[Unison-hackers] mutex

[Benjamin Pierce]

> There is a crash using the Mac GUI that seems to occur when one thread
> updates a reconItem (actually stateItem.whatHappened), and another
> thread reads from it at the same time (try scrolling up with the
> up-arrow through the reconitem table while a sync is in progress).  It
> should be easily fixable with a mutex, available as part of the ocaml
> threads libraries.  However, I see that significant effort has gone
> into avoiding use of those libraries in Unison, and using lwt/*
> instead.  Is there an alternative mutex implementation or workaround I
> can use?

The history of the lightweight threads (lwt) stuff is like this.   
Originally, Unison was completely single-threaded.  When we began  
using it seriously, we found that the transfer speed was  
unsatisfactory because of all the handshaking involved in our (rather  
simple and paranoid) data transfer protocol.  So we tried  
multithreading the transfer phase.  Unfortunately, we discovered (as  
so many have before) that trying to add threads to an application  
that was not designed to be threadsafe from the start was just too  
hard for us (even too hard for Jerome, which is saying something!).   
So we moved to an intermediate position where we rolled our own  
"lightweight" thread package that does its own, non-preemptive  
scheduling.

My guess is that protecting the Unison thread(s) from the GUI thread  
with a regular mutex should work well.

Regards,

     Benjamin