wasi-tests: add path_exists test

this *should* reproduce the report at
https://github.com/bytecodealliance/wasmtime/issues/2642 but it does
not, so the problem has something to do with invoking via wasmtime cli?

crates/test-programs/wasi-tests/src/bin/path_exists.rs

@@ -0,0 +1,79 @@
+use std::{env, process};
+use wasi_tests::{create_file, open_scratch_directory};
+
+unsafe fn test_path_exists(dir_fd: wasi::Fd) {
+    // Create a temporary directory
+    wasi::path_create_directory(dir_fd, "subdir").expect("create directory");
+
+    // Check directory exists:
+    let file_stat = wasi::path_filestat_get(dir_fd, 0, "subdir").expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_DIRECTORY);
+
+    // Should still exist with symlink follow flag:
+    let file_stat = wasi::path_filestat_get(dir_fd, wasi::LOOKUPFLAGS_SYMLINK_FOLLOW, "subdir")
+        .expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_DIRECTORY);
+
+    // Create a file:
+    create_file(dir_fd, "subdir/file");
+    // Check directory exists:
+    let file_stat = wasi::path_filestat_get(dir_fd, 0, "subdir/file").expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_REGULAR_FILE);
+
+    // Should still exist with symlink follow flag:
+    let file_stat =
+        wasi::path_filestat_get(dir_fd, wasi::LOOKUPFLAGS_SYMLINK_FOLLOW, "subdir/file")
+            .expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_REGULAR_FILE);
+
+    // Create a symlink to a file:
+    wasi::path_symlink("subdir/file", dir_fd, "link1").expect("create symlink");
+    // Check symlink exists:
+    let file_stat = wasi::path_filestat_get(dir_fd, 0, "link1").expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_SYMBOLIC_LINK);
+
+    // Should still exist with symlink follow flag, pointing to regular file
+    let file_stat = wasi::path_filestat_get(dir_fd, wasi::LOOKUPFLAGS_SYMLINK_FOLLOW, "link1")
+        .expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_REGULAR_FILE);
+
+    // Create a symlink to a dir:
+    wasi::path_symlink("subdir", dir_fd, "link2").expect("create symlink");
+    // Check symlink exists:
+    let file_stat = wasi::path_filestat_get(dir_fd, 0, "link2").expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_SYMBOLIC_LINK);
+
+    // Should still exist with symlink follow flag, pointing to directory
+    let file_stat = wasi::path_filestat_get(dir_fd, wasi::LOOKUPFLAGS_SYMLINK_FOLLOW, "link2")
+        .expect("reading file stats");
+    assert_eq!(file_stat.filetype, wasi::FILETYPE_DIRECTORY);
+
+    wasi::path_unlink_file(dir_fd, "link1").expect("clean up");
+    wasi::path_unlink_file(dir_fd, "link2").expect("clean up");
+    wasi::path_unlink_file(dir_fd, "subdir/file").expect("clean up");
+    wasi::path_remove_directory(dir_fd, "subdir").expect("clean up");
+}
+
+fn main() {
+    /*
+    let mut args = env::args();
+    let prog = args.next().unwrap();
+    let arg = if let Some(arg) = args.next() {
+        arg
+    } else {
+        eprintln!("usage: {} <scratch directory>", prog);
+        process::exit(1);
+    };
+
+    // Open scratch directory
+    let dir_fd = match open_scratch_directory(&arg) {
+        Ok(dir_fd) => dir_fd,
+        Err(err) => {
+            eprintln!("{}", err);
+            process::exit(1)
+        }
+    };
+    */
+    // Run the tests.
+    unsafe { test_path_exists(3) }
+}