Re: tablecmds.c and lock hierarchy

On Wed, Aug 5, 2015 at 11:47 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> On Tue, Aug 04, 2015 at 07:35:43AM +0100, Simon Riggs wrote:
>> On 4 August 2015 at 05:56, Michael Paquier <michael(dot)paquier(at)gmail(dot)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.

Some sub-commands are using ShareRowExclusiveLock... So this one is better :)
Assert(locklevel==ShareUpdateExclusiveLock || locklevel >=
ShareRowExclusiveLock);
Or we simply list all the locks allowed individually... But that's a
minor point.
--
Michael