[wasi-common]: move filetime module to yanix (#1255)

* Move filetime module to yanix

I've noticed that we could replace every occurrence of `crate::Result`
in `filetime` mods with `io::Result`, so I thought why not move it
to `yanix` and get rid off a lot of unnecessary code duplication
within `wasi-common`. Now, ideally I'd have our `filetime` modifications
backported to Alex's [`filetime`] crate, but one step at a time
(apologies Alex, I was meant to backport this ages ago, just didn't
find the time yet... :-().

Anyway, this commit does just that; i.e., moves the `filetime` modules
into `yanix` which seems a better fit for this type of code.

[`filetime`]: https://github.com/alexcrichton/filetime

There is one caveat here. On Emscripten, converting between `filetime::Filetime`
and `libc::timespec` appears to be lossy, at least as far as the
types are concerned. Now, `filetime::Filetime`'s seconds field is
`i64` while nanoseconds field is `u32`, while Emscripten's
`libc::timespec` requires both to be `i32` width. This might actually
not be a problem since I don't think it's possible to fill `filetime::Filetime`
struct with values of width wider than `i32` since Emscripten is 32bit
but just to be on the safe side, we do a `TryInto` conversion, log
the error (if any), and return `libc::EOVERFLOW`.

* Run cargo fmt

* Use i64::from instead of as cast

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

@@ -0,0 +1,34 @@
+//! This module consists of helper types and functions for dealing
+//! with setting the file times specific to Emscripten.
+use crate::filetime::FileTime;
+use crate::from_success_code;
+use std::fs::File;
+use std::io::Result;
+
+pub(crate) const UTIME_NOW: i32 = 1_073_741_823;
+pub(crate) const UTIME_OMIT: i32 = 1_073_741_822;
+
+/// Wrapper for `utimensat` syscall. In Emscripten, there is no point in dynamically resolving
+/// if `utimensat` is available as it always was and will be.
+pub fn utimensat(
+    dirfd: &File,
+    path: &str,
+    atime: FileTime,
+    mtime: FileTime,
+    symlink_nofollow: bool,
+) -> Result<()> {
+    use crate::filetime::to_timespec;
+    use std::ffi::CString;
+    use std::os::unix::prelude::*;
+
+    let flags = if symlink_nofollow {
+        libc::AT_SYMLINK_NOFOLLOW
+    } else {
+        0
+    };
+    let p = CString::new(path.as_bytes())?;
+    let times = [to_timespec(&atime)?, to_timespec(&mtime)?];
+    from_success_code(unsafe {
+        libc::utimensat(dirfd.as_raw_fd(), p.as_ptr(), times.as_ptr(), flags)
+    })
+}

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

@@ -4,6 +4,7 @@ pub(crate) mod dir;
 pub(crate) mod fadvise;
 #[path = "../linux/file.rs"]
 pub(crate) mod file;
+pub(crate) mod filetime;
 
 use crate::dir::SeekLoc;
 use std::convert::TryInto;