Add support for wasi_snapshot_preview1. (#592)

* Add support for wasi_snapshot_preview1.

This adds support for the new ABI, while preserving compatibility
support for the old ABI.

* Fix compilation on platforms where nlink_t isn't 64-bit.

* rustfmt

* Fix Windows build errors.

crates/wasi-common/src/old/snapshot_0/sys/unix/dir.rs

@@ -0,0 +1,216 @@
+// Based on src/dir.rs from nix
+#![allow(unused)] // temporarily, until BSD catches up with this change
+use crate::old::snapshot_0::hostcalls_impl::FileType;
+use libc;
+use nix::{errno::Errno, Error, Result};
+use std::os::unix::io::{AsRawFd, IntoRawFd, RawFd};
+use std::{ffi, ptr};
+
+#[cfg(target_os = "linux")]
+use libc::{dirent64 as dirent, readdir64_r as readdir_r};
+
+#[cfg(not(target_os = "linux"))]
+use libc::{dirent, readdir_r};
+
+/// An open directory.
+///
+/// This is a lower-level interface than `std::fs::ReadDir`. Notable differences:
+///    * can be opened from a file descriptor (as returned by `openat`, perhaps before knowing
+///      if the path represents a file or directory).
+///    * implements `AsRawFd`, so it can be passed to `fstat`, `openat`, etc.
+///      The file descriptor continues to be owned by the `Dir`, so callers must not keep a `RawFd`
+///      after the `Dir` is dropped.
+///    * can be iterated through multiple times without closing and reopening the file
+///      descriptor. Each iteration rewinds when finished.
+///    * returns entries for `.` (current directory) and `..` (parent directory).
+///    * returns entries' names as a `CStr` (no allocation or conversion beyond whatever libc
+///      does).
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+pub(crate) struct Dir(ptr::NonNull<libc::DIR>);
+
+impl Dir {
+    /// Converts from a descriptor-based object, closing the descriptor on success or failure.
+    #[inline]
+    pub(crate) fn from<F: IntoRawFd>(fd: F) -> Result<Self> {
+        unsafe { Self::from_fd(fd.into_raw_fd()) }
+    }
+
+    /// Converts from a file descriptor, closing it on success or failure.
+    unsafe fn from_fd(fd: RawFd) -> Result<Self> {
+        let d = libc::fdopendir(fd);
+        if d.is_null() {
+            let e = Error::last();
+            libc::close(fd);
+            return Err(e);
+        };
+        // Always guaranteed to be non-null by the previous check
+        Ok(Self(ptr::NonNull::new(d).unwrap()))
+    }
+
+    /// Set the position of the directory stream, see `seekdir(3)`.
+    #[cfg(not(target_os = "android"))]
+    pub(crate) fn seek(&mut self, loc: SeekLoc) {
+        unsafe { libc::seekdir(self.0.as_ptr(), loc.0) }
+    }
+
+    /// Reset directory stream, see `rewinddir(3)`.
+    pub(crate) fn rewind(&mut self) {
+        unsafe { libc::rewinddir(self.0.as_ptr()) }
+    }
+
+    /// Get the current position in the directory stream.
+    ///
+    /// If this location is given to `Dir::seek`, the entries up to the previously returned
+    /// will be omitted and the iteration will start from the currently pending directory entry.
+    #[cfg(not(target_os = "android"))]
+    #[allow(dead_code)]
+    pub(crate) fn tell(&self) -> SeekLoc {
+        let loc = unsafe { libc::telldir(self.0.as_ptr()) };
+        SeekLoc(loc)
+    }
+}
+
+// `Dir` is not `Sync`. With the current implementation, it could be, but according to
+// https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html,
+// future versions of POSIX are likely to obsolete `readdir_r` and specify that it's unsafe to
+// call `readdir` simultaneously from multiple threads.
+//
+// `Dir` is safe to pass from one thread to another, as it's not reference-counted.
+unsafe impl Send for Dir {}
+
+impl AsRawFd for Dir {
+    fn as_raw_fd(&self) -> RawFd {
+        unsafe { libc::dirfd(self.0.as_ptr()) }
+    }
+}
+
+impl Drop for Dir {
+    fn drop(&mut self) {
+        unsafe { libc::closedir(self.0.as_ptr()) };
+    }
+}
+
+pub(crate) struct IntoIter(Dir);
+impl Iterator for IntoIter {
+    type Item = Result<Entry>;
+    fn next(&mut self) -> Option<Self::Item> {
+        unsafe {
+            // Note: POSIX specifies that portable applications should dynamically allocate a
+            // buffer with room for a `d_name` field of size `pathconf(..., _PC_NAME_MAX)` plus 1
+            // for the NUL byte. It doesn't look like the std library does this; it just uses
+            // fixed-sized buffers (and libc's dirent seems to be sized so this is appropriate).
+            // Probably fine here too then.
+            //
+            // See `impl Iterator for ReadDir` [1] for more details.
+            // [1] https://github.com/rust-lang/rust/blob/master/src/libstd/sys/unix/fs.rs
+            let mut ent = std::mem::MaybeUninit::<dirent>::uninit();
+            let mut result = ptr::null_mut();
+            if let Err(e) = Errno::result(readdir_r(
+                (self.0).0.as_ptr(),
+                ent.as_mut_ptr(),
+                &mut result,
+            )) {
+                return Some(Err(e));
+            }
+            if result.is_null() {
+                None
+            } else {
+                assert_eq!(result, ent.as_mut_ptr(), "readdir_r specification violated");
+                Some(Ok(Entry(ent.assume_init())))
+            }
+        }
+    }
+}
+
+impl IntoIterator for Dir {
+    type IntoIter = IntoIter;
+    type Item = Result<Entry>;
+
+    fn into_iter(self) -> IntoIter {
+        IntoIter(self)
+    }
+}
+
+/// A directory entry, similar to `std::fs::DirEntry`.
+///
+/// Note that unlike the std version, this may represent the `.` or `..` entries.
+#[derive(Copy, Clone, Debug, Eq, Hash, PartialEq)]
+#[repr(transparent)]
+pub(crate) struct Entry(dirent);
+
+pub(crate) type Type = FileType;
+
+impl Entry {
+    /// Returns the inode number (`d_ino`) of the underlying `dirent`.
+    #[cfg(any(
+        target_os = "android",
+        target_os = "emscripten",
+        target_os = "fuchsia",
+        target_os = "haiku",
+        target_os = "ios",
+        target_os = "l4re",
+        target_os = "linux",
+        target_os = "macos",
+        target_os = "solaris"
+    ))]
+    pub(crate) fn ino(&self) -> u64 {
+        self.0.d_ino.into()
+    }
+
+    /// Returns the inode number (`d_fileno`) of the underlying `dirent`.
+    #[cfg(not(any(
+        target_os = "android",
+        target_os = "emscripten",
+        target_os = "fuchsia",
+        target_os = "haiku",
+        target_os = "ios",
+        target_os = "l4re",
+        target_os = "linux",
+        target_os = "macos",
+        target_os = "solaris"
+    )))]
+    pub(crate) fn ino(&self) -> u64 {
+        u64::from(self.0.d_fileno)
+    }
+
+    /// Returns the bare file name of this directory entry without any other leading path component.
+    pub(crate) fn file_name(&self) -> &ffi::CStr {
+        unsafe { ::std::ffi::CStr::from_ptr(self.0.d_name.as_ptr()) }
+    }
+
+    /// Returns the type of this directory entry, if known.
+    ///
+    /// See platform `readdir(3)` or `dirent(5)` manpage for when the file type is known;
+    /// notably, some Linux filesystems don't implement this. The caller should use `stat` or
+    /// `fstat` if this returns `None`.
+    pub(crate) fn file_type(&self) -> FileType {
+        match self.0.d_type {
+            libc::DT_CHR => Type::CharacterDevice,
+            libc::DT_DIR => Type::Directory,
+            libc::DT_BLK => Type::BlockDevice,
+            libc::DT_REG => Type::RegularFile,
+            libc::DT_LNK => Type::Symlink,
+            /* libc::DT_UNKNOWN | libc::DT_SOCK | libc::DT_FIFO */ _ => Type::Unknown,
+        }
+    }
+
+    #[cfg(target_os = "linux")]
+    pub(crate) fn seek_loc(&self) -> SeekLoc {
+        unsafe { SeekLoc::from_raw(self.0.d_off) }
+    }
+}
+
+#[cfg(not(target_os = "android"))]
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct SeekLoc(libc::c_long);
+
+#[cfg(not(target_os = "android"))]
+impl SeekLoc {
+    pub(crate) unsafe fn from_raw(loc: i64) -> Self {
+        Self(loc.into())
+    }
+
+    pub(crate) fn to_raw(&self) -> i64 {
+        self.0.into()
+    }
+}