Re: alter enum add value if not exists

On Thu, Aug 23, 2012 at 1:35 PM, Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:
>
> On 08/23/2012 06:47 AM, Magnus Hagander wrote:
>>
>> On Mon, Aug 20, 2012 at 4:52 PM, Andrew Dunstan <andrew(at)dunslane(dot)net>
>> wrote:
>>>
>>> Here is a patch for this feature, which should alleviate some of the woes
>>> caused by adding labels not being transactional (and thus not allowing
>>> for
>>> the catching of errors).
>>
>> I haven't actually checked the code in detail, but if it's not
>> transactional, how does it actually prevent race conditions? Doesn't
>> it at least have to do it's check *after* the enum is locked?
>
>
>
> Well, you can't remove a label, and if the test succeeds it results in your
> doing nothing, so my possibly naive thinking was that that wasn't necessary.
> But I could easily be wrong :-)

Ah, good point.
But still: what if:

Session A checks if the label is present, it's not.
Session B checks if the label is present, it's not.
Session A locks the enum, and adds the label, then releases lock.
Session B locks the enum, and tries to add it -- and you still get a failure.

It doesn't break, of course ,since it's protected by the unique index.
But aren't you at risk of getting the very error message you're trying
to avoid?

Or am I missing something? (I probably am :D - I still haven't looked
at it in detail)

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/