bpo-42979: Enhance abstract.c assertions checking slot result (GH-24352)

* bpo-42979: Enhance abstract.c assertions checking slot result

Add _Py_CheckSlotResult() function which fails with a fatal error if
a slot function succeeded with an exception set or failed with no
exception set: write the slot name, the type name and the current
exception (if an exception is set).

2 files changed, 39 insertions, 14 deletions

diff --git a/Objects/abstract.c b/Objects/abstract.c
index 44ed5b3932..cc452eaaf3 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -1,11 +1,12 @@
 /* Abstract Object Interface (many thanks to Jim Fulton) */
 
 #include "Python.h"
-#include "pycore_unionobject.h"      // _Py_UnionType && _Py_Union()
 #include "pycore_abstract.h"      // _PyIndex_Check()
 #include "pycore_ceval.h"         // _Py_EnterRecursiveCall()
+#include "pycore_object.h"        // _Py_CheckSlotResult()
 #include "pycore_pyerrors.h"      // _PyErr_Occurred()
 #include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "pycore_unionobject.h"   // _Py_UnionType && _Py_Union()
 #include <ctype.h>
 #include <stddef.h>               // offsetof()
 #include "longintrepr.h"
@@ -61,7 +62,7 @@ PyObject_Size(PyObject *o)
     m = Py_TYPE(o)->tp_as_sequence;
     if (m && m->sq_length) {
         Py_ssize_t len = m->sq_length(o);
-        assert(len >= 0 || PyErr_Occurred());
+        assert(_Py_CheckSlotResult(o, "__len__", len >= 0));
         return len;
     }
 
@@ -160,7 +161,7 @@ PyObject_GetItem(PyObject *o, PyObject *key)
     m = Py_TYPE(o)->tp_as_mapping;
     if (m && m->mp_subscript) {
         PyObject *item = m->mp_subscript(o, key);
-        assert((item != NULL) ^ (PyErr_Occurred() != NULL));
+        assert(_Py_CheckSlotResult(o, "__getitem__", item != NULL));
         return item;
     }
 
@@ -1564,7 +1565,7 @@ PySequence_Size(PyObject *s)
     m = Py_TYPE(s)->tp_as_sequence;
     if (m && m->sq_length) {
         Py_ssize_t len = m->sq_length(s);
-        assert(len >= 0 || PyErr_Occurred());
+        assert(_Py_CheckSlotResult(s, "__len__", len >= 0));
         return len;
     }
 
@@ -1708,8 +1709,8 @@ PySequence_GetItem(PyObject *s, Py_ssize_t i)
         if (i < 0) {
             if (m->sq_length) {
                 Py_ssize_t l = (*m->sq_length)(s);
+                assert(_Py_CheckSlotResult(s, "__len__", l >= 0));
                 if (l < 0) {
-                    assert(PyErr_Occurred());
                     return NULL;
                 }
                 i += l;
@@ -1762,8 +1763,8 @@ PySequence_SetItem(PyObject *s, Py_ssize_t i, PyObject *o)
         if (i < 0) {
             if (m->sq_length) {
                 Py_ssize_t l = (*m->sq_length)(s);
+                assert(_Py_CheckSlotResult(s, "__len__", l >= 0));
                 if (l < 0) {
-                    assert(PyErr_Occurred());
                     return -1;
                 }
                 i += l;
@@ -1795,8 +1796,8 @@ PySequence_DelItem(PyObject *s, Py_ssize_t i)
         if (i < 0) {
             if (m->sq_length) {
                 Py_ssize_t l = (*m->sq_length)(s);
+                assert(_Py_CheckSlotResult(s, "__len__", l >= 0));
                 if (l < 0) {
-                    assert(PyErr_Occurred());
                     return -1;
                 }
                 i += l;
@@ -2145,7 +2146,7 @@ PyMapping_Size(PyObject *o)
     m = Py_TYPE(o)->tp_as_mapping;
     if (m && m->mp_length) {
         Py_ssize_t len = m->mp_length(o);
-        assert(len >= 0 || PyErr_Occurred());
+        assert(_Py_CheckSlotResult(o, "__len__", len >= 0));
         return len;
     }
 
diff --git a/Objects/call.c b/Objects/call.c
index 35b06a9b9c..1fb85efab6 100644
--- a/Objects/call.c
+++ b/Objects/call.c
@@ -39,16 +39,16 @@ _Py_CheckFunctionResult(PyThreadState *tstate, PyObject *callable,
         if (!_PyErr_Occurred(tstate)) {
             if (callable)
                 _PyErr_Format(tstate, PyExc_SystemError,
-                              "%R returned NULL without setting an error",
+                              "%R returned NULL without setting an exception",
                               callable);
             else
                 _PyErr_Format(tstate, PyExc_SystemError,
-                              "%s returned NULL without setting an error",
+                              "%s returned NULL without setting an exception",
                               where);
 #ifdef Py_DEBUG
             /* Ensure that the bug is caught in debug mode.
                Py_FatalError() logs the SystemError exception raised above. */
-            Py_FatalError("a function returned NULL without setting an error");
+            Py_FatalError("a function returned NULL without setting an exception");
 #endif
             return NULL;
         }
@@ -60,17 +60,17 @@ _Py_CheckFunctionResult(PyThreadState *tstate, PyObject *callable,
             if (callable) {
                 _PyErr_FormatFromCauseTstate(
                     tstate, PyExc_SystemError,
-                    "%R returned a result with an error set", callable);
+                    "%R returned a result with an exception set", callable);
             }
             else {
                 _PyErr_FormatFromCauseTstate(
                     tstate, PyExc_SystemError,
-                    "%s returned a result with an error set", where);
+                    "%s returned a result with an exception set", where);
             }
 #ifdef Py_DEBUG
             /* Ensure that the bug is caught in debug mode.
                Py_FatalError() logs the SystemError exception raised above. */
-            Py_FatalError("a function returned a result with an error set");
+            Py_FatalError("a function returned a result with an exception set");
 #endif
             return NULL;
         }
@@ -79,6 +79,30 @@ _Py_CheckFunctionResult(PyThreadState *tstate, PyObject *callable,
 }
 
 
+int
+_Py_CheckSlotResult(PyObject *obj, const char *slot_name, int success)
+{
+    PyThreadState *tstate = _PyThreadState_GET();
+    if (!success) {
+        if (!_PyErr_Occurred(tstate)) {
+            _Py_FatalErrorFormat(__func__,
+                                 "Slot %s of type %s failed "
+                                 "without setting an exception",
+                                 slot_name, Py_TYPE(obj)->tp_name);
+        }
+    }
+    else {
+        if (_PyErr_Occurred(tstate)) {
+            _Py_FatalErrorFormat(__func__,
+                                 "Slot %s of type %s succeeded "
+                                 "with an exception set",
+                                 slot_name, Py_TYPE(obj)->tp_name);
+        }
+    }
+    return 1;
+}
+
+
 /* --- Core PyObject call functions ------------------------------- */
 
 /* Call a callable Python object without any arguments */