path_get refactor and implementation of missing path_ hostcalls on Windows (#41)

* Move path_get outside of sys module

* Add implementation of readlinkat

* Clean up path_open; use OpenOptions as much as possible

* Enable close_preopen test

* Implement path_create_directory; fix path_open

* Refactor path concatenation onto a descriptor

* Implement path_remove_directory

* Implement path_unlink_file

* Rewrite path_open using specific access mask

* Fix error mapping when unlinking file

* Fix readlinkat to pass nofollow_errors testcase

* Clean up winerror to WASI conversion

* Spoof creating dangling symlinks on windows (hacky!)

* Add positive testcase for readlink

* Implement path_readlink (for nonzero buffers for now)

* Clean up

* Add Symlink struct immitating *nix symlink

* Fix path_readlink

* Augment interesting_paths testcase with trailing slashes example

* Encapsulate path_get return value as PathGet struct

* Remove dangling symlink emulation

* Extract dangling symlinks into its own testcase

This way, we can re-enable nofollow_errors testcase
on Windows also.

* Return __WASI_ENOTCAPABLE if user lacks perms to symlink

src/sys/mod.rs

@@ -4,14 +4,16 @@ use cfg_if::cfg_if;
 cfg_if! {
     if #[cfg(unix)] {
         mod unix;
-        pub use self::unix::*;
+        pub(crate) use self::unix::*;
+        pub use self::unix::preopen_dir;
 
         pub(crate) fn errno_from_host(err: i32) -> host::__wasi_errno_t {
             host_impl::errno_from_nix(nix::errno::from_i32(err))
         }
     } else if #[cfg(windows)] {
         mod windows;
-        pub use self::windows::*;
+        pub(crate) use self::windows::*;
+        pub use self::windows::preopen_dir;
 
         pub(crate) fn errno_from_host(err: i32) -> host::__wasi_errno_t {
             host_impl::errno_from_win(winx::winerror::WinError::from_u32(err as u32))