From: MenTaLguY <mental@...>
Date: 2007-01-20T02:48:38+09:00
Subject: Re: [ANN] fastthread 0.6.2

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On Fri, 2007-01-19 at 13:03 +0900, Tim Pease wrote:

> Does fastthread allow you to see who has the lock on the mutex?

fastthread is just a re-implementation of the standard API offered by
Mutex, ConditionVariable, Queue and SizedQueue in thread.rb.  That isn't
an ability offered by stdlib's Mutex.

> Sync allows you to do this,

Not safely, actually.  Why do you need to do that?

> and it allows me to write re-entrant methods much more easily.

Yes, since Sync locks are reentrant, but personally I would avoid Sync
because it has some bugs, memory leaks, and the implementation isn't
very nice generally.

If do you need both a reentrant lock and the ability to see who's
currently holding it, what I'd suggest instead is writing your own lock
class along these lines:

 class MyLock
   def initialize
     @lock =3D Mutex.new
     @ready =3D ConditionVariable.new
     @count =3D 0
   end

   def lock
     @lock.synchronize do
       @ready.wait @lock while @owner and @owner !=3D Thread.current
       @owner =3D Thread.current
       @count +=3D 1
     end
     self
   end

   def unlock
     @lock.synchronize do
       return self unless @owner
       @count -=3D 1
       if @count.zero?
         @owner =3D nil
         @ready.signal
       end
     end
     self
   end

   # safely pass the current holder of the lock to a block
   def with_locker
     @lock.synchronize { yield @owner }
   end
 end

The above code should work fine with or without fastthread.  fastthread
would just make it a little faster (I'd recommend making it an optional
require, as described in the announcement).

I'd strongly recommend against writing code which checks the current
owner of a lock, by the way, but if you really need to do it (I don't
know your specific requirements), something like MyLock#with_locker is
probably the safest way to do so.

-mental

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