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.

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.

> 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.