branch locking mk2.

[Robert Collins]

On Wed, 2006-02-08 at 14:04 -0600, John A Meinel wrote:
> Robert Collins wrote:
> > Martin mentioned doing a new lock type on list a few days back, I have
> > chatted with him about that, and his ideas. The follow is a brain dump
> > of that for discussion and what we think would be nice for discussion,
> > critique etc.
> > 
> > 
> > A side note, but an important one : this cannot simply replace the
> > existing locks, we need to keeo the current lock code for compatibility
> > with extant clients. It requires a format change to each object that
> > uses this to introduce it.
> > 
> > 
> > The desired characteristics are:
> > * Locks are not reentrant
> > * Stale locks can be guessed at by a heuristic
> > * Lost locks can be broken by any client
> > * Failed lock operations leave little or no mess
> > * Deadlocks are avoided by having a timeout always in use, clients
> > desiring indefinite waits can retry or set a silly big timeout.
> 
> reentrant --- meaning the client itself deals with reentrance, rather
> than the lock itself. (Like it is currently done).
> 
> I like the 'create a dir in pending, rename it to locked' portion of
> your proposal.
> But I don't know what 'released' gives you. Other than being able to
> determine *after the fact*, that someone broke your lock. I suppose that
> is important, but by then, the data is probably corrupted (better than
> not knowing it was corrupted, I guess).
>
> Anyway, it seems like quite a bit of work, that still doesn't handle
> true transactions. I think the pending=>locked is good, locked=>released
> seems like extra work that doesn't help much.

There are two objects involved in the lock record: a directory and a
file. Removing them is a non atomic operation. Releasing the lock should
be an atomic operation. All the benefits of 'released' lie in making
lock releasing atomic.

Rob


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