bpo-46009: Do not exhaust generator when send() method raises (GH-29986)

Doc/library/dis.rst

@@ -1170,9 +1170,8 @@ All of the following opcodes use their arguments.
 
 .. opcode:: GEN_START (kind)
 
-    Pops TOS. If TOS was not ``None``, raises an exception. The ``kind``
-    operand corresponds to the type of generator or coroutine and determines
-    the error message. The legal kinds are 0 for generator, 1 for coroutine,
+    Pops TOS. The ``kind`` operand corresponds to the type of generator or
+    coroutine. The legal kinds are 0 for generator, 1 for coroutine,
     and 2 for async generator.
 
    .. versionadded:: 3.10

Lib/test/test_generators.py

@@ -162,6 +162,14 @@ class GeneratorTest(unittest.TestCase):
             with self.assertRaises((TypeError, pickle.PicklingError)):
                 pickle.dumps(g, proto)
 
+    def test_send_non_none_to_new_gen(self):
+        def f():
+            yield 1
+        g = f()
+        with self.assertRaises(TypeError):
+            g.send(0)
+        self.assertEqual(next(g), 1)
+
 
 class ExceptionTest(unittest.TestCase):
     # Tests for the issue #23353: check that the currently handled exception

Misc/NEWS.d/next/Core and Builtins/2021-12-08-11-06-53.bpo-46009.cL8pH0.rst

@@ -0,0 +1,5 @@
+Restore behavior from 3.9 and earlier when sending non-None to newly started
+generator. In 3.9 this did not affect the state of the generator. In 3.10.0
+and 3.10.1 ``gen_func().send(0)`` is equivalent to
+``gen_func().throw(TypeError(...)`` which exhausts the generator. In 3.10.2
+onward, the behavior has been reverted to that of 3.9.

Objects/genobject.c

@@ -157,6 +157,19 @@ gen_send_ex2(PyGenObject *gen, PyObject *arg, PyObject **presult,
     PyObject *result;
 
     *presult = NULL;
+    if (frame->f_lasti < 0 && arg && arg != Py_None) {
+        const char *msg = "can't send non-None value to a "
+                            "just-started generator";
+        if (PyCoro_CheckExact(gen)) {
+            msg = NON_INIT_CORO_MSG;
+        }
+        else if (PyAsyncGen_CheckExact(gen)) {
+            msg = "can't send non-None value to a "
+                    "just-started async generator";
+        }
+        PyErr_SetString(PyExc_TypeError, msg);
+        return PYGEN_ERROR;
+    }
     if (gen->gi_frame_valid && _PyFrame_IsExecuting(frame)) {
         const char *msg = "generator already executing";
         if (PyCoro_CheckExact(gen)) {

Python/ceval.c

@@ -2714,25 +2714,9 @@ check_eval_breaker:
 
         TARGET(GEN_START) {
             PyObject *none = POP();
+            assert(none == Py_None);
+            assert(oparg < 3);
             Py_DECREF(none);
-            if (!Py_IsNone(none)) {
-                if (oparg > 2) {
-                    _PyErr_SetString(tstate, PyExc_SystemError,
-                        "Illegal kind for GEN_START");
-                }
-                else {
-                    static const char *gen_kind[3] = {
-                        "generator",
-                        "coroutine",
-                        "async generator"
-                    };
-                    _PyErr_Format(tstate, PyExc_TypeError,
-                        "can't send non-None value to a "
-                                "just-started %s",
-                                gen_kind[oparg]);
-                }
-                goto error;
-            }
             DISPATCH();
         }