Waiting for a file lock without blocking other threads

ruby-talk
1

Is there a way to wait for a file lock without blocking other threads in
the same ruby process? The best I could come up with is

 if Thread.list.size == 1
   flock(LOCK_EX)
 else
   period = 0.001
   until flock(LOCK_SH|LOCK_NB)
     sleep period
     period *= 2 if period < 1
   end
 end

(I’m using the exponential so that it checks frequently for a while, but
gradually uses less and less cpu, down to once per second.)

I guess this really boils down to not having native threads. What I was
looking for was something like #select, but for file locks. But I can’t
see how that would be implemented without the ruby interpreter being
thread-safe.

A related question:

There are warnings that flock doesn’t work correctly with Linux NFS, and
that fcntl should be used instead. Anyone have any experience with this?

2

Oops, a typo. I meant to get the same kind of lock in each case, whether
it be LOCK_SH or LOCK_EX.