Any thoughts on this idea? Is it flawed? I am trying to write it up in a more formal generalized manner and was hoping to get at least one "it seems sane" before I do.
Thanks, -Martin On Monday, December 31, 2012 03:30:53 am Martin Fick wrote: > On Thursday, December 27, 2012 04:11:51 pm Martin Fick wrote: > > It concerns me that git uses any locking at all, even > > for refs since it has the potential to leave around > > stale locks. > > ... > > [a previous not so great attempt to fix this] > > ... > > I may have finally figured out a working loose ref update > mechanism which I think can avoid stale locks. > Unfortunately it requires atomic directory renames and > universally unique identifiers (uuids). These may be > no-go criteria? But I figure it is worth at least > exploring this idea because of the potential benefits? > > The general approach is to setup a transaction and either > commit or abort it. A transaction can be setup by > renaming an appropriately setup directory to the > "ref.lock" name. If the rename succeeds, the transaction > is begun. Any actor can abort the transaction (up until > it is committed) by simply deleting the "ref.lock" > directory, so it is not at risk of going stale. However, > once the actor who sets up the transaction commits it, > deleting the "ref.lock" directory simply aids in cleaning > it up for the next transaction (instead of aborting it). > > One important piece of the transaction is the use of > uuids. The uuids provide a mechanism to tie the atomic > commit pieces to the transactions and thus to prevent > long sleeping process from inadvertently performing > actions which could be out of date when they wake finally > up. In each case, the atomic commit piece is the > renaming of a file. For the create and update pieces, a > file is renamed from the "ref.lock" dir to the "ref" file > resulting in an update to the sha for the ref. However, > in the delete case, the "ref" file is instead renamed to > end up in the "ref.lock" directory resulting in a delete > of the ref. This scheme does not affect the way refs are > read today, > > To prepare for a transaction, an actor first generates a > uuid (an exercise I will delay for now). Next, a tmp > directory named after the uuid is generated in the parent > directory for the ref to be updated, perhaps something > like: ".lock_uuid". In this directory is places either a > file or a directory named after the uuid, something like: > ".lock_uuid/,uuid". In the case of a create or an > update, the new sha is written to this file. In the case > of a delete, it is a directory. > > Once the tmp directory is setup, the initiating actor > attempts to start the transaction by renaming the tmp > directory to "ref.lock". If the rename fails, the update > fails. If the rename succeeds, the actor can then attempt > to commit the transaction (before another actor aborts > it). > > In the case of a create, the actor verifies that "ref" > does not currently exist, and then renames the now named > "ref.lock/uuid" file to "ref". On success, the ref was > created. > > In the case of an update, the actor verifies that "ref" > currently contains the old sha, and then also renames the > now named "ref.lock/uuid" file to "ref". On success, the > ref was updated. > > In the case of a delete, the actor may verify that "ref" > currently contains the sha to "prune" if it needs to, and > then renames the "ref" file to "ref.lock/uuid/delete". On > success, the ref was deleted. > > Whether successful or not, the actor may now simply delete > the "ref.lock" directory, clearing the way for a new > transaction. Any other actor may delete this directory at > any time also, likely either on conflict (if they are > attempting to initiate a transaction), or after a grace > period just to cleanup the FS. Any actor may also safely > cleanup the tmp directories, preferably also after a grace > period. > > One neat part about this scheme is that I believe it would > be backwards compatible with the current locking > mechanism since the transaction directory will simply > appear to be a lock to older clients. And the old lock > file should continue to lock out these newer > transactions. > > Due to this backwards compatibility, I believe that this > could be incrementally employed today without affecting > very much. It could be deployed in place of any updates > which only hold ref.locks to update the loose ref. So > for example I think it could replace step 4a below from > Michael Haggerty's description of today's loose ref > pruning during > > ref packing: > > * Pack references: > ... > > > 4. prune_refs(): for each ref in the ref_to_prune list, > > > > call prune_ref(): > > a. Lock the reference using lock_ref_sha1(), > > verifying that the recorded SHA1 is still valid. If > > it is, unlink the loose reference file then free > > the lock; otherwise leave the loose reference file > > untouched. > > I think it would also therefore be able to replace the > loose ref locking in Michael's new ref-packing scheme as > well as the locking in Michael's new ref deletion scheme > (again steps > > 4): > > * Delete reference foo: > ... > > > 4. Delete loose ref for "foo": > > a. Acquire the lock $GIT_DIR/refs/heads/foo.lock > > > > b. Unlink $GIT_DIR/refs/heads/foo if it is > > unchanged. > > > > If it is changed, leave it untouched. If it is > > deleted, > > > > that is OK too. > > > > c. Release lock $GIT_DIR/refs/heads/foo.lock > > ... > > > * Pack references: > ... > > > 4. prune_refs(): for each ref in the ref_to_prune > > list, > > > > call prune_ref(): > > a. Lock the loose reference using lock_ref_sha1(), > > > > verifying that the recorded SHA1 is still valid > > > > b. If it is, unlink the loose reference file > > > > (otherwise, leave it untouched) > > > > c. Release the lock on the loose reference > > To be honest, I suspect I missed something obvious because > this seems almost too simple to work. I am ashamed that > it took me so long to come up with (of course, I will be > even more ashamed :( when it is shown to be flawed!) > This scheme also feels extensible. if there are no > obvious flaws in it, I will try to post solutions for ref > packing and for multiple repository/ref transactions also > soon. > > I welcome any comments/criticisms, > > -Martin > -- > To unsubscribe from this list: send the line "unsubscribe > git" in the body of a message to > majord...@vger.kernel.org More majordomo info at > http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
