Issue #24400: Introduce a distinct type for 'async def' coroutines.

Summary of changes:

1. Coroutines now have a distinct, separate from generators
   type at the C level: PyGen_Type, and a new typedef PyCoroObject.
   PyCoroObject shares the initial segment of struct layout with
   PyGenObject, making it possible to reuse existing generators
   machinery.  The new type is exposed as 'types.CoroutineType'.

   As a consequence of having a new type, CO_GENERATOR flag is
   no longer applied to coroutines.

2. Having a separate type for coroutines made it possible to add
   an __await__ method to the type.  Although it is not used by the
   interpreter (see details on that below), it makes coroutines
   naturally (without using __instancecheck__) conform to
   collections.abc.Coroutine and collections.abc.Awaitable ABCs.

   [The __instancecheck__ is still used for generator-based
   coroutines, as we don't want to add __await__ for generators.]

3. Add new opcode: GET_YIELD_FROM_ITER.  The opcode is needed to
   allow passing native coroutines to the YIELD_FROM opcode.

   Before this change, 'yield from o' expression was compiled to:

      (o)
      GET_ITER
      LOAD_CONST
      YIELD_FROM

   Now, we use GET_YIELD_FROM_ITER instead of GET_ITER.

   The reason for adding a new opcode is that GET_ITER is used
   in some contexts (such as 'for .. in' loops) where passing
   a coroutine object is invalid.

4. Add two new introspection functions to the inspec module:
   getcoroutinestate(c) and getcoroutinelocals(c).

5. inspect.iscoroutine(o) is updated to test if 'o' is a native
   coroutine object.  Before this commit it used abc.Coroutine,
   and it was requested to update inspect.isgenerator(o) to use
   abc.Generator; it was decided, however, that inspect functions
   should really be tailored for checking for native types.

6. sys.set_coroutine_wrapper(w) API is updated to work with only
   native coroutines.  Since types.coroutine decorator supports
   any type of callables now, it would be confusing that it does
   not work for all types of coroutines.

7. Exceptions logic in generators C implementation was updated
   to raise clearer messages for coroutines:

   Before: TypeError("generator raised StopIteration")
   After: TypeError("coroutine raised StopIteration")

2 files changed, 274 insertions, 57 deletions

diff --git a/Objects/genobject.c b/Objects/genobject.c
index 5d3b66c..40340b4 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -27,8 +27,7 @@ _PyGen_Finalize(PyObject *self)
     /* If `gen` is a coroutine, and if it was never awaited on,
        issue a RuntimeWarning. */
     if (gen->gi_code != NULL
-            && ((PyCodeObject *)gen->gi_code)->co_flags & (CO_COROUTINE
-                                                       | CO_ITERABLE_COROUTINE)
+            && ((PyCodeObject *)gen->gi_code)->co_flags & CO_COROUTINE
             && gen->gi_frame != NULL
             && gen->gi_frame->f_lasti == -1
             && !PyErr_Occurred()
@@ -86,8 +85,10 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
     PyObject *result;
 
     if (gen->gi_running) {
-        PyErr_SetString(PyExc_ValueError,
-                        "generator already executing");
+        char *msg = "generator already executing";
+        if (PyCoro_CheckExact(gen))
+            msg = "coroutine already executing";
+        PyErr_SetString(PyExc_ValueError, msg);
         return NULL;
     }
     if (f == NULL || f->f_stacktop == NULL) {
@@ -99,9 +100,12 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
 
     if (f->f_lasti == -1) {
         if (arg && arg != Py_None) {
-            PyErr_SetString(PyExc_TypeError,
-                            "can't send non-None value to a "
-                            "just-started generator");
+            char *msg = "can't send non-None value to a "
+                        "just-started generator";
+            if (PyCoro_CheckExact(gen))
+                msg = "can't send non-None value to a "
+                      "just-started coroutine";
+            PyErr_SetString(PyExc_TypeError, msg);
             return NULL;
         }
     } else {
@@ -151,14 +155,16 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
               (CO_FUTURE_GENERATOR_STOP | CO_COROUTINE | CO_ITERABLE_COROUTINE))
         {
             PyObject *exc, *val, *val2, *tb;
+            char *msg = "generator raised StopIteration";
+            if (PyCoro_CheckExact(gen))
+                msg = "coroutine raised StopIteration";
             PyErr_Fetch(&exc, &val, &tb);
             PyErr_NormalizeException(&exc, &val, &tb);
             if (tb != NULL)
                 PyException_SetTraceback(val, tb);
             Py_DECREF(exc);
             Py_XDECREF(tb);
-            PyErr_SetString(PyExc_RuntimeError,
-                "generator raised StopIteration");
+            PyErr_SetString(PyExc_RuntimeError, msg);
             PyErr_Fetch(&exc, &val2, &tb);
             PyErr_NormalizeException(&exc, &val2, &tb);
             Py_INCREF(val);
@@ -288,9 +294,11 @@ gen_close(PyGenObject *gen, PyObject *args)
         PyErr_SetNone(PyExc_GeneratorExit);
     retval = gen_send_ex(gen, Py_None, 1);
     if (retval) {
+        char *msg = "generator ignored GeneratorExit";
+        if (PyCoro_CheckExact(gen))
+            msg = "coroutine ignored GeneratorExit";
         Py_DECREF(retval);
-        PyErr_SetString(PyExc_RuntimeError,
-                        "generator ignored GeneratorExit");
+        PyErr_SetString(PyExc_RuntimeError, msg);
         return NULL;
     }
     if (PyErr_ExceptionMatches(PyExc_StopIteration)
@@ -432,12 +440,6 @@ failed_throw:
 static PyObject *
 gen_iternext(PyGenObject *gen)
 {
-    if (((PyCodeObject*)gen->gi_code)->co_flags & CO_COROUTINE) {
-        PyErr_SetString(PyExc_TypeError,
-                        "coroutine-objects do not support iteration");
-        return NULL;
-    }
-
     return gen_send_ex(gen, NULL, 0);
 }
 
@@ -494,14 +496,8 @@ _PyGen_FetchStopIterationValue(PyObject **pvalue) {
 static PyObject *
 gen_repr(PyGenObject *gen)
 {
-    if (PyGen_CheckCoroutineExact(gen)) {
-        return PyUnicode_FromFormat("<coroutine object %S at %p>",
-                                    gen->gi_qualname, gen);
-    }
-    else {
-        return PyUnicode_FromFormat("<generator object %S at %p>",
-                                    gen->gi_qualname, gen);
-    }
+    return PyUnicode_FromFormat("<generator object %S at %p>",
+                                gen->gi_qualname, gen);
 }
 
 static PyObject *
@@ -537,19 +533,6 @@ gen_get_qualname(PyGenObject *op)
     return op->gi_qualname;
 }
 
-static PyObject *
-gen_get_iter(PyGenObject *gen)
-{
-    if (((PyCodeObject*)gen->gi_code)->co_flags & CO_COROUTINE) {
-        PyErr_SetString(PyExc_TypeError,
-                        "coroutine-objects do not support iteration");
-        return NULL;
-    }
-
-    Py_INCREF(gen);
-    return (PyObject *)gen;
-}
-
 static int
 gen_set_qualname(PyGenObject *op, PyObject *value)
 {
@@ -619,7 +602,7 @@ PyTypeObject PyGen_Type = {
     0,                                          /* tp_clear */
     0,                                          /* tp_richcompare */
     offsetof(PyGenObject, gi_weakreflist),      /* tp_weaklistoffset */
-    (getiterfunc)gen_get_iter,                  /* tp_iter */
+    PyObject_SelfIter,                          /* tp_iter */
     (iternextfunc)gen_iternext,                 /* tp_iternext */
     gen_methods,                                /* tp_methods */
     gen_memberlist,                             /* tp_members */
@@ -645,10 +628,11 @@ PyTypeObject PyGen_Type = {
     _PyGen_Finalize,                            /* tp_finalize */
 };
 
-PyObject *
-PyGen_NewWithQualName(PyFrameObject *f, PyObject *name, PyObject *qualname)
+static PyObject *
+gen_new_with_qualname(PyTypeObject *type, PyFrameObject *f,
+                      PyObject *name, PyObject *qualname)
 {
-    PyGenObject *gen = PyObject_GC_New(PyGenObject, &PyGen_Type);
+    PyGenObject *gen = PyObject_GC_New(PyGenObject, type);
     if (gen == NULL) {
         Py_DECREF(f);
         return NULL;
@@ -674,9 +658,15 @@ PyGen_NewWithQualName(PyFrameObject *f, PyObject *name, PyObject *qualname)
 }
 
 PyObject *
+PyGen_NewWithQualName(PyFrameObject *f, PyObject *name, PyObject *qualname)
+{
+    return gen_new_with_qualname(&PyGen_Type, f, name, qualname);
+}
+
+PyObject *
 PyGen_New(PyFrameObject *f)
 {
-    return PyGen_NewWithQualName(f, NULL, NULL);
+    return gen_new_with_qualname(&PyGen_Type, f, NULL, NULL);
 }
 
 int
@@ -697,6 +687,25 @@ PyGen_NeedsFinalizing(PyGenObject *gen)
     return 0;
 }
 
+/* Coroutine Object */
+
+typedef struct {
+    PyObject_HEAD
+    PyCoroObject *cw_coroutine;
+} PyCoroWrapper;
+
+static int
+gen_is_coroutine(PyObject *o)
+{
+    if (PyGen_CheckExact(o)) {
+        PyCodeObject *code = (PyCodeObject *)((PyGenObject*)o)->gi_code;
+        if (code->co_flags & CO_ITERABLE_COROUTINE) {
+            return 1;
+        }
+    }
+    return 0;
+}
+
 /*
  *   This helper function returns an awaitable for `o`:
  *     - `o` if `o` is a coroutine-object;
@@ -706,13 +715,13 @@ PyGen_NeedsFinalizing(PyGenObject *gen)
  *   an awaitable and returns NULL.
  */
 PyObject *
-_PyGen_GetAwaitableIter(PyObject *o)
+_PyCoro_GetAwaitableIter(PyObject *o)
 {
     unaryfunc getter = NULL;
     PyTypeObject *ot;
 
-    if (PyGen_CheckCoroutineExact(o)) {
-        /* Fast path. It's a central function for 'await'. */
+    if (PyCoro_CheckExact(o) || gen_is_coroutine(o)) {
+        /* 'o' is a coroutine. */
         Py_INCREF(o);
         return o;
     }
@@ -724,22 +733,19 @@ _PyGen_GetAwaitableIter(PyObject *o)
     if (getter != NULL) {
         PyObject *res = (*getter)(o);
         if (res != NULL) {
-            if (!PyIter_Check(res)) {
+            if (PyCoro_CheckExact(res) || gen_is_coroutine(res)) {
+                /* __await__ must return an *iterator*, not
+                   a coroutine or another awaitable (see PEP 492) */
+                PyErr_SetString(PyExc_TypeError,
+                                "__await__() returned a coroutine");
+                Py_CLEAR(res);
+            } else if (!PyIter_Check(res)) {
                 PyErr_Format(PyExc_TypeError,
                              "__await__() returned non-iterator "
                              "of type '%.100s'",
                              Py_TYPE(res)->tp_name);
                 Py_CLEAR(res);
             }
-            else {
-                if (PyGen_CheckCoroutineExact(res)) {
-                    /* __await__ must return an *iterator*, not
-                       a coroutine or another awaitable (see PEP 492) */
-                    PyErr_SetString(PyExc_TypeError,
-                                    "__await__() returned a coroutine");
-                    Py_CLEAR(res);
-                }
-            }
         }
         return res;
     }
@@ -747,6 +753,211 @@ _PyGen_GetAwaitableIter(PyObject *o)
     PyErr_Format(PyExc_TypeError,
                  "object %.100s can't be used in 'await' expression",
                  ot->tp_name);
-
     return NULL;
 }
+
+static PyObject *
+coro_repr(PyCoroObject *coro)
+{
+    return PyUnicode_FromFormat("<coroutine object %S at %p>",
+                                coro->cr_qualname, coro);
+}
+
+static PyObject *
+coro_await(PyCoroObject *coro)
+{
+    PyCoroWrapper *cw = PyObject_GC_New(PyCoroWrapper, &_PyCoroWrapper_Type);
+    if (cw == NULL) {
+        return NULL;
+    }
+    Py_INCREF(coro);
+    cw->cw_coroutine = coro;
+    _PyObject_GC_TRACK(cw);
+    return (PyObject *)cw;
+}
+
+static PyGetSetDef coro_getsetlist[] = {
+    {"__name__", (getter)gen_get_name, (setter)gen_set_name,
+     PyDoc_STR("name of the coroutine")},
+    {"__qualname__", (getter)gen_get_qualname, (setter)gen_set_qualname,
+     PyDoc_STR("qualified name of the coroutine")},
+    {NULL} /* Sentinel */
+};
+
+static PyMemberDef coro_memberlist[] = {
+    {"cr_frame",     T_OBJECT, offsetof(PyCoroObject, cr_frame),    READONLY},
+    {"cr_running",   T_BOOL,   offsetof(PyCoroObject, cr_running),  READONLY},
+    {"cr_code",      T_OBJECT, offsetof(PyCoroObject, cr_code),     READONLY},
+    {NULL}      /* Sentinel */
+};
+
+PyDoc_STRVAR(coro_send_doc,
+"send(arg) -> send 'arg' into coroutine,\n\
+return next yielded value or raise StopIteration.");
+
+PyDoc_STRVAR(coro_throw_doc,
+"throw(typ[,val[,tb]]) -> raise exception in coroutine,\n\
+return next yielded value or raise StopIteration.");
+
+PyDoc_STRVAR(coro_close_doc,
+"close() -> raise GeneratorExit inside coroutine.");
+
+static PyMethodDef coro_methods[] = {
+    {"send",(PyCFunction)_PyGen_Send, METH_O, coro_send_doc},
+    {"throw",(PyCFunction)gen_throw, METH_VARARGS, coro_throw_doc},
+    {"close",(PyCFunction)gen_close, METH_NOARGS, coro_close_doc},
+    {NULL, NULL}        /* Sentinel */
+};
+
+static PyAsyncMethods coro_as_async = {
+    (unaryfunc)coro_await,                      /* am_await */
+    0,                                          /* am_aiter */
+    0                                           /* am_anext */
+};
+
+PyTypeObject PyCoro_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "coroutine",                                /* tp_name */
+    sizeof(PyCoroObject),                       /* tp_basicsize */
+    0,                                          /* tp_itemsize */
+    /* methods */
+    (destructor)gen_dealloc,                    /* tp_dealloc */
+    0,                                          /* tp_print */
+    0,                                          /* tp_getattr */
+    0,                                          /* tp_setattr */
+    &coro_as_async,                             /* tp_as_async */
+    (reprfunc)coro_repr,                        /* tp_repr */
+    0,                                          /* tp_as_number */
+    0,                                          /* tp_as_sequence */
+    0,                                          /* tp_as_mapping */
+    0,                                          /* tp_hash */
+    0,                                          /* tp_call */
+    0,                                          /* tp_str */
+    PyObject_GenericGetAttr,                    /* tp_getattro */
+    0,                                          /* tp_setattro */
+    0,                                          /* tp_as_buffer */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+        Py_TPFLAGS_HAVE_FINALIZE,               /* tp_flags */
+    0,                                          /* tp_doc */
+    (traverseproc)gen_traverse,                 /* tp_traverse */
+    0,                                          /* tp_clear */
+    0,                                          /* tp_richcompare */
+    offsetof(PyCoroObject, cr_weakreflist),     /* tp_weaklistoffset */
+    0,                                          /* tp_iter */
+    0,                                          /* tp_iternext */
+    coro_methods,                               /* tp_methods */
+    coro_memberlist,                            /* tp_members */
+    coro_getsetlist,                            /* tp_getset */
+    0,                                          /* tp_base */
+    0,                                          /* tp_dict */
+    0,                                          /* tp_descr_get */
+    0,                                          /* tp_descr_set */
+    0,                                          /* tp_dictoffset */
+    0,                                          /* tp_init */
+    0,                                          /* tp_alloc */
+    0,                                          /* tp_new */
+    0,                                          /* tp_free */
+    0,                                          /* tp_is_gc */
+    0,                                          /* tp_bases */
+    0,                                          /* tp_mro */
+    0,                                          /* tp_cache */
+    0,                                          /* tp_subclasses */
+    0,                                          /* tp_weaklist */
+    0,                                          /* tp_del */
+    0,                                          /* tp_version_tag */
+    _PyGen_Finalize,                            /* tp_finalize */
+};
+
+static void
+coro_wrapper_dealloc(PyCoroWrapper *cw)
+{
+    _PyObject_GC_UNTRACK((PyObject *)cw);
+    Py_CLEAR(cw->cw_coroutine);
+    PyObject_GC_Del(cw);
+}
+
+static PyObject *
+coro_wrapper_iternext(PyCoroWrapper *cw)
+{
+    return gen_send_ex((PyGenObject *)cw->cw_coroutine, NULL, 0);
+}
+
+static PyObject *
+coro_wrapper_send(PyCoroWrapper *cw, PyObject *arg)
+{
+    return gen_send_ex((PyGenObject *)cw->cw_coroutine, arg, 0);
+}
+
+static PyObject *
+coro_wrapper_throw(PyCoroWrapper *cw, PyObject *args)
+{
+    return gen_throw((PyGenObject *)cw->cw_coroutine, args);
+}
+
+static PyObject *
+coro_wrapper_close(PyCoroWrapper *cw, PyObject *args)
+{
+    return gen_close((PyGenObject *)cw->cw_coroutine, args);
+}
+
+static int
+coro_wrapper_traverse(PyCoroWrapper *cw, visitproc visit, void *arg)
+{
+    Py_VISIT((PyObject *)cw->cw_coroutine);
+    return 0;
+}
+
+static PyMethodDef coro_wrapper_methods[] = {
+    {"send",(PyCFunction)coro_wrapper_send, METH_O, send_doc},
+    {"throw",(PyCFunction)coro_wrapper_throw, METH_VARARGS, throw_doc},
+    {"close",(PyCFunction)coro_wrapper_close, METH_NOARGS, close_doc},
+    {NULL, NULL}        /* Sentinel */
+};
+
+PyTypeObject _PyCoroWrapper_Type = {
+    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    "coroutine_wrapper",
+    sizeof(PyCoroWrapper),                      /* tp_basicsize */
+    0,                                          /* tp_itemsize */
+    (destructor)coro_wrapper_dealloc,           /* destructor tp_dealloc */
+    0,                                          /* tp_print */
+    0,                                          /* tp_getattr */
+    0,                                          /* tp_setattr */
+    0,                                          /* tp_as_async */
+    0,                                          /* tp_repr */
+    0,                                          /* tp_as_number */
+    0,                                          /* tp_as_sequence */
+    0,                                          /* tp_as_mapping */
+    0,                                          /* tp_hash */
+    0,                                          /* tp_call */
+    0,                                          /* tp_str */
+    PyObject_GenericGetAttr,                    /* tp_getattro */
+    0,                                          /* tp_setattro */
+    0,                                          /* tp_as_buffer */
+    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,    /* tp_flags */
+    "A wrapper object implementing __await__ for coroutines.",
+    (traverseproc)coro_wrapper_traverse,        /* tp_traverse */
+    0,                                          /* tp_clear */
+    0,                                          /* tp_richcompare */
+    0,                                          /* tp_weaklistoffset */
+    PyObject_SelfIter,                          /* tp_iter */
+    (iternextfunc)coro_wrapper_iternext,        /* tp_iternext */
+    coro_wrapper_methods,                       /* tp_methods */
+    0,                                          /* tp_members */
+    0,                                          /* tp_getset */
+    0,                                          /* tp_base */
+    0,                                          /* tp_dict */
+    0,                                          /* tp_descr_get */
+    0,                                          /* tp_descr_set */
+    0,                                          /* tp_dictoffset */
+    0,                                          /* tp_init */
+    0,                                          /* tp_alloc */
+    0,                                          /* tp_new */
+    PyObject_Del,                               /* tp_free */
+};
+
+PyObject *
+PyCoro_New(PyFrameObject *f, PyObject *name, PyObject *qualname)
+{
+    return gen_new_with_qualname(&PyCoro_Type, f, name, qualname);
+}
diff --git a/Objects/object.c b/Objects/object.c
index be12be5..11b7bff 100644
--- a/Objects/object.c
+++ b/Objects/object.c
@@ -1726,6 +1726,12 @@ _Py_ReadyTypes(void)
 
     if (PyType_Ready(&PySeqIter_Type) < 0)
         Py_FatalError("Can't initialize sequence iterator type");
+
+    if (PyType_Ready(&PyCoro_Type) < 0)
+        Py_FatalError("Can't initialize coroutine type");
+
+    if (PyType_Ready(&_PyCoroWrapper_Type) < 0)
+        Py_FatalError("Can't initialize coroutine wrapper type");
 }