Allocation on macOS is not working, while it does with fs2

[nazar-pc]

I just replaced some of fs2 usage with fs4 and got following error when calling file.allocate() on macOS from multiple users:

No space left on device (os error 28)

In one example it happened with ~750GB file and then file.allocate() was called with the same file size again, which failed with ^ error.

Here is what implementation looks in fs2, it is much more verbose than in fs4:
https://github.com/danburkert/fs2-rs/blob/9a340454a8292df025de368fc4b310bb736f382f/src/unix.rs#L108-L137

I'll be switching to fs2 for that use case for now.

[al8n]

Hi, this is because in fs2, if the contiguous allocation fails, then try non-contiguous allocation. rustix does not provide the same functionality (fcntl_preallocate) as libc

I review the source code of rustix's fallocate, it already tries to allocate non-contiguous when contiguous allocation fails, so it is weird.

The fallocate source code in rustix is the same as what fs2's allocate does.

#[cfg(apple)]
pub(crate) fn fallocate(
    fd: BorrowedFd<'_>,
    mode: FallocateFlags,
    offset: u64,
    len: u64,
) -> io::Result<()> {
    let offset: i64 = offset.try_into().map_err(|_e| io::Errno::INVAL)?;
    let len = len as i64;

    assert!(mode.is_empty());

    let new_len = offset.checked_add(len).ok_or(io::Errno::FBIG)?;
    let mut store = c::fstore_t {
        fst_flags: c::F_ALLOCATECONTIG,
        fst_posmode: c::F_PEOFPOSMODE,
        fst_offset: 0,
        fst_length: new_len,
        fst_bytesalloc: 0,
    };
    unsafe {
        if c::fcntl(borrowed_fd(fd), c::F_PREALLOCATE, &store) == -1 {
            // Unable to allocate contiguous disk space; attempt to allocate
            // non-contiguously.
            store.fst_flags = c::F_ALLOCATEALL;
            let _ = ret_c_int(c::fcntl(borrowed_fd(fd), c::F_PREALLOCATE, &store))?;
        }
        ret(c::ftruncate(borrowed_fd(fd), new_len))
    }
}