From: Zardosht Kasheff
Date: February 26 2011 12:06am
Subject: Re: transaction commit/rollback and handler::external_lock(F_UNLCK)
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Hello Konstantin,

Thanks for your reply. It is the 5.1 case that I am particularly
interested in. I do not see where InnoDB is maintaining this count and
auto committing the transaction.

Also, I always see handlerton->commit/rollback being called at the end
of statement. This is what I did for the simplest example:
create temporary table foo (a int);
Lock tables foo read;
select * from foo;
unlock tables;

I saw innobase_commit get called during "lock tables foo read" and
during "select * from foo;"

So I guess I still dont see an example where
handlerton->commit/rollback is not called. Is there a chance this has
been fixed in 5.1 as well?

Thanks
-Zardosht

On Fri, Feb 25, 2011 at 6:57 PM, Konstantin Osipov
<kostja.osipov@stripped> wrote:
> * Zardosht Kasheff <zardosht@stripped> [11/02/26 02:41]:
>> In ha_ndbcluster::external_lock, in mysql 5.1.52, I see the following
>> comment under the case where lock_type =3D=3D F_UNLCK:
>> =A0 =A0 =A0 =A0 =A0 /*
>> =A0 =A0 =A0 =A0 =A0 =A0 Unlock is done without a transaction commit / ro=
llback.
>> =A0 =A0 =A0 =A0 =A0 =A0 This happens if the thread didn't update any row=
s
>> =A0 =A0 =A0 =A0 =A0 =A0 We must in this case close the transaction to re=
lease resources
>> =A0 =A0 =A0 =A0 =A0 */
>>
>> But I do not see this anywhere else in any other handler.
>>
>> Is there a scenario where a transaction that has been registered via
>> trans_register_ha does NOT have handlerton->commit or
>> handlerton->rollback getting called, and therefore requireing
>> ha_ndbcluster::external_lock to execute:
>> =A0 =A0 =A0 =A0 =A0 ndb->closeTransaction(thd_ndb->trans);
>> =A0 =A0 =A0 =A0 =A0 thd_ndb->trans=3D NULL;
>
> In 5.5 handlerton->commit/rollback is guaranteed to be eventually
> called for any registered handler.
>
> However, this doesn't always happen in order.
>
> One example, when ha->external_lock(F_UNLCK) is called before
> handlerton->commit/rollback is mysql_unlock_some_tables and
> mysql_unlock_read_tables().
>
> MySQL can call these functions before end of statement,
> (commit/rollback is done at end of statement), and they
> in turn call handler::external_lock.
>
>> My guess is that no, this is not required, otherwise other storage
>> engines would do this, but I would like to confirm.
>
> InnoDB counts calls to external_lock(F_LOCK) and
> external_lock(F_UNLOCK). When it drops to zero, it auto-commits
> the statement transaction. This is necessary in 5.0 and 5.1,
> where handlerton->commit/rollback is not guaranteed to be called
> at end of statement (try, for the simplest example, to create a
> temporary table and lock it with LOCK TABLES, and then execute
> some statements).
>
>
>
>
> --
>