branch locking mk2.

[John A Meinel]

Robert Collins wrote:
> 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
> 
> 
good enough for me then. +1 all around.


John
=:->

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