coroutines: Error when awaiting on coroutine that's being awaited

Issue #25888

Include/genobject.h

@@ -43,6 +43,7 @@ PyAPI_FUNC(PyObject *) PyGen_NewWithQualName(struct _frame *,
 PyAPI_FUNC(int) PyGen_NeedsFinalizing(PyGenObject *);
 PyAPI_FUNC(int) _PyGen_FetchStopIterationValue(PyObject **);
 PyObject *_PyGen_Send(PyGenObject *, PyObject *);
+PyObject *_PyGen_yf(PyGenObject *);
 PyAPI_FUNC(void) _PyGen_Finalize(PyObject *self);
 
 #ifndef Py_LIMITED_API

Lib/test/test_coroutines.py

@@ -942,6 +942,24 @@ async def coro2():
         with self.assertRaises(Marker):
             c.throw(ZeroDivisionError)
 
+    def test_await_15(self):
+        @types.coroutine
+        def nop():
+            yield
+
+        async def coroutine():
+            await nop()
+
+        async def waiter(coro):
+            await coro
+
+        coro = coroutine()
+        coro.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    "coroutine is being awaited already"):
+            waiter(coro).send(None)
+
     def test_with_1(self):
         class Manager:
             def __init__(self, name):

Objects/genobject.c

@@ -267,8 +267,8 @@ gen_close_iter(PyObject *yf)
     return 0;
 }
 
-static PyObject *
+PyObject *
-gen_yf(PyGenObject *gen)
+_PyGen_yf(PyGenObject *gen)
 {
     PyObject *yf = NULL;
     PyFrameObject *f = gen->gi_frame;
@@ -290,7 +290,7 @@ static PyObject *
 gen_close(PyGenObject *gen, PyObject *args)
 {
     PyObject *retval;
-    PyObject *yf = gen_yf(gen);
+    PyObject *yf = _PyGen_yf(gen);
     int err = 0;
 
     if (yf) {
@@ -330,7 +330,7 @@ gen_throw(PyGenObject *gen, PyObject *args)
     PyObject *typ;
     PyObject *tb = NULL;
     PyObject *val = NULL;
-    PyObject *yf = gen_yf(gen);
+    PyObject *yf = _PyGen_yf(gen);
     _Py_IDENTIFIER(throw);
 
     if (!PyArg_UnpackTuple(args, "throw", 1, 3, &typ, &val, &tb))
@@ -564,7 +564,7 @@ gen_set_qualname(PyGenObject *op, PyObject *value)
 static PyObject *
 gen_getyieldfrom(PyGenObject *gen)
 {
-    PyObject *yf = gen_yf(gen);
+    PyObject *yf = _PyGen_yf(gen);
     if (yf == NULL)
         Py_RETURN_NONE;
     return yf;
@@ -799,7 +799,7 @@ coro_await(PyCoroObject *coro)
 static PyObject *
 coro_get_cr_await(PyCoroObject *coro)
 {
-    PyObject *yf = gen_yf((PyGenObject *) coro);
+    PyObject *yf = _PyGen_yf((PyGenObject *) coro);
     if (yf == NULL)
         Py_RETURN_NONE;
     return yf;

Python/ceval.c

@@ -2021,6 +2021,21 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 
             Py_DECREF(iterable);
 
+            if (iter != NULL && PyCoro_CheckExact(iter)) {
+                PyObject *yf = _PyGen_yf((PyGenObject*)iter);
+                if (yf != NULL) {
+                    /* `iter` is a coroutine object that is being
+                       awaited, `yf` is a pointer to the current awaitable
+                       being awaited on. */
+                    Py_DECREF(yf);
+                    Py_CLEAR(iter);
+                    PyErr_SetString(
+                        PyExc_RuntimeError,
+                        "coroutine is being awaited already");
+                    /* The code below jumps to `error` if `iter` is NULL. */
+                }
+            }
+
             SET_TOP(iter); /* Even if it's NULL */
 
             if (iter == NULL) {