6 files changed, 68 insertions, 10 deletions

diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 3e8fd82..f9733b2 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1085,6 +1085,20 @@ always available.
    If called twice, the new wrapper replaces the previous one.  The function
    is thread-specific.
 
+   The *wrapper* callable cannot define new coroutines directly or indirectly::
+
+        def wrapper(coro):
+            async def wrap(coro):
+                return await coro
+            return wrap(coro)
+        sys.set_coroutine_wrapper(wrapper)
+
+        async def foo(): pass
+
+        # The following line will fail with a RuntimeError, because
+        # `wrapper` creates a `wrap(coro)` coroutine:
+        foo()
+
    See also :func:`get_coroutine_wrapper`.
 
    .. versionadded:: 3.5
diff --git a/Include/ceval.h b/Include/ceval.h
index e558594..9f4d3f1 100644
--- a/Include/ceval.h
+++ b/Include/ceval.h
@@ -23,8 +23,9 @@ PyAPI_FUNC(PyObject *) PyEval_CallMethod(PyObject *obj,
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(void) PyEval_SetProfile(Py_tracefunc, PyObject *);
 PyAPI_FUNC(void) PyEval_SetTrace(Py_tracefunc, PyObject *);
-PyAPI_FUNC(void) _PyEval_SetCoroutineWrapper(PyObject *wrapper);
+PyAPI_FUNC(void) _PyEval_SetCoroutineWrapper(PyObject *);
 PyAPI_FUNC(PyObject *) _PyEval_GetCoroutineWrapper(void);
+PyAPI_FUNC(PyObject *) _PyEval_ApplyCoroutineWrapper(PyObject *);
 #endif
 
 struct _frame; /* Avoid including frameobject.h */
diff --git a/Include/pystate.h b/Include/pystate.h
index 2ee81df..a2fd803 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -135,6 +135,7 @@ typedef struct _ts {
     void *on_delete_data;
 
     PyObject *coroutine_wrapper;
+    int in_coroutine_wrapper;
 
     /* XXX signal handlers should also be here */
 
diff --git a/Lib/test/test_coroutines.py b/Lib/test/test_coroutines.py
index e79896a..670852d 100644
--- a/Lib/test/test_coroutines.py
+++ b/Lib/test/test_coroutines.py
@@ -995,6 +995,26 @@ class SysSetCoroWrapperTest(unittest.TestCase):
             sys.set_coroutine_wrapper(1)
         self.assertIsNone(sys.get_coroutine_wrapper())
 
+    def test_set_wrapper_3(self):
+        async def foo():
+            return 'spam'
+
+        def wrapper(coro):
+            async def wrap(coro):
+                return await coro
+            return wrap(coro)
+
+        sys.set_coroutine_wrapper(wrapper)
+        try:
+            with self.assertRaisesRegex(
+                RuntimeError,
+                "coroutine wrapper.*\.wrapper at 0x.*attempted to "
+                "recursively wrap <coroutine.*\.wrap"):
+
+                foo()
+        finally:
+            sys.set_coroutine_wrapper(None)
+
 
 class CAPITest(unittest.TestCase):
 
diff --git a/Python/ceval.c b/Python/ceval.c
index bb2c0b9..2a1db17 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -3921,7 +3921,6 @@ _PyEval_EvalCodeWithName(PyObject *_co, PyObject *globals, PyObject *locals,
 
     if (co->co_flags & CO_GENERATOR) {
         PyObject *gen;
-        PyObject *coroutine_wrapper;
 
         /* Don't need to keep the reference to f_back, it will be set
          * when the generator is resumed. */
@@ -3935,14 +3934,9 @@ _PyEval_EvalCodeWithName(PyObject *_co, PyObject *globals, PyObject *locals,
         if (gen == NULL)
             return NULL;
 
-        if (co->co_flags & (CO_COROUTINE | CO_ITERABLE_COROUTINE)) {
-            coroutine_wrapper = _PyEval_GetCoroutineWrapper();
-            if (coroutine_wrapper != NULL) {
-                PyObject *wrapped =
-                            PyObject_CallFunction(coroutine_wrapper, "N", gen);
-                gen = wrapped;
-            }
-        }
+        if (co->co_flags & (CO_COROUTINE | CO_ITERABLE_COROUTINE))
+            return _PyEval_ApplyCoroutineWrapper(gen);
+
         return gen;
     }
 
@@ -4408,6 +4402,33 @@ _PyEval_GetCoroutineWrapper(void)
 }
 
 PyObject *
+_PyEval_ApplyCoroutineWrapper(PyObject *gen)
+{
+    PyObject *wrapped;
+    PyThreadState *tstate = PyThreadState_GET();
+    PyObject *wrapper = tstate->coroutine_wrapper;
+
+    if (tstate->in_coroutine_wrapper) {
+        assert(wrapper != NULL);
+        PyErr_Format(PyExc_RuntimeError,
+                     "coroutine wrapper %.150R attempted "
+                     "to recursively wrap %.150R",
+                     wrapper,
+                     gen);
+        return NULL;
+    }
+
+    if (wrapper == NULL) {
+        return gen;
+    }
+
+    tstate->in_coroutine_wrapper = 1;
+    wrapped = PyObject_CallFunction(wrapper, "N", gen);
+    tstate->in_coroutine_wrapper = 0;
+    return wrapped;
+}
+
+PyObject *
 PyEval_GetBuiltins(void)
 {
     PyFrameObject *current_frame = PyEval_GetFrame();
diff --git a/Python/pystate.c b/Python/pystate.c
index 4ac05d6..7e0267a 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -213,6 +213,7 @@ new_threadstate(PyInterpreterState *interp, int init)
         tstate->on_delete_data = NULL;
 
         tstate->coroutine_wrapper = NULL;
+        tstate->in_coroutine_wrapper = 0;
 
         if (init)
             _PyThreadState_Init(tstate);