refine the bad path

Author: PragmaTwice

mlir/lib/Bindings/Python/Pass.cpp

@@ -196,7 +196,14 @@ void mlir::python::populatePassManagerSubmodule(nb::module_ &m) {
               nb::delattr(callable, mlirExternalPassAttr);
             };
             nb::setattr(run, "signal_pass_failure", nb::cpp_function([run]() {
-                          nb::capsule cap = run.attr(mlirExternalPassAttr);
+                          nb::capsule cap;
+                          try {
+                            cap = run.attr(mlirExternalPassAttr);
+                          } catch (nb::python_error &e) {
+                            throw std::runtime_error(
+                                "signal_pass_failure() should always be called "
+                                "from the __call__ method");
+                          }
                           mlirExternalPassSignalFailure(
                               MlirExternalPass{cap.data()});
                         }));

mlir/test/python/python_pass.py

@@ -103,3 +103,9 @@ def __call__(self, m):
             pm.run(module)
         except Exception as e:
             print(f"caught exception: {e}")
+
+        # CHECK: caught exception: signal_pass_failure() should always be called from the __call__ method
+        try:
+            custom_pass_that_fails.signal_pass_failure()
+        except Exception as e:
+            print(f"caught exception: {e}")