fd_allocate on Windows

[sunfishcode]

I have not found a way to implement fd_allocate on Windows in a non-racy way. SetFileInformationByHandle and SetEndOfFile can change a file's size, but will also silently truncate a file if it happens to be longer than the requested length. WriteFile can change a file's size, but needs to write at least one byte in order to do so.

One option would be to remove fd_allocate from WASI. Use cases where fd_allocate is being used to increase a file's size may be able to use filestat_set_size or pwrite themselves. It may make sense to have wasi-libc provide an optional emulated version of posix_fallocate that works in terms of fileestat_set_size -- it would be racy, but some applications would be ok with that, and applications are in a better position to make that determination than WASI implementations.

Another option would be to specify that fd_allocate can truncate files. This would differ from posix_fallocate, but would be more portable. It may make sense for wasi-libc to provide an optional emulated version of posix_fallocate which checks the file length and adjusts the requested length before calling fd_allocate -- it would be racy, but as above, some applications would be ok with that.

[sunfishcode]

There doesn't appear to be a way to implement fd_allocate in a non-racy way on macOS either.

[yamt]

One option would be to remove fd_allocate from WASI.

it makes sense as the functionality is rarely used at best.

otherwise, another option would be to provide an api w/o changing the size instead.
say, fd_preallocate.

• fallocate with FALLOC_FL_KEEP_SIZE for linux
• probably fcntl F_PREALLOCATE for macOS
• for platforms w/o the functionality, an no-op implementation is not too bad. (at least it would be safer than fd_allocate.)