use_file: Use AtomicI32 instead of AtomicUsize to avoid conversions.

All the targets that use this support AtomicI32.

Author: briansmith

src/use_file.rs

@@ -7,7 +7,7 @@ use core::{
     cell::UnsafeCell,
     ffi::c_void,
     mem::MaybeUninit,
-    sync::atomic::{AtomicUsize, Ordering::Relaxed},
+    sync::atomic::{AtomicI32, Ordering::Relaxed},
 };
 
 /// For all platforms, we use `/dev/urandom` rather than `/dev/random`.
@@ -17,7 +17,7 @@ use core::{
 ///   - On AIX, /dev/urandom will "provide cryptographically secure output".
 ///   - On Haiku and QNX Neutrino they are identical.
 const FILE_PATH: &[u8] = b"/dev/urandom\0";
-const FD_UNINIT: usize = usize::max_value();
+const FD_UNINIT: i32 = i32::MIN;
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
     let fd = get_rng_fd()?;
@@ -30,11 +30,12 @@ pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
 // bytes. The file will be opened exactly once. All subsequent calls will
 // return the same file descriptor. This file descriptor is never closed.
 fn get_rng_fd() -> Result<libc::c_int, Error> {
-    static FD: AtomicUsize = AtomicUsize::new(FD_UNINIT);
+    const _: () = assert!(core::mem::size_of::<libc::c_int>() == core::mem::size_of::<i32>());
+    static FD: AtomicI32 = AtomicI32::new(FD_UNINIT);
     fn get_fd() -> Option<libc::c_int> {
         match FD.load(Relaxed) {
             FD_UNINIT => None,
-            val => Some(val as libc::c_int),
+            val => Some(val),
         }
     }
 
@@ -59,8 +60,9 @@ fn get_rng_fd() -> Result<libc::c_int, Error> {
 
     let fd = open_readonly(FILE_PATH)?;
     // The fd always fits in a usize without conflicting with FD_UNINIT.
-    debug_assert!(fd >= 0 && (fd as usize) < FD_UNINIT);
-    FD.store(fd as usize, Relaxed);
+    debug_assert!(fd >= 0);
+    const _: () = assert!(FD_UNINIT < 0);
+    FD.store(fd, Relaxed);
 
     Ok(fd)
 }