Auto merge of rust-lang#119436 - semarie:openbsd-available_parallelism, r=Mark-Simulacrum

openbsd: available_parallelism: use the right API

use the standard `sysconf(_SC_NPROCESSORS_ONLN)` way to get the number of  available processors (capable of running processes), and fallback to `sysctl([CTL_HW, HW_NCPU])` (number of CPUs configured) only on error.

it permits to differenciate CPUs online (capable of running processes) vs CPUs configured (not necessary  capable of running processes).

while here, use the common code path for BSDs for doing that, and avoid code  duplication.

Problem initially reported to me by Jiri Navratil.

Author: bors

library/std/src/sys/unix/thread.rs

@@ -354,7 +354,12 @@ pub fn available_parallelism() -> io::Result<NonZeroUsize> {
                     Ok(unsafe { NonZeroUsize::new_unchecked(count) })
                 }
             }
-        } else if #[cfg(any(target_os = "freebsd", target_os = "dragonfly", target_os = "netbsd"))] {
+        } else if #[cfg(any(
+                   target_os = "freebsd",
+                   target_os = "dragonfly",
+                   target_os = "openbsd",
+                   target_os = "netbsd",
+               ))] {
             use crate::ptr;
 
             #[cfg(target_os = "freebsd")]
@@ -427,31 +432,6 @@ pub fn available_parallelism() -> io::Result<NonZeroUsize> {
                     return Err(io::const_io_error!(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform"));
                 }
             }
-            Ok(unsafe { NonZeroUsize::new_unchecked(cpus as usize) })
-        } else if #[cfg(target_os = "openbsd")] {
-            use crate::ptr;
-
-            let mut cpus: libc::c_uint = 0;
-            let mut cpus_size = crate::mem::size_of_val(&cpus);
-            let mut mib = [libc::CTL_HW, libc::HW_NCPU, 0, 0];
-
-            let res = unsafe {
-                libc::sysctl(
-                    mib.as_mut_ptr(),
-                    2,
-                    &mut cpus as *mut _ as *mut _,
-                    &mut cpus_size as *mut _ as *mut _,
-                    ptr::null_mut(),
-                    0,
-                )
-            };
-
-            // Handle errors if any.
-            if res == -1 {
-                return Err(io::Error::last_os_error());
-            } else if cpus == 0 {
-                return Err(io::const_io_error!(io::ErrorKind::NotFound, "The number of hardware threads is not known for the target platform"));
-            }
 
             Ok(unsafe { NonZeroUsize::new_unchecked(cpus as usize) })
         } else if #[cfg(target_os = "nto")] {