Dangerous usage of "O" format string in _asynciomodule.c

[vstinner]

BPO	28920
Nosy	@gvanrossum, @vstinner, @1st1
Files	_asyncio.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-12-15.08:10:13.455>
created_at = <Date 2016-12-09.13:38:13.443>
labels = ['3.7', 'expert-asyncio']
title = 'Dangerous usage of "O" format string in _asynciomodule.c'
updated_at = <Date 2016-12-15.08:10:13.454>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2016-12-15.08:10:13.454>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2016-12-15.08:10:13.455>
closer = 'vstinner'
components = ['asyncio']
creation = <Date 2016-12-09.13:38:13.443>
creator = 'vstinner'
dependencies = []
files = ['45815']
hgrepos = []
issue_num = 28920
keywords = ['patch']
message_count = 5.0
messages = ['282778', '282779', '282781', '282783', '283250']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'vstinner', 'python-dev', 'yselivanov']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue28920'
versions = ['Python 3.6', 'Python 3.7']

[vstinner]

The new _asyncio module of Python 3.6 uses the _PyObject_CallMethodId() function to call functions. This function has a weird behaviour when using the format string "O": if the object is a tuple, the tuple is unpacked.

_PyObject_CallMethodId(obj, &PyId_meth, "O", tuple, NULL) calls obj.meth(*tuple) instead of obj.meth(tuple).

I only found one function which may have the bug: task_call_step(). But it seems like this function cannot be called with a tuple as "arg", only with an exception object.

But just in case, I would suggest to replace:
_PyObject_CallMethodId(obj, nameid, "O", arg);
with
_PyObject_CallMethodIdObjArgs(obj, nameid, arg, NULL);

Note: _PyObject_CallMethodId() is called with a NULL terminal in the argument list, but the NULL is useless. A terminator is only required by _PyObject_CallMethodIdObjArgs(). Yeah, Python has a wide choice of functions to call a callable object, with funny APIs... And I'm adding new ones to Python 3.7 ;-)

[vstinner]

Previous similar bug in generators: issue bpo-21209.

[vstinner]

The issue was in fixed in Python 3.7 with a change made for optimization, not directly to fix this issue:

changeset: 105547:b29c719d5992
tag: tip
user: Victor Stinner <victor.stinner@gmail.com>
date: Fri Dec 09 14:24:02 2016 +0100
files: Modules/_asynciomodule.c
description:
Use _PyObject_CallMethodIdObjArgs() in _asyncio

Issue bpo-28915: Replace _PyObject_CallMethodId() with
_PyObject_CallMethodIdObjArgs() when the format string was only made of "O"
formats, PyObject* arguments.

_PyObject_CallMethodIdObjArgs() avoids the creation of a temporary tuple and
doesn't have to parse a format string.

[vstinner]

Minimum patch for Python 3.6: only modify calls using the "O" format.

[python-dev]

New changeset f2745b64d8b7 by Victor Stinner in branch '3.6':
_asyncio uses _PyObject_CallMethodIdObjArgs()
https://hg.python.org/cpython/rev/f2745b64d8b7