thread::available_parallelism for wasm32-wasip1-threads

The target has limited POSIX support and provides the sysconf
function which allows querying the number of available
CPUs.

library/std/src/sys/pal/wasi/thread.rs

@@ -2,7 +2,6 @@
 
 use crate::ffi::CStr;
 use crate::num::NonZero;
-use crate::sys::unsupported;
 use crate::time::Duration;
 use crate::{io, mem};
 
@@ -34,6 +33,8 @@ pub struct pthread_attr_t {
             #[allow(non_camel_case_types)]
             pub type pthread_t = *mut ffi::c_void;
 
+            pub const _SC_NPROCESSORS_ONLN: ffi::c_int = 84;
+
             extern "C" {
                 pub fn pthread_create(
                     native: *mut pthread_t,
@@ -121,7 +122,7 @@ extern "C" fn thread_start(main: *mut libc::c_void) -> *mut libc::c_void {
             }
         } else {
             pub unsafe fn new(_stack: usize, _p: Box<dyn FnOnce()>) -> io::Result<Thread> {
-                unsupported()
+                crate::sys::unsupported()
             }
         }
     }
@@ -187,5 +188,15 @@ pub fn join(self) {
 }
 
 pub fn available_parallelism() -> io::Result<NonZero<usize>> {
-    unsupported()
+    cfg_if::cfg_if! {
+        if #[cfg(target_feature = "atomics")] {
+            match unsafe { libc::sysconf(libc::_SC_NPROCESSORS_ONLN) } {
+                -1 => Err(io::Error::last_os_error()),
+                0 => Err(io::Error::UNKNOWN_THREAD_COUNT),
+                cpus => Ok(unsafe { NonZero::new_unchecked(cpus as usize) }),
+            }
+        } else {
+            crate::sys::unsupported()
+        }
+    }
 }