Issue #25887: Raise a RuntimeError when a coroutine is awaited more than once.

Doc/reference/datamodel.rst

@@ -2316,6 +2316,10 @@ Coroutines also have the methods listed below, which are analogous to
 those of generators (see :ref:`generator-methods`).  However, unlike
 generators, coroutines do not directly support iteration.
 
+.. versionchanged:: 3.5.2
+   It is a :exc:`RuntimeError` to await on a coroutine more than once.
+
+
 .. method:: coroutine.send(value)
 
    Starts or resumes execution of the coroutine.  If *value* is ``None``,

Lib/test/test_coroutines.py

@@ -569,6 +569,147 @@ class CoroutineTest(unittest.TestCase):
                                     "coroutine ignored GeneratorExit"):
             c.close()
 
+    def test_func_15(self):
+        # See http://bugs.python.org/issue25887 for details
+
+        async def spammer():
+            return 'spam'
+        async def reader(coro):
+            return await coro
+
+        spammer_coro = spammer()
+
+        with self.assertRaisesRegex(StopIteration, 'spam'):
+            reader(spammer_coro).send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            reader(spammer_coro).send(None)
+
+    def test_func_16(self):
+        # See http://bugs.python.org/issue25887 for details
+
+        @types.coroutine
+        def nop():
+            yield
+        async def send():
+            await nop()
+            return 'spam'
+        async def read(coro):
+            await nop()
+            return await coro
+
+        spammer = send()
+
+        reader = read(spammer)
+        reader.send(None)
+        reader.send(None)
+        with self.assertRaisesRegex(Exception, 'ham'):
+            reader.throw(Exception('ham'))
+
+        reader = read(spammer)
+        reader.send(None)
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            reader.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            reader.throw(Exception('wat'))
+
+    def test_func_17(self):
+        # See http://bugs.python.org/issue25887 for details
+
+        async def coroutine():
+            return 'spam'
+
+        coro = coroutine()
+        with self.assertRaisesRegex(StopIteration, 'spam'):
+            coro.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            coro.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            coro.throw(Exception('wat'))
+
+        # Closing a coroutine shouldn't raise any exception even if it's
+        # already closed/exhausted (similar to generators)
+        coro.close()
+        coro.close()
+
+    def test_func_18(self):
+        # See http://bugs.python.org/issue25887 for details
+
+        async def coroutine():
+            return 'spam'
+
+        coro = coroutine()
+        await_iter = coro.__await__()
+        it = iter(await_iter)
+
+        with self.assertRaisesRegex(StopIteration, 'spam'):
+            it.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            it.send(None)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            # Although the iterator protocol requires iterators to
+            # raise another StopIteration here, we don't want to do
+            # that.  In this particular case, the iterator will raise
+            # a RuntimeError, so that 'yield from' and 'await'
+            # expressions will trigger the error, instead of silently
+            # ignoring the call.
+            next(it)
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            it.throw(Exception('wat'))
+
+        with self.assertRaisesRegex(RuntimeError,
+                                    'cannot reuse already awaited coroutine'):
+            it.throw(Exception('wat'))
+
+        # Closing a coroutine shouldn't raise any exception even if it's
+        # already closed/exhausted (similar to generators)
+        it.close()
+        it.close()
+
+    def test_func_19(self):
+        CHK = 0
+
+        @types.coroutine
+        def foo():
+            nonlocal CHK
+            yield
+            try:
+                yield
+            except GeneratorExit:
+                CHK += 1
+
+        async def coroutine():
+            await foo()
+
+        coro = coroutine()
+
+        coro.send(None)
+        coro.send(None)
+
+        self.assertEqual(CHK, 0)
+        coro.close()
+        self.assertEqual(CHK, 1)
+
+        for _ in range(3):
+            # Closing a coroutine shouldn't raise any exception even if it's
+            # already closed/exhausted (similar to generators)
+            coro.close()
+            self.assertEqual(CHK, 1)
+
     def test_cr_await(self):
         @types.coroutine
         def a():

Misc/NEWS

@@ -69,6 +69,9 @@ Core and Builtins
 
 - Issue #25660: Fix TAB key behaviour in REPL with readline.
 
+- Issue #25887: Raise a RuntimeError when a coroutine object is awaited
+  more than once.
+
 
 Library
 -------

Objects/genobject.c

@@ -78,7 +78,7 @@ gen_dealloc(PyGenObject *gen)
 }
 
 static PyObject *
-gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
+gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
 {
     PyThreadState *tstate = PyThreadState_GET();
     PyFrameObject *f = gen->gi_frame;
@@ -92,9 +92,18 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
         return NULL;
     }
     if (f == NULL || f->f_stacktop == NULL) {
-        /* Only set exception if called from send() */
+        if (PyCoro_CheckExact(gen) && !closing) {
-        if (arg && !exc)
+            /* `gen` is an exhausted coroutine: raise an error,
+               except when called from gen_close(), which should
+               always be a silent method. */
+            PyErr_SetString(
+                PyExc_RuntimeError,
+                "cannot reuse already awaited coroutine");
+        } else if (arg && !exc) {
+            /* `gen` is an exhausted generator:
+               only set exception if called from send(). */
             PyErr_SetNone(PyExc_StopIteration);
+        }
         return NULL;
     }
 
@@ -220,7 +229,7 @@ return next yielded value or raise StopIteration.");
 PyObject *
 _PyGen_Send(PyGenObject *gen, PyObject *arg)
 {
-    return gen_send_ex(gen, arg, 0);
+    return gen_send_ex(gen, arg, 0, 0);
 }
 
 PyDoc_STRVAR(close_doc,
@@ -292,7 +301,7 @@ gen_close(PyGenObject *gen, PyObject *args)
     }
     if (err == 0)
         PyErr_SetNone(PyExc_GeneratorExit);
-    retval = gen_send_ex(gen, Py_None, 1);
+    retval = gen_send_ex(gen, Py_None, 1, 1);
     if (retval) {
         char *msg = "generator ignored GeneratorExit";
         if (PyCoro_CheckExact(gen))
@@ -336,7 +345,7 @@ gen_throw(PyGenObject *gen, PyObject *args)
             gen->gi_running = 0;
             Py_DECREF(yf);
             if (err < 0)
-                return gen_send_ex(gen, Py_None, 1);
+                return gen_send_ex(gen, Py_None, 1, 0);
             goto throw_here;
         }
         if (PyGen_CheckExact(yf)) {
@@ -369,10 +378,10 @@ gen_throw(PyGenObject *gen, PyObject *args)
             /* Termination repetition of YIELD_FROM */
             gen->gi_frame->f_lasti++;
             if (_PyGen_FetchStopIterationValue(&val) == 0) {
-                ret = gen_send_ex(gen, val, 0);
+                ret = gen_send_ex(gen, val, 0, 0);
                 Py_DECREF(val);
             } else {
-                ret = gen_send_ex(gen, Py_None, 1);
+                ret = gen_send_ex(gen, Py_None, 1, 0);
             }
         }
         return ret;
@@ -426,7 +435,7 @@ throw_here:
     }
 
     PyErr_Restore(typ, val, tb);
-    return gen_send_ex(gen, Py_None, 1);
+    return gen_send_ex(gen, Py_None, 1, 0);
 
 failed_throw:
     /* Didn't use our arguments, so restore their original refcounts */
@@ -440,7 +449,7 @@ failed_throw:
 static PyObject *
 gen_iternext(PyGenObject *gen)
 {
-    return gen_send_ex(gen, NULL, 0);
+    return gen_send_ex(gen, NULL, 0, 0);
 }
 
 /*
@@ -901,13 +910,13 @@ coro_wrapper_dealloc(PyCoroWrapper *cw)
 static PyObject *
 coro_wrapper_iternext(PyCoroWrapper *cw)
 {
-    return gen_send_ex((PyGenObject *)cw->cw_coroutine, NULL, 0);
+    return gen_send_ex((PyGenObject *)cw->cw_coroutine, NULL, 0, 0);
 }
 
 static PyObject *
 coro_wrapper_send(PyCoroWrapper *cw, PyObject *arg)
 {
-    return gen_send_ex((PyGenObject *)cw->cw_coroutine, arg, 0);
+    return gen_send_ex((PyGenObject *)cw->cw_coroutine, arg, 0, 0);
 }
 
 static PyObject *