From c724bae51cd0580cd493f319f3b14c2e1a28f3b6 Mon Sep 17 00:00:00 2001
From: Yury Selivanov <yselivanov@sprymix.com>
Date: Wed, 2 Mar 2016 11:30:46 -0500
Subject: [PATCH] coroutines: Error when awaiting on coroutine that's being
 awaited

Issue #25888
---
 Include/genobject.h         |  1 +
 Lib/test/test_coroutines.py | 18 ++++++++++++++++++
 Objects/genobject.c         | 12 ++++++------
 Python/ceval.c              | 15 +++++++++++++++
 4 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/Include/genobject.h b/Include/genobject.h
index 4c718617232..30cb0232344 100644
--- a/Include/genobject.h
+++ b/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
diff --git a/Lib/test/test_coroutines.py b/Lib/test/test_coroutines.py
index 954a9a1db2f..187348d48e1 100644
--- a/Lib/test/test_coroutines.py
+++ b/Lib/test/test_coroutines.py
@@ -942,6 +942,24 @@ class CoroutineTest(unittest.TestCase):
         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):
diff --git a/Objects/genobject.c b/Objects/genobject.c
index 82019774d9c..f74d044dcf7 100644
--- a/Objects/genobject.c
+++ b/Objects/genobject.c
@@ -267,8 +267,8 @@ gen_close_iter(PyObject *yf)
     return 0;
 }
 
-static PyObject *
-gen_yf(PyGenObject *gen)
+PyObject *
+_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;
diff --git a/Python/ceval.c b/Python/ceval.c
index aee0e6bc852..f9ecad46cf8 100644
--- a/Python/ceval.c
+++ b/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) {