QN: Usage and purpose of file leases (F_SETLEASE, F_GETLEASE)

[Michael Kerrisk]

Gidday,

I've had a look through various LK archives, and elsewhere, and read the
sources, and there seems to be no documentation to be found on Linux file
leases (as set by fcntl(fd, F_SETLEASE, lease-type)).

My question(s): what are file leases, what applications currently use them,
and what are they used for?

After reading the relevant sources and doing a done a bit of
experimentation, I can see that setting a lease can allow one process to
find out if another process opens or truncates a specified file, through the
generation of a SIGIO signal.  That picture is very sketchy (e.g., there is
some system for downgrading a write lease to a read lease after a certain
timeout interval, but I haven't nailed down the details), and probably
incomplete - can anyone fill it out, or point me to documentation on file
leases.

Ultimately, I'd like to add material on file leases to the fcntl() man page.

Cheers

Michael