Cleanup wasm32-wasi target

This change ensures that we only compile our WASI implementation for
32-bit targets. The interaction between the WASI proposal and the
memory64 proposal is not yet clear, [wasmtime does not yet support](
bytecodealliance/wasmtime#3594 (comment))
using WASI with memory64, and many of the interfaces use 32-bit values
for pointers.

This change also reduces the use of `unsafe` from the wasi
implementation. As noted in #253, changes to `Errno` mean that we can't
get the error message from the raw error code, but we can avoid using
unsafe when converting this code to a NonZeroU32. This handling also
makes WASI behave more like our other targets, which also manually check
that errno is non-zero.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/lib.rs

@@ -238,7 +238,7 @@ cfg_if! {
     } else if #[cfg(target_os = "openbsd")] {
         mod util_libc;
         #[path = "openbsd.rs"] mod imp;
-    } else if #[cfg(target_os = "wasi")] {
+    } else if #[cfg(all(target_arch = "wasm32", target_os = "wasi"))] {
         #[path = "wasi.rs"] mod imp;
     } else if #[cfg(all(target_arch = "x86_64", target_os = "hermit"))] {
         #[path = "rdrand.rs"] mod imp;

src/wasi.rs

@@ -8,12 +8,18 @@
 
 //! Implementation for WASI
 use crate::Error;
-use core::{mem::MaybeUninit, num::NonZeroU32};
-use wasi::wasi_snapshot_preview1::random_get;
+use core::{
+    mem::MaybeUninit,
+    num::{NonZeroU16, NonZeroU32},
+};
+use wasi::random_get;
 
 pub fn getrandom_inner(dest: &mut [MaybeUninit<u8>]) -> Result<(), Error> {
-    match unsafe { random_get(dest.as_mut_ptr() as i32, dest.len() as i32) } {
-        0 => Ok(()),
-        err => Err(unsafe { NonZeroU32::new_unchecked(err as u32) }.into()),
-    }
+    unsafe { random_get(dest.as_mut_ptr() as *mut u8, dest.len()) }.map_err(|e| {
+        // The WASI errno will always be non-zero, but we check just in case.
+        match NonZeroU16::new(e.raw()) {
+            Some(r) => Error::from(NonZeroU32::from(r)),
+            None => Error::ERRNO_NOT_POSITIVE,
+        }
+    })
 }