Use NULL in the exception stack to indicate an exception was handled

pR0Ps · pR0Ps · commit c62f8ae1955a · 2023-12-19T22:17:32.000-05:00
Previously, both `NULL` and `Py_None` would be used interchangeably to indicate that an exception is no longer being handled. By ensuring that only `NULL` is used, this opens up the possibility to use `Py_None` to indicate a cleared exception. The difference here would be that clearing would indicate that no exception is currently being handled vs. handling would indicate that the next exception in the stack is currently being handled. This functionality will be used to patch up some edge cases in how the exception context interacts with exceptions thrown into coroutines. This is implemented in this commit by changing code that could add `Py_None` to the exception stack to indicate that an exception is no longer being handled to add `NULL` instead. An assert was also added to ensure that `Py_None` is no longer added to the exception stack. See gh-111676 for context.
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-12-19-22-03-43.gh-issue-111375.M9vuA6.rst b/Misc/NEWS.d/next/Core and Builtins/2023-12-19-22-03-43.gh-issue-111375.M9vuA6.rst
@@ -0,0 +1,2 @@
+Only use ``NULL`` in the exception stack to indicate an exception was
+handled. Patch by Carey Metcalfe.
diff --git a/Objects/frameobject.c b/Objects/frameobject.c
@@ -811,7 +811,7 @@ frame_setlineno(PyFrameObject *f, PyObject* p_new_lineno, void *Py_UNUSED(ignore
             PyObject *exc = _PyFrame_StackPop(f->f_frame);
             assert(PyExceptionInstance_Check(exc) || exc == Py_None);
             PyThreadState *tstate = _PyThreadState_GET();
-            Py_XSETREF(tstate->exc_info->exc_value, exc);
+            Py_XSETREF(tstate->exc_info->exc_value, exc == Py_None ? NULL : exc);
         }
         else {
             PyObject *v = _PyFrame_StackPop(f->f_frame);
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
@@ -1100,7 +1100,7 @@ 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
@@ -126,6 +126,7 @@ _PyErr_GetTopmostException(PyThreadState *tstate)
     {
         exc_info = exc_info->previous_item;
     }
+    assert(!Py_IsNone(exc_info->exc_value));
     return exc_info;
 }
 
@@ -592,7 +593,7 @@ PyErr_GetHandledException(void)
 void
 _PyErr_SetHandledException(PyThreadState *tstate, PyObject *exc)
 {
-    Py_XSETREF(tstate->exc_info->exc_value, Py_XNewRef(exc));
+    Py_XSETREF(tstate->exc_info->exc_value, Py_XNewRef(exc == Py_None ? NULL : exc));
 }
 
 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
`@@ -0,0 +1,2 @@`
	`1`	+Only use ``NULL`` in the exception stack to indicate an exception was
	`2`	`+handled. Patch by Carey Metcalfe.`
Original file line number	Diff line number	Diff line change
`@@ -811,7 +811,7 @@ frame_setlineno(PyFrameObject f, PyObject p_new_lineno, void *Py_UNUSED(ignore`
`811`	`811`	`PyObject *exc = _PyFrame_StackPop(f->f_frame);`
`812`	`812`	`assert(PyExceptionInstance_Check(exc) \|\| exc == Py_None);`
`813`	`813`	`PyThreadState *tstate = _PyThreadState_GET();`
`814`		`- Py_XSETREF(tstate->exc_info->exc_value, exc);`
	`814`	`+ Py_XSETREF(tstate->exc_info->exc_value, exc == Py_None ? NULL : exc);`
`815`	`815`	`}`
`816`	`816`	`else {`
`817`	`817`	`PyObject *v = _PyFrame_StackPop(f->f_frame);`
Original file line number	Diff line number	Diff line change
`@@ -1100,7 +1100,7 @@ dummy_func(`
`1100`	`1100`
`1101`	`1101`	`inst(POP_EXCEPT, (exc_value -- )) {`
`1102`	`1102`	`_PyErr_StackItem *exc_info = tstate->exc_info;`
`1103`		`- Py_XSETREF(exc_info->exc_value, exc_value);`
	`1103`	`+ Py_XSETREF(exc_info->exc_value, exc_value == Py_None ? NULL : exc_value);`
`1104`	`1104`	`}`
`1105`	`1105`
`1106`	`1106`	`inst(RERAISE, (values[oparg], exc -- values[oparg])) {`
Original file line number	Diff line number	Diff line change
`@@ -126,6 +126,7 @@ _PyErr_GetTopmostException(PyThreadState *tstate)`
`126`	`126`	`{`
`127`	`127`	`exc_info = exc_info->previous_item;`
`128`	`128`	`}`
	`129`	`+ assert(!Py_IsNone(exc_info->exc_value));`
`129`	`130`	`return exc_info;`
`130`	`131`	`}`
`131`	`132`
`@@ -592,7 +593,7 @@ PyErr_GetHandledException(void)`
`592`	`593`	`void`
`593`	`594`	`_PyErr_SetHandledException(PyThreadState tstate, PyObject exc)`
`594`	`595`	`{`
`595`		`- Py_XSETREF(tstate->exc_info->exc_value, Py_XNewRef(exc));`
	`596`	`+ Py_XSETREF(tstate->exc_info->exc_value, Py_XNewRef(exc == Py_None ? NULL : exc));`
`596`	`597`	`}`
`597`	`598`
`598`	`599`	`void`