xdg-document-portal: implement flock

[lufia]

I implemented flock operation of FUSE.

This is needed for Steam client.

• Steam prints "flock (FILE) LOCK_SH failed. errno = 38" errors for each the game that was moved to the external disk ValveSoftware/steam-for-linux#10829

Tested

I tested whether the new xdg-document-portal accepts flock(2) syscall with flock(1).

$ umount /run/user/60331/doc
$ meson compile -C _build
$ ./_build/document-portal/xdg-document-portal &

$ cd /run/user/60331/doc
$ touch lock.L
$ flock -n lock.L sleep 100

Then, within 100 seconds, I ran flock(1) in other terminal session.

$ flock -n lock.L ls || echo BAD
BAD

I checked blocking operations are rejected.

$ flock lock.L ls
flock: lock.L: Function not implemented

$ flock -s lock.L ls
flock: lock.L: Function not implemented

[swick]

Looks like this will block the entire document portal when any file is blocked due to flock.

[lufia]

Hi, @swick
I mitigated not to block entire document-portal by rejecting blocking operations of flock.
Steam client works well in my Arch Linux box.

@hfiguiere Thanks for your suggestion!

[GeorgesStavracas]

Thanks for the patch. Please squash the suggestion commit into a single commit.

[swick]

Supporting flock only for certain option is very suspicious to me. This needs review from someone who knows about fuse and filesystems.