gh-118702: Implement vectorcall for BaseException (#118703)

* BaseException_vectorcall() now creates a tuple from 'args' array.
* Creation an exception using BaseException_vectorcall() is now a
  single function call, rather than having to call
  BaseException_new() and then BaseException_init().
  Calling BaseException_init() is inefficient since it overrides
  the 'args' attribute.
* _PyErr_SetKeyError() now uses PyObject_CallOneArg() to create the
  KeyError instance to use BaseException_vectorcall().

Lib/test/test_exceptions.py

@@ -1817,6 +1817,29 @@ class ExceptionTests(unittest.TestCase):
         rc, _, err = script_helper.assert_python_ok("-c", code)
         self.assertIn(b'MemoryError', err)
 
+    def test_keyerror_context(self):
+        # Make sure that _PyErr_SetKeyError() chains exceptions
+        try:
+            err1 = None
+            err2 = None
+            try:
+                d = {}
+                try:
+                    raise ValueError("bug")
+                except Exception as exc:
+                    err1 = exc
+                    d[1]
+            except Exception as exc:
+                err2 = exc
+
+            self.assertIsInstance(err1, ValueError)
+            self.assertIsInstance(err2, KeyError)
+            self.assertEqual(err2.__context__, err1)
+        finally:
+            # Break any potential reference cycle
+            exc1 = None
+            exc2 = None
+
 
 class NameErrorTests(unittest.TestCase):
     def test_name_error_has_name(self):

Objects/exceptions.c

@@ -78,6 +78,40 @@ BaseException_init(PyBaseExceptionObject *self, PyObject *args, PyObject *kwds)
     return 0;
 }
 
+
+static PyObject *
+BaseException_vectorcall(PyObject *type_obj, PyObject * const*args,
+                         size_t nargsf, PyObject *kwnames)
+{
+    PyTypeObject *type = _PyType_CAST(type_obj);
+    if (!_PyArg_NoKwnames(type->tp_name, kwnames)) {
+        return NULL;
+    }
+
+    PyBaseExceptionObject *self;
+    self = (PyBaseExceptionObject *)type->tp_alloc(type, 0);
+    if (!self) {
+        return NULL;
+    }
+
+    // The dict is created on the fly in PyObject_GenericSetAttr()
+    self->dict = NULL;
+    self->notes = NULL;
+    self->traceback = NULL;
+    self->cause = NULL;
+    self->context = NULL;
+    self->suppress_context = 0;
+
+    self->args = _PyTuple_FromArray(args, PyVectorcall_NARGS(nargsf));
+    if (!self->args) {
+        Py_DECREF(self);
+        return NULL;
+    }
+
+    return (PyObject *)self;
+}
+
+
 static int
 BaseException_clear(PyBaseExceptionObject *self)
 {
@@ -486,6 +520,7 @@ static PyTypeObject _PyExc_BaseException = {
     (initproc)BaseException_init, /* tp_init */
     0,                          /* tp_alloc */
     BaseException_new,          /* tp_new */
+    .tp_vectorcall = BaseException_vectorcall,
 };
 /* the CPython API expects exceptions to be (PyObject *) - both a hold-over
 from the previous implementation and also allowing Python objects to be used
@@ -3675,6 +3710,11 @@ _PyExc_InitTypes(PyInterpreterState *interp)
         if (_PyStaticType_InitBuiltin(interp, exc) < 0) {
             return -1;
         }
+        if (exc->tp_new == BaseException_new
+            && exc->tp_init == (initproc)BaseException_init)
+        {
+            exc->tp_vectorcall = BaseException_vectorcall;
+        }
     }
     return 0;
 }

Python/errors.c

@@ -257,13 +257,14 @@ void
 _PyErr_SetKeyError(PyObject *arg)
 {
     PyThreadState *tstate = _PyThreadState_GET();
-    PyObject *tup = PyTuple_Pack(1, arg);
-    if (!tup) {
+    PyObject *exc = PyObject_CallOneArg(PyExc_KeyError, arg);
+    if (!exc) {
         /* caller will expect error to be set anyway */
         return;
     }
-    _PyErr_SetObject(tstate, PyExc_KeyError, tup);
-    Py_DECREF(tup);
+
+    _PyErr_SetObject(tstate, (PyObject*)Py_TYPE(exc), exc);
+    Py_DECREF(exc);
 }
 
 void