problem?

• I have searched the existing issues

Version number

main at aa46003 (7.0.0-alpha.2-post); also on #9789

What happened

A chain of about 480 or more ordinary nodes between an Iterate and an If branch input makes GraphExecutionState.next() raise RecursionError: maximum recursion depth exceeded. The chain below uses 1000 nodes; 400 completes.

What you expected to happen

Graph length should not be limited by the Python recursion limit.

How to reproduce the problem

g = Graph()
g.add_node(CollectionConcatInvocation(id="conditions", first=[True, False]))
g.add_node(IterateInvocation(id="iterate"))
g.add_node(MarkedAnyTypeTestInvocation(id="false_branch", marker="f"))
g.add_node(IfInvocation(id="if"))
g.add_node(CollectInvocation(id="collect"))
g.add_edge(create_edge("conditions", "collection", "iterate", "collection"))
g.add_edge(create_edge("iterate", "item", "if", "condition"))
g.add_edge(create_edge("iterate", "item", "false_branch", "value"))
g.add_edge(create_edge("false_branch", "value", "if", "false_input"))
previous = ("iterate", "item")
for i in range(1000):
    g.add_node(AnyTypeTestInvocation(id=f"n{i}"))
    g.add_edge(create_edge(previous[0], previous[1], f"n{i}", "value"))
    previous = (f"n{i}", "value")
g.add_edge(create_edge(previous[0], previous[1], "if", "true_input"))
g.add_edge(create_edge("if", "value", "collect", "item"))

Additional context

The recursion is in graph_if_runtime._get_effective_iteration_paths_for_edge. It recurses once per unprepared upstream node while inferring an If's frames.

Under the real session processor the caller stack is deeper, so the practical limit is somewhat lower than in tests. An iterative walk (or memoising in topological order, as _get_frame_depths in #9789 does) would remove the limit.

Found while reviewing #9789.