fix[next-dace]: return no successors for terminal blocks in find_successor_state

src/gt4py/next/program_processors/runners/dace/transformations/utils.py

@@ -634,7 +634,14 @@ def _impl(
         curr_out_edges = [oedge.dst for oedge in graph.out_edges(state)]
 
         # End recursion if we found some successor edges or we have reached the top.
-        if len(curr_out_edges) > 0 or graph.parent_graph is state.sdfg:
+        #  Note that if `graph` is the root region (e.g. for a terminal state at
+        #  the top level of the SDFG) its `parent_graph` is `None`, not the SDFG
+        #  itself, so that case must be handled explicitly.
+        if (
+            len(curr_out_edges) > 0
+            or graph.parent_graph is state.sdfg
+            or graph.parent_graph is None
+        ):
             return curr_out_edges
         elif isinstance(graph.parent_graph, dace.sdfg.state.ConditionalBlock):
             # For conditional we go two levels up, because there is nothing

tests/next_tests/unit_tests/program_processor_tests/runners_tests/dace_tests/transformation_tests/test_utils.py

@@ -0,0 +1,54 @@
+# GT4Py - GridTools Framework
+#
+# Copyright (c) 2014-2024, ETH Zurich
+# All rights reserved.
+#
+# Please, refer to the LICENSE file in the root directory.
+# SPDX-License-Identifier: BSD-3-Clause
+
+import dace
+from dace.sdfg.state import LoopRegion
+
+from gt4py.next.program_processors.runners.dace.transformations import (
+    utils as gtx_transformations_utils,
+)
+
+from . import util
+
+
+def test_find_successor_state():
+    sdfg = dace.SDFG(util.unique_name("find_successor_state"))
+    state1 = sdfg.add_state(is_start_block=True)
+    state2 = sdfg.add_state_after(state1)
+    sdfg.validate()
+
+    assert gtx_transformations_utils.find_successor_state(state1) == [state2]
+
+    # `state2` is a terminal control-flow block of the SDFG, thus it has no
+    #  successor. However, the function must not walk above the root region
+    #  (previously this crashed with an `AttributeError`).
+    assert gtx_transformations_utils.find_successor_state(state2) == []
+
+
+def test_find_successor_state_terminal_loop_region():
+    """Terminal inside a `LoopRegion` that is itself terminal.
+
+    The successor of the last state of the loop body is not expressible, thus
+    the function must return an empty list and not walk above the root region
+    (previously this crashed with an `AttributeError`).
+    """
+    sdfg = dace.SDFG(util.unique_name("find_successor_state_terminal_loop_region"))
+    loop = LoopRegion(
+        label="scan_loop",
+        condition_expr="i < 2",
+        loop_var="i",
+        initialize_expr="i = 0",
+        update_expr="i = i + 1",
+    )
+    sdfg.add_node(loop, is_start_block=True)
+    body_state1 = loop.add_state("body_state1", is_start_block=True)
+    body_state2 = loop.add_state_after(body_state1)
+    sdfg.validate()
+
+    assert gtx_transformations_utils.find_successor_state(body_state1) == [body_state2]
+    assert gtx_transformations_utils.find_successor_state(body_state2) == []