Fixes to use lfs_filebd on windows platforms

[m8ddin]

There are two issues, when using the file-based block device emulation
on Windows Platforms:

1. There is no fsync implementation available. This needs to be mapped
   to a Windows-specific FlushFileBuffers system call.
2. The block device file needs to be opened as binary file (O_BINARY)
   The corresponding flag is not required for Linux.

[geky]

Hi @m8ddin, thanks for the PR.

A request, could these be changed to explicit ifdef statements?

#ifdef _WIN32
bd->fd = open(path, O_RDWR | O_CREAT | O_BINARY, 0666);
#else
bd->fd = open(path, O_RDWR | O_CREAT, 0666);
#endif

This would just help future work to know what exact idiosyncrasies are required for each platform.

---

I also want to highlight that we can't easily test these platform specific changes in CI, so future regressions are possible. This unfortunately may break in the future without warning.

[m8ddin]

Hi @geky !
Thanks for reviewing the PR so quickly! Of course, I can add explicit ifdefs. Will add another commit today.

[geky]

Hi @m8ddin, thanks for the update.

Sorry, I meant that each platform specific function should be an ifdef, both the O_BINARY and FlushFileBuffers, instead of aliasing fsync. Consider what would happen if Windows adds fsync in the future (or more likely fsync is provided by mingw or some other toolchain on Windows).

#ifdef _WIN32
int err = FlushFileBuffers((HANDLE)_get_osfhandle(bd->fd)) ? 0 : -1;
#else
int err = fsync(bd->fd);
#endif

[m8ddin]

Makes sense 👍

[geky]

Thanks for this! I will bring this in next minor release, which shouldn't be too far away.