[wasi-common] Clean up fd_filestat_get implementation (#757)

* Clean up fd_filestat_get implementation

This commit does 4 things:
* Adds `yanix::file::fstat`, a wrapper around `libc::fstat`.
* It essentially reverts 89fbde2 for Unix hosts -- in other words,
  it brings back the use of `fstat` to obtain `libc::stat` from a
  file descriptor, rather than relying on `std::fs::Metadata`. This
  way, we reuse `host_impl::filestat_from_nix` in
  `hostcalls_impl::fd_filestat_get` implementation rather than
  unnecessarily duplicate code for converting filestats into
  `__wasi_filestat_t`.
* Moves `crate::helpers::systemtime_to_timestamp` to Windows `host_impl`
  module. It does the same thing with helpers which assist in converting
  `std::fs::Metadata` into `__wasi_filestat_t`. This should retain symmetry
  between *nix and Windows impls.
* Makes timestamp conversions in `host_impl::filestat_from_nix` fallible.

* Backport changes to snapshot0

* Signal no overflow with `from` rather than `as` cast

crates/wasi-common/src/sys/windows/host_impl.rs

@@ -1,7 +1,12 @@
 //! WASI host types specific to Windows host.
+use crate::host::FileType;
 use crate::{wasi, Error, Result};
+use std::convert::TryInto;
 use std::ffi::OsStr;
+use std::fs::{self, File};
+use std::io;
 use std::os::windows::ffi::OsStrExt;
+use std::time::{SystemTime, UNIX_EPOCH};
 
 pub(crate) fn errno_from_win(error: winx::winerror::WinError) -> wasi::__wasi_errno_t {
     // TODO: implement error mapping between Windows and WASI
@@ -34,6 +39,60 @@ pub(crate) fn errno_from_win(error: winx::winerror::WinError) -> wasi::__wasi_er
     }
 }
 
+pub(crate) fn filetype_from_std(ftype: &fs::FileType) -> FileType {
+    if ftype.is_file() {
+        FileType::RegularFile
+    } else if ftype.is_dir() {
+        FileType::Directory
+    } else if ftype.is_symlink() {
+        FileType::Symlink
+    } else {
+        FileType::Unknown
+    }
+}
+
+fn num_hardlinks(file: &File) -> io::Result<u64> {
+    Ok(winx::file::get_fileinfo(file)?.nNumberOfLinks.into())
+}
+
+fn device_id(file: &File) -> io::Result<u64> {
+    Ok(winx::file::get_fileinfo(file)?.dwVolumeSerialNumber.into())
+}
+
+pub(crate) fn file_serial_no(file: &File) -> io::Result<u64> {
+    let info = winx::file::get_fileinfo(file)?;
+    let high = info.nFileIndexHigh;
+    let low = info.nFileIndexLow;
+    let no = (u64::from(high) << 32) | u64::from(low);
+    Ok(no)
+}
+
+fn change_time(file: &File) -> io::Result<i64> {
+    winx::file::change_time(file)
+}
+
+fn systemtime_to_timestamp(st: SystemTime) -> Result<u64> {
+    st.duration_since(UNIX_EPOCH)
+        .map_err(|_| Error::EINVAL)? // date earlier than UNIX_EPOCH
+        .as_nanos()
+        .try_into()
+        .map_err(Into::into) // u128 doesn't fit into u64
+}
+
+pub(crate) fn filestat_from_win(file: &File) -> Result<wasi::__wasi_filestat_t> {
+    let metadata = file.metadata()?;
+    Ok(wasi::__wasi_filestat_t {
+        dev: device_id(file)?,
+        ino: file_serial_no(file)?,
+        nlink: num_hardlinks(file)?.try_into()?, // u64 doesn't fit into u32
+        size: metadata.len(),
+        atim: systemtime_to_timestamp(metadata.accessed()?)?,
+        ctim: change_time(file)?.try_into()?, // i64 doesn't fit into u64
+        mtim: systemtime_to_timestamp(metadata.modified()?)?,
+        filetype: filetype_from_std(&metadata.file_type()).to_wasi(),
+    })
+}
+
 /// Creates owned WASI path from OS string.
 ///
 /// NB WASI spec requires OS string to be valid UTF-8. Otherwise,

crates/wasi-common/src/sys/windows/hostcalls_impl/fs.rs

@@ -3,7 +3,6 @@
 use super::fs_helpers::*;
 use crate::ctx::WasiCtx;
 use crate::fdentry::FdEntry;
-use crate::helpers::systemtime_to_timestamp;
 use crate::host::{Dirent, FileType};
 use crate::hostcalls_impl::{fd_filestat_set_times_impl, PathGet};
 use crate::sys::fdentry_impl::determine_type_rights;
@@ -249,10 +248,10 @@ fn dirent_from_path<P: AsRef<Path>>(
         .open(path)?;
     let ty = file.metadata()?.file_type();
     Ok(Dirent {
-        ftype: filetype_from_std(&ty),
+        ftype: host_impl::filetype_from_std(&ty),
         name: name.to_owned(),
         cookie,
-        ino: file_serial_no(&file)?,
+        ino: host_impl::file_serial_no(&file)?,
     })
 }
 
@@ -305,8 +304,8 @@ pub(crate) fn fd_readdir(
 
         Ok(Dirent {
             name: path_from_host(dir.file_name())?,
-            ftype: filetype_from_std(&dir.file_type()?),
-            ino: File::open(dir.path()).and_then(|f| file_serial_no(&f))?,
+            ftype: host_impl::filetype_from_std(&dir.file_type()?),
+            ino: File::open(dir.path()).and_then(|f| host_impl::file_serial_no(&f))?,
             cookie: no,
         })
     });
@@ -429,50 +428,8 @@ pub(crate) fn path_rename(resolved_old: PathGet, resolved_new: PathGet) -> Resul
     })
 }
 
-pub(crate) fn num_hardlinks(file: &File, _metadata: &Metadata) -> io::Result<u64> {
-    Ok(winx::file::get_fileinfo(file)?.nNumberOfLinks.into())
-}
-
-pub(crate) fn device_id(file: &File, _metadata: &Metadata) -> io::Result<u64> {
-    Ok(winx::file::get_fileinfo(file)?.dwVolumeSerialNumber.into())
-}
-
-pub(crate) fn file_serial_no(file: &File) -> io::Result<u64> {
-    let info = winx::file::get_fileinfo(file)?;
-    let high = info.nFileIndexHigh;
-    let low = info.nFileIndexLow;
-    let no = ((high as u64) << 32) | (low as u64);
-    Ok(no)
-}
-
-pub(crate) fn change_time(file: &File, _metadata: &Metadata) -> io::Result<i64> {
-    winx::file::change_time(file)
-}
-
-pub(crate) fn fd_filestat_get_impl(file: &std::fs::File) -> Result<wasi::__wasi_filestat_t> {
-    let metadata = file.metadata()?;
-    Ok(wasi::__wasi_filestat_t {
-        dev: device_id(file, &metadata)?,
-        ino: file_serial_no(file)?,
-        nlink: num_hardlinks(file, &metadata)?.try_into()?, // u64 doesn't fit into u32
-        size: metadata.len(),
-        atim: systemtime_to_timestamp(metadata.accessed()?)?,
-        ctim: change_time(file, &metadata)?.try_into()?, // i64 doesn't fit into u64
-        mtim: systemtime_to_timestamp(metadata.modified()?)?,
-        filetype: filetype_from_std(&metadata.file_type()).to_wasi(),
-    })
-}
-
-pub(crate) fn filetype_from_std(ftype: &std::fs::FileType) -> FileType {
-    if ftype.is_file() {
-        FileType::RegularFile
-    } else if ftype.is_dir() {
-        FileType::Directory
-    } else if ftype.is_symlink() {
-        FileType::Symlink
-    } else {
-        FileType::Unknown
-    }
+pub(crate) fn fd_filestat_get(file: &std::fs::File) -> Result<wasi::__wasi_filestat_t> {
+    host_impl::filestat_from_win(file)
 }
 
 pub(crate) fn path_filestat_get(
@@ -481,7 +438,7 @@ pub(crate) fn path_filestat_get(
 ) -> Result<wasi::__wasi_filestat_t> {
     let path = resolved.concatenate()?;
     let file = File::open(path)?;
-    fd_filestat_get_impl(&file)
+    host_impl::filestat_from_win(&file)
 }
 
 pub(crate) fn path_filestat_set_times(

crates/wasi-common/src/sys/windows/hostcalls_impl/misc.rs

@@ -1,7 +1,6 @@
 #![allow(non_camel_case_types)]
 #![allow(unused_unsafe)]
 #![allow(unused)]
-use crate::helpers::systemtime_to_timestamp;
 use crate::hostcalls_impl::{ClockEventData, FdEventData};
 use crate::memory::*;
 use crate::sys::host_impl;