bpo-29587: Make gen.throw() chain exceptions with yield from (GH-19858)

The previous commits on bpo-29587 got exception chaining working
with gen.throw() in the `yield` case. This patch also gets the
`yield from` case working.

As a consequence, implicit exception chaining now also works in
the asyncio scenario of awaiting on a task when an exception is
already active.

Tests are included for both the asyncio case and the pure
generator-only case.

Lib/test/test_asyncio/test_tasks.py

@@ -466,6 +466,33 @@ async def inner2():
  t = outer()
  self.assertEqual(self.loop.run_until_complete(t), 1042)
 
+ def test_exception_chaining_after_await(self):
+ # Test that when awaiting on a task when an exception is already
+ # active, if the task raises an exception it will be chained
+ # with the original.
+ loop = asyncio.new_event_loop()
+ self.set_event_loop(loop)
+
+ async def raise_error():
+ raise ValueError
+
+ async def run():
+ try:
+ raise KeyError(3)
+ except Exception as exc:
+ task = self.new_task(loop, raise_error())
+ try:
+ await task
+ except Exception as exc:
+ self.assertEqual(type(exc), ValueError)
+ chained = exc.__context__
+ self.assertEqual((type(chained), chained.args),
+ (KeyError, (3,)))
+
+ task = self.new_task(loop, run())
+ loop.run_until_complete(task)
+ loop.close()
+
  def test_cancel(self):
 
  def gen():

Lib/test/test_generators.py

@@ -318,7 +318,7 @@ def g():
 
 class GeneratorThrowTest(unittest.TestCase):
 
- def test_exception_context_set(self):
+ def test_exception_context_with_yield(self):
  def f():
  try:
  raise KeyError('a')
@@ -332,6 +332,23 @@ def f():
  context = cm.exception.__context__
  self.assertEqual((type(context), context.args), (KeyError, ('a',)))
 
+ def test_exception_context_with_yield_from(self):
+ def f():
+ yield
+
+ def g():
+ try:
+ raise KeyError('a')
+ except Exception:
+ yield from f()
+
+ gen = g()
+ gen.send(None)
+ with self.assertRaises(ValueError) as cm:
+ gen.throw(ValueError)
+ context = cm.exception.__context__
+ self.assertEqual((type(context), context.args), (KeyError, ('a',)))
+
  def test_throw_after_none_exc_type(self):
  def g():
  try:

Objects/genobject.c

@@ -217,6 +217,18 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc, int closing)
  assert(f->f_back == NULL);
  f->f_back = tstate->frame;
 
+ _PyErr_StackItem *gi_exc_state = &gen->gi_exc_state;
+ if (exc && gi_exc_state->exc_type != NULL &&
+ gi_exc_state->exc_type != Py_None)
+ {
+ Py_INCREF(gi_exc_state->exc_type);
+ Py_XINCREF(gi_exc_state->exc_value);
+ Py_XINCREF(gi_exc_state->exc_traceback);
+ _PyErr_ChainExceptions(gi_exc_state->exc_type,
+ gi_exc_state->exc_value,
+ gi_exc_state->exc_traceback);
+ }
+
  gen->gi_running = 1;
  gen->gi_exc_state.previous_item = tstate->exc_info;
  tstate->exc_info = &gen->gi_exc_state;
@@ -512,16 +524,6 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
  }
 
  PyErr_Restore(typ, val, tb);
-
- _PyErr_StackItem *gi_exc_state = &gen->gi_exc_state;
- if (gi_exc_state->exc_type != NULL && gi_exc_state->exc_type != Py_None) {
- Py_INCREF(gi_exc_state->exc_type);
- Py_XINCREF(gi_exc_state->exc_value);
- Py_XINCREF(gi_exc_state->exc_traceback);
- _PyErr_ChainExceptions(gi_exc_state->exc_type,
- gi_exc_state->exc_value,
- gi_exc_state->exc_traceback);
- }
  return gen_send_ex(gen, Py_None, 1, 0);
 
 failed_throw: