Add yanix crate and replace nix with yanix in wasi-common (#649)

* Add yanix crate

This commit adds `yanix` crate as a Unix dependency for `wasi-common`.
`yanix` stands for Yet Another Nix crate and is exactly what the name
suggests: a crate in the spirit of the `nix` crate, but which takes a different
approach, using lower-level interfaces with less abstraction, so that it fits
better with its main use case, implementation of WASI syscalls.

* Replace nix with yanix crate

Having introduced `yanix` crate as an in-house replacement for the
`nix` crate, this commit makes the necessary changes to `wasi-common`
to depend _only_ on `yanix` crate.

* Address review comments

* make `fd_dup` unsafe
* rename `get_fd` to `get_fd_flags`, etc.
* reuse `io::Error::last_os_error()` to get the last errno value

* Address more comments

* make all `fcntl` fns unsafe
* adjust `wasi-common` impl appropriately

* Make all fns operating on RawFd unsafe

* Fix linux build

* Address more comments

crates/wasi-common/src/host.rs

@@ -5,7 +5,8 @@
 #![allow(non_snake_case)]
 
 use crate::wasi::*;
-use std::{io, slice};
+use crate::{Error, Result};
+use std::{convert::TryInto, io, mem, slice};
 use wig::witx_host_types;
 
 witx_host_types!("snapshot" "wasi_snapshot_preview1");
@@ -20,6 +21,64 @@ pub(crate) unsafe fn iovec_to_host_mut(iovec: &mut __wasi_iovec_t) -> io::IoSlic
     io::IoSliceMut::new(slice)
 }
 
+#[allow(dead_code)] // trouble with sockets
+#[derive(Clone, Copy, Debug)]
+#[repr(u8)]
+pub(crate) enum FileType {
+    Unknown = __WASI_FILETYPE_UNKNOWN,
+    BlockDevice = __WASI_FILETYPE_BLOCK_DEVICE,
+    CharacterDevice = __WASI_FILETYPE_CHARACTER_DEVICE,
+    Directory = __WASI_FILETYPE_DIRECTORY,
+    RegularFile = __WASI_FILETYPE_REGULAR_FILE,
+    SocketDgram = __WASI_FILETYPE_SOCKET_DGRAM,
+    SocketStream = __WASI_FILETYPE_SOCKET_STREAM,
+    Symlink = __WASI_FILETYPE_SYMBOLIC_LINK,
+}
+
+impl FileType {
+    pub(crate) fn to_wasi(&self) -> __wasi_filetype_t {
+        *self as __wasi_filetype_t
+    }
+}
+
+#[derive(Debug, Clone)]
+pub(crate) struct Dirent {
+    pub name: String,
+    pub ftype: FileType,
+    pub ino: u64,
+    pub cookie: __wasi_dircookie_t,
+}
+
+impl Dirent {
+    /// Serialize the directory entry to the format define by `__wasi_fd_readdir`,
+    /// so that the serialized entries can be concatenated by the implementation.
+    pub fn to_wasi_raw(&self) -> Result<Vec<u8>> {
+        let name = self.name.as_bytes();
+        let namlen = name.len();
+        let dirent_size = mem::size_of::<__wasi_dirent_t>();
+        let offset = dirent_size.checked_add(namlen).ok_or(Error::EOVERFLOW)?;
+
+        let mut raw = Vec::<u8>::with_capacity(offset);
+        raw.resize(offset, 0);
+
+        let sys_dirent = raw.as_mut_ptr() as *mut __wasi_dirent_t;
+        unsafe {
+            *sys_dirent = __wasi_dirent_t {
+                d_namlen: namlen.try_into()?,
+                d_ino: self.ino,
+                d_next: self.cookie,
+                d_type: self.ftype.to_wasi(),
+            };
+        }
+
+        let sys_name = unsafe { sys_dirent.offset(1) as *mut u8 };
+        let sys_name = unsafe { slice::from_raw_parts_mut(sys_name, namlen) };
+        sys_name.copy_from_slice(&name);
+
+        Ok(raw)
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;