When accessing the topmost exception, treat Py_None as an exception

pR0Ps · pR0Ps · commit 36b6177aa0bb · 2023-11-06T16:12:26.000-05:00
This allows `PyErr_SetHandledException(NULL)` to mask the entire
exception chain and "clear" the current exception state as documented in
`Doc/c-api/exceptions.rst`.

The change in behavior is implemented in `_PyErr_GetTopmostException`.
Instead of traversing the exception chain until a non-`NULL`, non-`None`
value is found, it now will return `None` as the topmost exception.

To support this change, places that add `Py_None` to the exception stack
to indicate that an exception is no longer being handled were changed to
add `NULL` instead.

This commit also reverts the previous change to `PyErr_SetHandledException`.
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -209,20 +209,56 @@ def test_cleared_exc_overridden_and_restored(self):
         self.assertIsNone(sys.exception())
 
     def test_clear_exc_in_generator(self):
-        def gen():
+        def inner():
             self.assertIsNone(sys.exception())
             yield
-            self.assertIsInstance(sys.exception(), ZeroDivisionError)
+            self.assertIsInstance(sys.exception(), ValueError)
             sys._set_exception(None)
             self.assertIsNone(sys.exception())
             yield
+            self.assertIsNone(sys.exception())
 
-        g = gen()
+        # with a single exception in exc_info stack
+        g = inner()
         next(g)
         try:
-            1/0
+            raise ValueError()
         except:
+            self.assertIsInstance(sys.exception(), ValueError)
             next(g)
+            self.assertIsInstance(sys.exception(), ValueError)
+        self.assertIsNone(sys.exception())
+        with self.assertRaises(StopIteration):
+            next(g)
+        self.assertIsNone(sys.exception())
+
+        # with multiple exceptions in exc_info stack by chaining generators
+        def outer():
+            g = inner()
+            self.assertIsNone(sys.exception())
+            yield next(g)
+            self.assertIsInstance(sys.exception(), TypeError)
+            try:
+                raise ValueError()
+            except:
+                self.assertIsInstance(sys.exception(), ValueError)
+                self.assertIsInstance(sys.exception().__context__, TypeError)
+                yield next(g)
+                self.assertIsInstance(sys.exception(), ValueError)
+            self.assertIsInstance(sys.exception(), TypeError)
+
+        g = outer()
+        next(g)
+        try:
+            raise TypeError()
+        except:
+            self.assertIsInstance(sys.exception(), TypeError)
+            next(g)
+            self.assertIsInstance(sys.exception(), TypeError)
+        self.assertIsNone(sys.exception())
+        with self.assertRaises(StopIteration):
+            next(g)
+        self.assertIsNone(sys.exception())
 
 class ExceptHookTest(unittest.TestCase):
 
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
@@ -810,7 +810,7 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno, void *Py_UNUSED(ignore
             PyObject *value = exc_info->exc_value;
             PyObject *exc = _PyFrame_StackPop(f->f_frame);
             assert(PyExceptionInstance_Check(exc) || exc == Py_None);
-            exc_info->exc_value = exc;
+            exc_info->exc_value = (exc == Py_None ? NULL : exc);
             Py_XDECREF(value);
         }
         else {
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
@@ -1073,7 +1073,7 @@ dummy_func(
 
         inst(POP_EXCEPT, (exc_value -- )) {
             _PyErr_StackItem *exc_info = tstate->exc_info;
-            Py_XSETREF(exc_info->exc_value, exc_value);
+            Py_XSETREF(exc_info->exc_value, exc_value == Py_None ? NULL : exc_value);
         }
 
         inst(RERAISE, (values[oparg], exc -- values[oparg])) {
diff --git a/Python/errors.c b/Python/errors.c
@@ -121,8 +121,7 @@ _PyErr_GetTopmostException(PyThreadState *tstate)
     _PyErr_StackItem *exc_info = tstate->exc_info;
     assert(exc_info);
 
-    while ((exc_info->exc_value == NULL || exc_info->exc_value == Py_None) &&
-           exc_info->previous_item != NULL)
+    while (exc_info->exc_value == NULL && exc_info->previous_item != NULL)
     {
         exc_info = exc_info->previous_item;
     }
@@ -592,8 +591,7 @@ PyErr_GetHandledException(void)
 void
 _PyErr_SetHandledException(PyThreadState *tstate, PyObject *exc)
 {
-    _PyErr_StackItem *exc_info = _PyErr_GetTopmostException(tstate);
-    Py_XSETREF(exc_info->exc_value, Py_XNewRef(exc));
+    Py_XSETREF(tstate->exc_info->exc_value, Py_XNewRef(exc ? exc : Py_None));
 }
 
 void
diff --git a/Python/executor_cases.c.h b/Python/executor_cases.c.h
diff --git a/Python/generated_cases.c.h b/Python/generated_cases.c.h

Original file line number	Diff line number	Diff line change
`@@ -810,7 +810,7 @@ frame_setlineno(PyFrameObject f, PyObject p_new_lineno, void *Py_UNUSED(ignore`
`810`	`810`	`PyObject *value = exc_info->exc_value;`
`811`	`811`	`PyObject *exc = _PyFrame_StackPop(f->f_frame);`
`812`	`812`	`assert(PyExceptionInstance_Check(exc) \|\| exc == Py_None);`
`813`		`- exc_info->exc_value = exc;`
	`813`	`+ exc_info->exc_value = (exc == Py_None ? NULL : exc);`
`814`	`814`	`Py_XDECREF(value);`
`815`	`815`	`}`
`816`	`816`	`else {`
Original file line number	Diff line number	Diff line change
`@@ -1073,7 +1073,7 @@ dummy_func(`
`1073`	`1073`
`1074`	`1074`	`inst(POP_EXCEPT, (exc_value -- )) {`
`1075`	`1075`	`_PyErr_StackItem *exc_info = tstate->exc_info;`
`1076`		`- Py_XSETREF(exc_info->exc_value, exc_value);`
	`1076`	`+ Py_XSETREF(exc_info->exc_value, exc_value == Py_None ? NULL : exc_value);`
`1077`	`1077`	`}`
`1078`	`1078`
`1079`	`1079`	`inst(RERAISE, (values[oparg], exc -- values[oparg])) {`
Original file line number	Diff line number	Diff line change
`@@ -121,8 +121,7 @@ _PyErr_GetTopmostException(PyThreadState *tstate)`
`121`	`121`	`_PyErr_StackItem *exc_info = tstate->exc_info;`
`122`	`122`	`assert(exc_info);`
`123`	`123`
`124`		`- while ((exc_info->exc_value == NULL \|\| exc_info->exc_value == Py_None) &&`
`125`		`- exc_info->previous_item != NULL)`
	`124`	`+ while (exc_info->exc_value == NULL && exc_info->previous_item != NULL)`
`126`	`125`	`{`
`127`	`126`	`exc_info = exc_info->previous_item;`
`128`	`127`	`}`
`@@ -592,8 +591,7 @@ PyErr_GetHandledException(void)`
`592`	`591`	`void`
`593`	`592`	`_PyErr_SetHandledException(PyThreadState tstate, PyObject exc)`
`594`	`593`	`{`
`595`		`- _PyErr_StackItem *exc_info = _PyErr_GetTopmostException(tstate);`
`596`		`- Py_XSETREF(exc_info->exc_value, Py_XNewRef(exc));`
	`594`	`+ Py_XSETREF(tstate->exc_info->exc_value, Py_XNewRef(exc ? exc : Py_None));`
`597`	`595`	`}`
`598`	`596`
`599`	`597`	`void`