Subject: Re: Lockless Refs? (Was [PATCH] refs: do not use cached refs in repack_without_ref)

From: Martin Fick <[email-masked]>
Date: Mon, 31 Dec 2012 03:30:53 -0700

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from inadvertently performing actions which could be out of 
date when they wake finally up.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