Re: tablecmds.c and lock hierarchy

On 4 August 2015 at 05:56, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
wrote:

> Hi all,
>
> As mentioned in the thread related to lowering locks of autovacuum
> reloptions in ALTER TABLE SET
> (
> http://www.postgresql.org/message-id/CAFcNs+oX7jVENC_3i54fDQ3ibmOGmknc2tMevdSmvojbSXGbGg@mail.gmail.com
> ),
> I have noticed the following code in
> AlterTableGetLockLevel(at)tablecmds(dot)c:
> /*
> * Take the greatest lockmode from any subcommand
> */
> if (cmd_lockmode > lockmode)
> lockmode = cmd_lockmode;
>
> The thing is that, as mentioned by Alvaro and Andres on this thread,
> we have no guarantee that the different relation locks compared have a
> monotone hierarchy and we may finish by taking a lock that does not
> behave as you would like to. We are now lucky enough that ALTER TABLE
> only uses ShareUpdateExclusiveLock, ShareRowExclusiveLock and
> AccessExclusiveLock that actually have a hierarchy so this is not a
> problem yet.
> However it may become a problem if we add in the future more lock
> modes and that are used by ALTER TABLE.
>

Please provide the link to the discussion of this. I don't see a problem
here right now that can't be solved by saying

Assert(locklevel==ShareUpdateExclusiveLock ||
locklevel>ShareRowExclusiveLock);

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