os.dup2 should return the new fd

[benjaminp]

BPO	32441
Nosy	@gpshead, @vstinner, @benjaminp, @xdegaye
PRs	return the new file descriptor from os.dup2 (closes bpo-32441) #5041
Files	without-dup3_works.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 2018-01-30.07:16:36.816>
created_at = <Date 2017-12-28.17:44:43.535>
labels = ['extension-modules', 'easy', '3.7']
title = 'os.dup2 should return the new fd'
updated_at = <Date 2018-01-30.07:16:36.815>
user = 'https://github.com/benjaminp'

bugs.python.org fields:

activity = <Date 2018-01-30.07:16:36.815>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-30.07:16:36.816>
closer = 'benjamin.peterson'
components = ['Extension Modules']
creation = <Date 2017-12-28.17:44:43.535>
creator = 'benjamin.peterson'
dependencies = []
files = ['47366']
hgrepos = []
issue_num = 32441
keywords = ['patch', 'easy']
message_count = 9.0
messages = ['309135', '309197', '309503', '309538', '309549', '309578', '309585', '310752', '311239']
nosy_count = 4.0
nosy_names = ['gregory.p.smith', 'vstinner', 'benjamin.peterson', 'xdegaye']
pr_nums = ['5041']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue32441'
versions = ['Python 3.7']

[benjaminp]

os.dup2 currently always None. However, the underlying standard Unix function returns the new file descriptor (i.e., the second argument) on success. We should follow that convention, too.

[benjaminp]

New changeset bbdb17d by Benjamin Peterson in branch 'master':
return the new file descriptor from os.dup2 (closes bpo-32441) (bpo-5041)
bbdb17d

[xdegaye]

gcc is a little bit lost and prints now the following (false) warning:

gcc -pthread -Wno-unused-result -Wsign-compare -g -Og -Wall -Wstrict-prototypes    -std=c99 -Wextra
-Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration   -I. -I./Include    -DPy_BUILD_CORE  -c ./Modules/posixmodule.c -o Modules/posixmodule.o./Modules/posixmodule.c: In function ‘os_dup2_impl’:
./Modules/posixmodule.c:7785:9: warning: ‘res’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     int res;
         ^~~

The following change fools gcc that does not print anymore the warning:

diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 47b79fcc79..90d73daf97 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -7845,7 +7845,7 @@ os_dup2_impl(PyObject *module, int fd, int fd2, int inheritable)
         }
     }
 
-    if (inheritable || dup3_works == 0)
+    if (inheritable || (!inheritable && dup3_works == 0))
     {
 #endif
         Py_BEGIN_ALLOW_THREADS

The change does not modify the behavior:

• dup3_works == 0 is equivalent to ((inheritable && dup3_works == 0) || (!inheritable && dup3_works == 0))
• (inheritable && dup3_works == 0) is always false
• hence dup3_works == 0 is equivalent to (!inheritable && dup3_works == 0)

[benjaminp]

I would just make a declaration a definition with a dummy value (0?) rather than complicating the branches.

[xdegaye]

Both the change in my previous post or using a definition for 'res' are not needed if 'dup3_works' is removed. That would make the code more clear for both gcc and a human reader. The attached patch removes it.

Using a definition for 'res' LGTM also.

[benjaminp]

I suspect the dup3_works variable is supposed to be static.

[xdegaye]

Good catch, the code makes much more sense now :-)

[vstinner]

The change introduced a warning. Can someone please take a look (and maybe fix it)?

./Modules/posixmodule.c: In function 'os_dup2_impl':
./Modules/posixmodule.c:7785:9: warning: 'res' may be used uninitialized in this function [-Wmaybe-uninitialized]
     int res;
         ^~~

[gpshead]

#5346 (merged) should fix that warning.