wasip3 filesystem: Opening dir with MUTATE_DIRECTORY but not READ doesn't preserve MUTATE_DIRECTORY

[wingo]

async fn test_flags_and_type(dir: &Descriptor) {
    let open = |path: &str, oflags: OpenFlags, fdflags: DescriptorFlags| -> _ {
        dir.open_at(PathFlags::empty(), path.to_string(), oflags, fdflags)
    };
    let d = open(".", OpenFlags::empty(), DescriptorFlags::MUTATE_DIRECTORY).await.unwrap();
    assert_eq!(d.get_flags().await,
               Ok(DescriptorFlags::READ | DescriptorFlags::MUTATE_DIRECTORY));
}

I would expect that the directory is opened with MUTATE_DIRECTORY. However it seems it is not:

openat2(3, ".", {flags=O_RDONLY|O_LARGEFILE|O_NOFOLLOW|O_CLOEXEC, resolve=RESOLVE_NO_MAGICLINKS|RESOLVE_BENEATH}, 24) = 12
statx(12, "", AT_STATX_SYNC_AS_STAT|AT_EMPTY_PATH, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFDIR|0755, stx_size=4096, ...}) = 0
fcntl(12, F_GETFL)                      = 0x28000 (flags O_RDONLY|O_LARGEFILE|O_NOFOLLOW)

It seems we only have READ:

thread '<unnamed>' (1) panicked at src/bin/filesystem.rs:620:5:
assertion `left == right` failed
  left: Ok(DescriptorFlags(READ))
 right: Ok(DescriptorFlags(READ | MUTATE_DIRECTORY))

[wingo]

It's weirder than that; MUTATE_DIRECTORY doesn't seem to be propagated at all:

    let d = open(".", OpenFlags::DIRECTORY,
                 DescriptorFlags::READ | DescriptorFlags::MUTATE_DIRECTORY).await.unwrap();
    assert_eq!(d.get_flags().await,
               Ok(DescriptorFlags::READ | DescriptorFlags::MUTATE_DIRECTORY));

Here the assertion fails; the flags that I get are just READ.

[alexcrichton]

I think MUTATE_DIRECTORY is entirely unimplemented in Wasmtime. I'm not seeing any references to it at all. Wasmtime does implement READ and WRITE and I suspect in the transition to p2 this was changed in the WITs but never updated in Wasmtime, so variuos bits and pieces which check for WRITE probably need to check for this instead.

[wingo]

Preopened directories do have MUTATE_DIRECTORY in wasmtime, fwiw.