Fix fatal error when GraphViz is missing

Ref. eng/recordflux/RecordFlux#1499

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Fatal error when comparing opaque fields (AdaCore/RecordFlux#1294, eng/recordflux/RecordFlux#1497)
+- Fatal error when GraphViz is missing (eng/recordflux/RecordFlux#1499)
 - Missing rejection of sequences of parameterized messages (eng/recordflux/RecordFlux#1439)
 
 ### Removed

rflx/graph.py

@@ -6,8 +6,9 @@
 from pathlib import Path
 from typing import Optional
 
-from pydotplus import Dot, Edge, Node
+from pydotplus import Dot, Edge, InvocationException, Node  # type: ignore[attr-defined]
 
+from rflx.error import RecordFluxError, Severity, Subsystem
 from rflx.expression import TRUE, UNDEFINED
 from rflx.identifier import ID
 from rflx.model import FINAL_STATE, AbstractSession, Link, Message
@@ -48,7 +49,20 @@ def write_graph(graph: Dot, filename: Path, fmt: str = "svg") -> None:
     log.info("Creating %s", filename)
 
     with filename.open("wb") as f:
-        graph.write(f, format=fmt)
+        try:
+            graph.write(f, format=fmt)
+        except InvocationException as e:
+            RecordFluxError(
+                [
+                    (e, Subsystem.GRAPH, Severity.ERROR, None),
+                    (
+                        "GraphViz is required for creating graphs",
+                        Subsystem.GRAPH,
+                        Severity.INFO,
+                        None,
+                    ),
+                ],
+            ).propagate()
 
 
 def create_message_graph(message: Message) -> Dot:

tests/unit/graph_test.py

@@ -1,7 +1,9 @@
 from pathlib import Path
 
-from pydotplus import Dot
+import pytest
+from pydotplus import Dot, InvocationException  # type: ignore[attr-defined]
 
+from rflx.error import RecordFluxError
 from rflx.expression import FALSE, TRUE, Equal, Greater, Less, Number, Pow, Sub, Variable
 from rflx.graph import create_message_graph, create_session_graph, write_graph
 from rflx.identifier import ID
@@ -244,3 +246,21 @@ def test_session_graph(tmp_path: Path) -> None:
         """
 
     assert_graph(create_session_graph(s, ignore=[r"^IGNORED_"]), expected_filtered, tmp_path)
+
+
+def test_missing_graphviz(monkeypatch: pytest.MonkeyPatch, tmp_path: Path) -> None:
+    def write_mock(self: object, path: object, format: str = "") -> object:  # noqa: ARG001, A002
+        raise InvocationException("GraphViz not found")
+
+    monkeypatch.setattr(Dot, "write", write_mock)
+
+    with pytest.raises(
+        RecordFluxError,
+        match=(
+            r"^"
+            r"graph: error: GraphViz not found\n"
+            r"graph: info: GraphViz is required for creating graphs"
+            r"$"
+        ),
+    ):
+        write_graph(Dot(""), tmp_path / "graph")