Allow WASI to open directories without O_DIRECTORY (#6163)

* Allow WASI to open directories without O_DIRECTORY

The `O_DIRECTORY` flag is a request that open should fail if the named
path is not a directory. Opening a path which turns out to be a
directory is not supposed to fail if this flag is not specified.
However, wasi-common required callers to use it when opening
directories.

With this PR, we always open the path the same way whether or not the
`O_DIRECTORY` flag is specified. However, after opening it, we `stat` it
to check whether it turned out to be a directory, and determine which
operations the file descriptor should support accordingly. In addition,
we explicitly check whether the precondition defined by `O_DIRECTORY` is
satisfied.

Closes #4947 and closes #4967, which were earlier attempts at fixing the
same issue, but which had race conditions.

prtest:full

* Add tests from #4967/#4947

This test was authored by Roman Volosatovs <rvolosatovs@riseup.net> as
part of #4947.

* Tests: Close FDs before trying to unlink files

On Windows, when opening a path which might be a directory using
`CreateFile`, cap-primitives also removes the `FILE_SHARE_DELETE` mode.

That means that if we implement WASI's `path_open` such that it always
uses `CreateFile` on Windows, for both files and directories, then
holding an open file handle prevents deletion of that file.

So I'm changing these test programs to make sure they've closed the
handle before trying to delete the file.

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

@@ -65,10 +65,9 @@ unsafe fn exec_fd_readdir(fd: wasi::Fd, cookie: wasi::Dircookie) -> (Vec<DirEntr
     (dirs, eof)
 }
 
-unsafe fn test_fd_readdir(dir_fd: wasi::Fd) {
+unsafe fn assert_empty_dir(dir_fd: wasi::Fd) {
     let stat = wasi::fd_filestat_get(dir_fd).expect("failed filestat");
 
-    // Check the behavior in an empty directory
     let (mut dirs, eof) = exec_fd_readdir(dir_fd, 0);
     assert!(eof, "expected to read the entire directory");
     dirs.sort_by_key(|d| d.name.clone());
@@ -91,6 +90,11 @@ unsafe fn test_fd_readdir(dir_fd: wasi::Fd) {
         dirs.next().is_none(),
         "the directory should be seen as empty"
     );
+}
+
+unsafe fn test_fd_readdir(dir_fd: wasi::Fd) {
+    // Check the behavior in an empty directory
+    assert_empty_dir(dir_fd);
 
     // Add a file and check the behavior
     let file_fd = wasi::path_open(
@@ -111,16 +115,33 @@ unsafe fn test_fd_readdir(dir_fd: wasi::Fd) {
         "file descriptor range check",
     );
 
-    let stat = wasi::fd_filestat_get(file_fd).expect("failed filestat");
+    let file_stat = wasi::fd_filestat_get(file_fd).expect("failed filestat");
     wasi::fd_close(file_fd).expect("closing a file");
 
+    wasi::path_create_directory(dir_fd, "nested").expect("create a directory");
+    let nested_fd = wasi::path_open(
+        dir_fd,
+        0,
+        "nested",
+        0,
+        wasi::RIGHTS_FD_READ | wasi::RIGHTS_FD_READDIR | wasi::RIGHTS_FD_FILESTAT_GET,
+        0,
+        0,
+    )
+    .expect("failed to open nested directory");
+    assert!(
+        nested_fd > file_fd,
+        "nested directory file descriptor range check",
+    );
+    let nested_stat = wasi::fd_filestat_get(nested_fd).expect("failed filestat");
+
     // Execute another readdir
     let (mut dirs, eof) = exec_fd_readdir(dir_fd, 0);
     assert!(eof, "expected to read the entire directory");
-    assert_eq!(dirs.len(), 3, "expected three entries");
+    assert_eq!(dirs.len(), 4, "expected four entries");
     // Save the data about the last entry. We need to do it before sorting.
-    let lastfile_cookie = dirs[1].dirent.d_next;
-    let lastfile_name = dirs[2].name.clone();
+    let lastfile_cookie = dirs[2].dirent.d_next;
+    let lastfile_name = dirs[3].name.clone();
     dirs.sort_by_key(|d| d.name.clone());
     let mut dirs = dirs.into_iter();
 
@@ -136,7 +157,16 @@ unsafe fn test_fd_readdir(dir_fd: wasi::Fd) {
         wasi::FILETYPE_REGULAR_FILE,
         "type for the real file"
     );
-    assert_eq!(dir.dirent.d_ino, stat.ino);
+    assert_eq!(dir.dirent.d_ino, file_stat.ino);
+    let dir = dirs.next().expect("fourth entry is None");
+    // check the directory info
+    assert_eq!(dir.name, "nested", "nested directory name doesn't match");
+    assert_eq!(
+        dir.dirent.d_type,
+        wasi::FILETYPE_DIRECTORY,
+        "type for the nested directory"
+    );
+    assert_eq!(dir.dirent.d_ino, nested_stat.ino);
 
     // check if cookie works as expected
     let (dirs, eof) = exec_fd_readdir(dir_fd, lastfile_cookie);
@@ -144,7 +174,12 @@ unsafe fn test_fd_readdir(dir_fd: wasi::Fd) {
     assert_eq!(dirs.len(), 1, "expected one entry");
     assert_eq!(dirs[0].name, lastfile_name, "name of the only entry");
 
+    // check if nested directory shows up as empty
+    assert_empty_dir(nested_fd);
+    wasi::fd_close(nested_fd).expect("closing a nested directory");
+
     wasi::path_unlink_file(dir_fd, "file").expect("removing a file");
+    wasi::path_remove_directory(dir_fd, "nested").expect("removing a nested directory");
 }
 
 unsafe fn test_fd_readdir_lots(dir_fd: wasi::Fd) {