Start to update the wasi crate in wasi tests (#675)

* Move `wasi` to `wasi_old` in wasi-tests

Leave space for the new `wasi` crate but allow us to incrementally
update tests.

* Update the big_random_buf test

* Update clock_time_get test

* Update close_preopen test

* Review comments

* Update to latest Wasmtime API

crates/test-programs/wasi-tests/src/lib.rs

@@ -4,8 +4,12 @@ pub mod wasi_wrappers;
 use libc;
 use std::ffi::CString;
 use std::io;
-use wasi::wasi_unstable;
+use wasi_old::wasi_unstable;
 
+/// Opens a fresh file descriptor for `path` where `path` should be a preopened
+/// directory. This is intended to be used with `wasi_unstable`, not with
+/// `wasi_snapshot_preview1`. This is getting phased out and will likely be
+/// deleted soon.
 pub fn open_scratch_directory(path: &str) -> Result<wasi_unstable::Fd, String> {
     // Open the scratch directory.
     let dir_fd: wasi_unstable::Fd = unsafe {
@@ -23,3 +27,33 @@ pub fn open_scratch_directory(path: &str) -> Result<wasi_unstable::Fd, String> {
         Ok(dir_fd)
     }
 }
+
+/// Same as `open_scratch_directory` above, except uses `wasi_snapshot_preview1`
+/// APIs instead of `wasi_unstable` ones.
+///
+/// This is intended to replace `open_scratch_directory` once all the tests are
+/// updated.
+pub fn open_scratch_directory_new(path: &str) -> Result<wasi::Fd, String> {
+    unsafe {
+        for i in 3.. {
+            let stat = match wasi::fd_prestat_get(i) {
+                Ok(s) => s,
+                Err(_) => break,
+            };
+            if stat.pr_type != wasi::PREOPENTYPE_DIR {
+                continue;
+            }
+            let mut dst = Vec::with_capacity(stat.u.dir.pr_name_len);
+            if wasi::fd_prestat_dir_name(i, dst.as_mut_ptr(), dst.capacity()).is_err() {
+                continue;
+            }
+            dst.set_len(stat.u.dir.pr_name_len);
+            if dst == path.as_bytes() {
+                return Ok(wasi::path_open(i, 0, ".", wasi::OFLAGS_DIRECTORY, 0, 0, 0)
+                    .expect("failed to open dir"));
+            }
+        }
+
+        Err(format!("failed to find scratch dir"))
+    }
+}