Remove "w" format of PyParse_ParseTuple()

[vstinner]

BPO	8850
Nosy	@loewis, @pitrou, @vstinner
Files	remove_w_format.patch remove_w_format-2.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 2010-06-25.00:03:40.859>
created_at = <Date 2010-05-29.02:24:18.472>
labels = ['interpreter-core']
title = 'Remove "w" format of PyParse_ParseTuple()'
updated_at = <Date 2010-06-25.00:03:40.858>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2010-06-25.00:03:40.858>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2010-06-25.00:03:40.859>
closer = 'vstinner'
components = ['Interpreter Core']
creation = <Date 2010-05-29.02:24:18.472>
creator = 'vstinner'
dependencies = []
files = ['17571', '17670']
hgrepos = []
issue_num = 8850
keywords = ['patch']
message_count = 8.0
messages = ['106702', '107193', '107324', '107367', '107755', '107812', '107829', '108572']
nosy_count = 3.0
nosy_names = ['loewis', 'pitrou', 'vstinner']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue8850'
versions = ['Python 3.2']

[vstinner]

"w" format is dangerous because it doesn't give the size of the buffer: the caller may write outside the buffer (buffer overflow).

"w*" and "w#" formats are fine.

It looks like "w" format is not used in trunk nor py3k (only w# and w*).

[vstinner]

Attached patch removes "w" format, cleanups the code for "w*" and "w#" formats, and update the documentation.

[pitrou]

I'd point out that "w#" is mostly useless too. It's supposed to return a read-write buffer, but as the doc says it also doesn't support "mutable objects", since it isn't able to properly lock/pin the buffer; therefore it probably doesn't support anything useful.

[vstinner]

See also bpo-8592: 'y' does not check for embedded NUL bytes.

[vstinner]

"w#" is not only useless but also not used in py3k source code. "w" is also not used. Only "w*" is used by fnctl and socket modules.

The problem with w# is that the caller cannot "release" the buffer and so we cannot lock the buffer. I don't know exactly what happens if you get a pointer to a bytearray, release the GIL and then use it, whereas another thread modifies and/or destroys the bytearray object. (I suppose that something bad will happen, like a segfault)

[vstinner]

New (improved) version of the patch:

• Remove also "w#" format
• Write tests for "w*" format

[vstinner]

See also bpo-8926.

[vstinner]

Commited to 3.2 (r82208), blocked in 3.1 (r82209).