Solaris: Fix error checking in sched_get_priority_ functions

[kulikjak]

BPO	41839
Nosy	@jstasiak, @kulikjak
PRs	bpo-41839: Fix error checking in sched_get_priority_ functions #22374

^{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 2020-09-23.09:44:51.444>
labels = ['3.8', 'type-bug', 'library', '3.9', '3.10']
title = 'Solaris: Fix error checking in sched_get_priority_ functions'
updated_at = <Date 2020-11-06.15:49:23.262>
user = 'https://github.com/kulikjak'

bugs.python.org fields:

activity = <Date 2020-11-06.15:49:23.262>
actor = 'kulikjak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-09-23.09:44:51.444>
creator = 'kulikjak'
dependencies = []
files = []
hgrepos = []
issue_num = 41839
keywords = ['patch']
message_count = 3.0
messages = ['377363', '380418', '380451']
nosy_count = 2.0
nosy_names = ['jstasiak', 'kulikjak']
pr_nums = ['22374']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41839'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[kulikjak]

Python presumes that any negative number returned from sched_get_priority_min or sched_get_priority_max indicates that error has occurred. However, neither Linux manual pages nor POSIX.1-2001 specification forbids negative values to be returned; only -1 has a special meaning for error.

On Solaris, sched_get_priority_min can, in some instances, return -60, and while this is a legitimate scheduling priority value, Python throws an OSError. Checking for -1 rather than all negative values fixes this issue.

[jstasiak]

I think negative values aren't possible on Linux if one's to trust the sched_get_priority_min man page:

   Linux allows the static priority range 1 to 99 for the SCHED_FIFO and
   SCHED_RR policies, and the priority 0 for the remaining policies.
   Scheduling priority ranges for the various policies are not
   alterable.

But yeah, it seems that negative values need to be accepted on Solaris.

[kulikjak]

Checking for -1 rather than all negative values fixes this issue.

To be 100% exact, it should be "checking for -1 and errno" (as other Jakub noted in the PR). Here is the standard for reference:
https://pubs.opengroup.org/onlinepubs/009695399/functions/sched_get_priority_min.html

Sticking to it should hopefully not break anything.