Enhancement: add `os.getdtablesize(..)` to `os` (`posix`) module

[ngie-eign]

BPO	39742
Nosy	@larryhastings, @ngie-eign

^{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 = None
created_at = <Date 2020-02-24.20:36:13.030>
labels = ['type-feature', 'library', '3.9']
title = 'Enhancement: add `os.getdtablesize(..)` to `os` (`posix`) module'
updated_at = <Date 2020-02-29.00:21:12.093>
user = 'https://github.com/ngie-eign'

bugs.python.org fields:

activity = <Date 2020-02-29.00:21:12.093>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-02-24.20:36:13.030>
creator = 'ngie'
dependencies = []
files = []
hgrepos = []
issue_num = 39742
keywords = []
message_count = 1.0
messages = ['362603']
nosy_count = 2.0
nosy_names = ['larry', 'ngie']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue39742'
versions = ['Python 3.9']

Linked PRs

• gh-83923: Added os.getdtablesize function to os module #124411

[ngie-eign]

getdtablesize({2,3}) is a wonderful library call/system call to have access to because it allows one to determine the descriptor limits at runtime in an easier manner than having to do the equivalent with os.sysconf(..):

>> os.sysconf(os.sysconf_names["SC_OPEN_MAX"])

This has been present in *BSD since time memorial [1] and in Linux since 2010 [2], so I think it would be a good addition to the os (posix) module.

I will submit a diff for this in a few days, if it's deemed acceptable to have in the posix module.

1. https://www.freebsd.org/cgi/man.cgi?query=getdtablesize&sektion=2&manpath=freebsd-release-ports
2. http://man7.org/linux/man-pages/man2/getdtablesize.2.html

[serhiy-storchaka]

There are already two ways to get the same value: os.sysconf("SC_OPEN_MAX") and resource.getrlimit(resource.RLIMIT_NOFILE). They are even more portable, because both functions are in POSIX, unlike getdtablesize().

For now, #124411 changes 8 files, has 15 commits and gathered 26 comments for 8 hours. It is obvious that the cost of this feature is pretty high. It may grow to hundreds of comments if we discuss it on discuss.python.org.

On other hand, the benefit is pretty small, taking into account existing alternatives. I suggest to close this issue.

[vstinner]

I don't think that it's worth it to add this function, see #124411 for the rationale.

You can use already resource.getrlimit(resource.RLIMIT_NOFILE)[0] and os.sysconf("SC_OPEN_MAX").