_overlapped: read_buffer and write_buffer are misused

[vstinner]

BPO	20449
Nosy	@gvanrossum, @pitrou, @vstinner, @larryhastings
Files	overlapped.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 2014-01-30.20:45:43.593>
created_at = <Date 2014-01-30.15:22:29.593>
labels = ['release-blocker']
title = '_overlapped: read_buffer and write_buffer are misused'
updated_at = <Date 2014-01-30.20:45:43.592>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2014-01-30.20:45:43.592>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2014-01-30.20:45:43.593>
closer = 'vstinner'
components = []
creation = <Date 2014-01-30.15:22:29.593>
creator = 'vstinner'
dependencies = []
files = ['33812']
hgrepos = []
issue_num = 20449
keywords = ['patch']
message_count = 2.0
messages = ['209721', '209737']
nosy_count = 5.0
nosy_names = ['gvanrossum', 'pitrou', 'vstinner', 'larry', 'sbt']
pr_nums = []
priority = 'release blocker'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue20449'
versions = ['Python 3.4']

[vstinner]

I found an issue in overlapped.c: read_buffer and write_buffer attributes are defined in an union, but they are sometimes use even if the type is not good. I found this issue while trying to get a BrokenPipeError on Windows when the stdin stream of a subprocess is full but the process exited.

Could you please review attached patch, especially the change in Overlapped_getresult?

Another option would be to use two real attributes (remove the union), but I like the usage of union :-)

All Tulip unit tests of my subprocess_stream pass on Linux and Windows with the patch.

[vstinner]

Patch commited as 73dbb884eb09 in Python 3.4. (No need to backport, it's a new module in Python 3.4).

Thanks for the review Richard.