Begin porting yanix to WASI.

This isn't complete yet, but subsequent steps will depend on Rust libstd
and libc bindings changes that are in flight.

crates/wasi-common/yanix/src/sys/mod.rs

@@ -18,6 +18,9 @@ cfg_if! {
                         target_os = "dragonfly"))] {
         mod bsd;
         pub(crate) use bsd::*;
+    } else if #[cfg(target_os = "wasi")] {
+        mod wasi;
+        pub(crate) use wasi::*;
     } else {
         compile_error!("yanix doesn't compile for this platform yet");
     }

crates/wasi-common/yanix/src/sys/wasi/fadvise.rs

@@ -0,0 +1,23 @@
+use crate::from_success_code;
+use std::io::Result;
+use std::os::wasi::prelude::*;
+
+#[derive(Debug, Copy, Clone)]
+#[repr(i32)]
+pub enum PosixFadviseAdvice {
+    Normal = libc::POSIX_FADV_NORMAL,
+    Sequential = libc::POSIX_FADV_SEQUENTIAL,
+    Random = libc::POSIX_FADV_RANDOM,
+    NoReuse = libc::POSIX_FADV_NOREUSE,
+    WillNeed = libc::POSIX_FADV_WILLNEED,
+    DontNeed = libc::POSIX_FADV_DONTNEED,
+}
+
+pub unsafe fn posix_fadvise(
+    fd: RawFd,
+    offset: libc::off_t,
+    len: libc::off_t,
+    advice: PosixFadviseAdvice,
+) -> Result<()> {
+    from_success_code(libc::posix_fadvise(fd, offset, len, advice as libc::c_int))
+}

crates/wasi-common/yanix/src/sys/wasi/file.rs

@@ -0,0 +1,21 @@
+use std::{
+    io::{Error, Result},
+    os::wasi::prelude::*,
+};
+
+pub unsafe fn isatty(fd: RawFd) -> Result<bool> {
+    let res = libc::isatty(fd);
+    if res == 1 {
+        // isatty() returns 1 if fd is an open file descriptor referring to a terminal...
+        Ok(true)
+    } else {
+        // ... otherwise 0 is returned, and errno is set to indicate the error.
+        let errno = Error::last_os_error();
+        let raw_errno = errno.raw_os_error().unwrap();
+        if raw_errno == libc::ENOTTY {
+            Ok(false)
+        } else {
+            Err(errno)
+        }
+    }
+}

crates/wasi-common/yanix/src/sys/wasi/filetime.rs

@@ -0,0 +1,33 @@
+use crate::filetime::FileTime;
+use crate::from_success_code;
+use std::fs::File;
+use std::io;
+
+pub fn utimensat(
+    dirfd: &File,
+    path: &str,
+    atime: FileTime,
+    mtime: FileTime,
+    symlink_nofollow: bool,
+) -> io::Result<()> {
+    use crate::filetime::to_timespec;
+    use std::ffi::CString;
+    use std::os::wasi::prelude::*;
+
+    let p = CString::new(path.as_bytes())?;
+    let times = [to_timespec(&atime)?, to_timespec(&mtime)?];
+    let flags = if symlink_nofollow {
+        libc::AT_SYMLINK_NOFOLLOW
+    } else {
+        0
+    };
+
+    from_success_code(unsafe {
+        libc::utimensat(
+            dirfd.as_raw_fd() as libc::c_int,
+            p.as_ptr(),
+            times.as_ptr(),
+            flags,
+        )
+    })
+}

crates/wasi-common/yanix/src/sys/wasi/mod.rs

@@ -0,0 +1,3 @@
+pub(crate) mod fadvise;
+pub(crate) mod file;
+pub(crate) mod filetime;