Re: tablecmds.c and lock hierarchy

On Tue, Aug 04, 2015 at 07:35:43AM +0100, Simon Riggs wrote:
> On 4 August 2015 at 05:56, Michael Paquier <michael.paquier@gmail.com> wrote:
> > 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);

Agreed; that addresses the foreseeable future of this threat.