Hello,

I'm interested in using the zsystem flock function in the zsh/system
module to lock files, with a timeout if the lock is already taken, but
currently the timeout value has a whole-second granularity and I'd like
to specify sub-second timeouts.

The attached patch (against zsh 5.7.1) modifies the bin_zsystem_flock()
function, allowing the -t option to take floating-point parameters, and
handles the timeout with microsecond granularity.  It also adds option
-p to control the retrying period, which was previously hardcoded to
1 second.

I kept the same function's logic of periodically retrying to acquire
the lock in non-blocking mode.  However, there may be a better way to
implement this timeout: set a timer such as setitimer(2) which sends
a SIGALRM on timeout, trap SIGALRM without SA_RESTART, then acquire the
lock in blocking mode; fcntl() will then block until SIGALRM is caught,
after which errno will be EINTR.  Unfortunately, lacking knowledge of
zsh internals especially signal handling, I'm not confident that I can
reimplement it this way by myself.

The patch modifies Src/Modules/system.c and also Src/utils.c (to add
a function akin to time(NULL) but that returns microseconds, using
clock_gettime(CLOCK_MONOTONIC) if supported, or else gettimeofday()).

I did not modify the documentation yet, not being sure of people being
interested in this feature, nor of whether I should edit just the
Doc/Zsh/mod_system.yo file, or also Doc/{zshmodules.1,zsh.texi} or
others.

Also, I'm afraid that I only did minimal testing.  I checked that
invoking zsystem flock with various values for -t and -p seems to do
what I intended.  Is there a more rigorous test framework for this
kind of thing?

					Thanks, best regards,
					Cedric Ware.