first patch for process-shared semaphore

[David Xu]

John Baldwin wrote:

> Yes, Solaris uses files in /tmp and Darwin uses special file descriptors 
> similar to what we do.  However, you will have to restrict the namespace if 
> you go the /tmp route to be safe I think similar to what Solaris does (no path 
> separators, just simple names like 'foo'). 

I will only allow names like /XXX, make sure to be compatible with
current implementation.

> You might also want to use the 
> same naming convention as Solaris if you go the /tmp route (I think they use a 
> path other than .semaphore under /tmp IIRC).

What is their name convention? The patch just create all semaphore files
in directory /tmp/.semaphore and semaphore name is not changed.

>  Not sure if we want to do 
> anything special to ensure that those particular set of files in /tmp always 
> get purged on reboot to avoid weird bugs with semaphores unexpectedly 
> persisting across reboots.
> 

Yes, this is an issue, I would like to purge it on reboot.