feat: implement slot_error_boundary decorator and add unit tests for session and device controllers

tests/test_slot_error_boundary.py

@@ -0,0 +1,99 @@
+"""Tests for slot_error_boundary decorator."""
+from __future__ import annotations
+
+import logging
+from unittest.mock import MagicMock
+
+from pygui.backend.utils import slot_error_boundary
+
+
+class _FakeSlot:
+    """Minimal QObject-like object to test the decorator against."""
+
+    def __init__(self, has_log_message=True):
+        self.logMessage = MagicMock() if has_log_message else None
+        self.__class__.__module__ = "test_module"
+
+
+class TestSlotErrorBoundary:
+
+    def test_passes_through_on_success(self):
+        @slot_error_boundary
+        def good(self) -> str:
+            return "ok"
+
+        result = good(_FakeSlot())
+        assert result == "ok"
+
+    def test_catches_and_logs_exception(self, caplog):
+        @slot_error_boundary
+        def bad(self) -> None:
+            raise ValueError("boom")
+
+        with caplog.at_level(logging.ERROR):
+            bad(_FakeSlot())
+
+        assert "ValueError" in caplog.text
+        assert "bad" in caplog.text
+
+    def test_emits_log_message_on_error(self):
+        obj = _FakeSlot(has_log_message=True)
+
+        @slot_error_boundary
+        def bad(self) -> None:
+            raise RuntimeError("fail")
+
+        bad(obj)
+
+        obj.logMessage.emit.assert_called_once()
+        emitted_msg = obj.logMessage.emit.call_args[0][0]
+        assert "bad" in emitted_msg
+        assert "fail" in emitted_msg
+
+    def test_returns_none_on_error(self):
+        @slot_error_boundary
+        def bad(self) -> None:
+            raise KeyError("missing")
+
+        result = bad(_FakeSlot())
+        assert result is None
+
+    def test_no_log_message_signal_is_ok(self):
+        """Controller without logMessage signal should not crash."""
+
+        class _NoLogMsg:
+            __module__ = "test_module"
+
+        @slot_error_boundary
+        def bad(self) -> None:
+            raise ValueError("boom")
+
+        # Should not raise — hasattr check guards the emit
+        result = bad(_NoLogMsg())
+        assert result is None
+
+    def test_preserves_function_name(self):
+        @slot_error_boundary
+        def named_method(self) -> None:
+            pass
+
+        assert named_method.__name__ == "named_method"
+
+    def test_passes_args_through(self):
+        @slot_error_boundary
+        def with_args(self, a: int, b: str) -> tuple[int, str]:
+            return (a, b)
+
+        result = with_args(_FakeSlot(), 42, "hello")
+        assert result == (42, "hello")
+
+    def test_exception_in_wrapper_does_not_leak(self):
+        """The decorator must never re-raise — QML would crash."""
+
+        @slot_error_boundary
+        def crashing(self) -> None:
+            raise TypeError("uncaught type error")
+
+        # Should return None, not raise
+        result = crashing(_FakeSlot())
+        assert result is None