bpo-42979: Use _Py_CheckSlotResult() to check slots result (GH-24356)

When Python is built in debug mode (with C assertions), calling a
type slot like sq_length (__len__() in Python) now fails with a fatal
error if the slot succeeded with an exception set, or failed with no
exception set. The error message contains the slot, the type name,
and the current exception (if an exception is set).

* Check the result of all slots using _Py_CheckSlotResult().
* No longer pass op_name to ternary_op() in release mode.
* Replace operator with dunder Python method name in error messages.
  For example, replace "*" with "__mul__".
* Fix compiler_exit_scope() when an exception is set.
* Fix bytearray.extend() when an exception is set: don't call
  bytearray_setslice() with an exception set.

1 files changed, 11 insertions, 7 deletions

diff --git a/Python/compile.c b/Python/compile.c
index 223c63637ff..d8fea9d047d 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -667,25 +667,29 @@ compiler_enter_scope(struct compiler *c, identifier name,
 static void
 compiler_exit_scope(struct compiler *c)
 {
-    Py_ssize_t n;
-    PyObject *capsule;
+    // Don't call PySequence_DelItem() with an exception raised
+    PyObject *exc_type, *exc_val, *exc_tb;
+    PyErr_Fetch(&exc_type, &exc_val, &exc_tb);
 
     c->c_nestlevel--;
     compiler_unit_free(c->u);
     /* Restore c->u to the parent unit. */
-    n = PyList_GET_SIZE(c->c_stack) - 1;
+    Py_ssize_t n = PyList_GET_SIZE(c->c_stack) - 1;
     if (n >= 0) {
-        capsule = PyList_GET_ITEM(c->c_stack, n);
+        PyObject *capsule = PyList_GET_ITEM(c->c_stack, n);
         c->u = (struct compiler_unit *)PyCapsule_GetPointer(capsule, CAPSULE_NAME);
         assert(c->u);
         /* we are deleting from a list so this really shouldn't fail */
-        if (PySequence_DelItem(c->c_stack, n) < 0)
-            Py_FatalError("compiler_exit_scope()");
+        if (PySequence_DelItem(c->c_stack, n) < 0) {
+            Py_FatalError("PySequence_DelItem failed");
+        }
         compiler_unit_check(c->u);
     }
-    else
+    else {
         c->u = NULL;
+    }
 
+    PyErr_Restore(exc_type, exc_val, exc_tb);
 }
 
 static int