Need sparse file support (POSIX lseek() SEEK_DATA, SEEK_HOLE)

[lionelconswork]

We need sparse file support, i.e. POSIX 2024 lseek() SEEK_DATA and SEEK_HOLE support.
Cygwin 3.6 already has support for this, so it only needs to be ported to MSYS2

References:

• https://pubs.opengroup.org/onlinepubs/9799919799/functions/lseek.html
• https://docs.oracle.com/cd/E86824_01/html/E54765/lseek-2.html
• https://man7.org/linux/man-pages/man2/lseek.2.html

[lazka]

We follow upstream cygwin closely, so once 3.6 is out we will have it too with a small delay: msys2/msys2-runtime#260

Is there anything else we need to do?

[lionelconswork]

Yes, add automated tests to ensure you never break sparse file support.
That includes:

• /usr/bin/dd
• /usr/bin/fallocate
• cp --sparse=auto
• /usr/bin/tar
• /usr/bin/bsdtar

https://learn.microsoft.com/en-us/windows-server/administration/windows-commands/fsutil-sparse can be used to list the ranges of allocated data sections, before and after copying a file with cp/mv/tar/bsdtar the data and hole ranges should be at the same position and size if both source and target filesystem have the same block size

[jeremyd2019]

I made sure fallocate got built in the last util-linux update. I wouldn't be surprised if coreutils/tar/libarchive needed a rebuild (at least) to pick up any new functionality in the runtime (possibly breaking the ability to downgrade the runtime, so this needs to be considered carefully). Perhaps even grabbing some additional patches from the Cygwin packages, since I seem to remember seeing some discussions about these things having issues on their mailing list.

[jeremyd2019]

Yes, add automated tests to ensure you never break sparse file support. That includes:

Pull requests to https://github.com/msys2/msys2-tests thoughtfully considered.