Re: [Bug][patch]: After dropping the last label from a property graph element, invoking pg_get_propgraphdef() triggers an assertion failure

On 19.06.26 14:05, Ashutosh Bapat wrote:
> On Tue, May 12, 2026 at 12:31 PM Ashutosh Bapat
> <ashutosh.bapat.oss@gmail.com> wrote:
>>
>> On Tue, May 12, 2026 at 7:05 AM Ashutosh Bapat
>> <ashutosh.bapat.oss@gmail.com> wrote:
>>>
>>> On Mon, May 4, 2026 at 8:16 PM Peter Eisentraut <peter@eisentraut.org> wrote:
>>>>
>>>> On 28.04.26 17:02, Ashutosh Bapat wrote:
>>>>> We are looking up element label catalogs twice in this patch - first
>>>>> to find the label to be dropped and then to find the number of labels
>>>>> associated with the given element. I combined these two into a single
>>>>> while loop.
>>>>
>>>> That looks okay, but I think the names of the local variables are now a
>>>> bit off.  I would expect elrel and elscan to refer to
>>>> pg_propgraph_element, not pg_propgraph_element_label.  Maybe use
>>>> ellabelrel etc.
>>>
>>> Done.
>>>
>>>>
>>>> Also, I think this code needs to think a bit about locking to handle the
>>>> situation where more than one DROP LABEL operation happens concurrently.
>>>>
>>>
>>> AlterPropGraph already takes ShareRowExclusiveLock at the beginning so
>>> only one label can be dropped at a time. I have added an isolation
>>> test to test the scenario. We could further add some more tests to
>>> make sure that properties can not be added to a label being dropped,
>>> adding label to an element being dropped, adding label to an element
>>> being added etc. Would that be an overkill?
>>
>> Here's the patchset without the extra tests.
> 
> I decided to go ahead and added those extra tests as a separate patch.
> They don't cover all the possible concurrent modifications, but the
> ones which may render a property or a label orphan.
> 
> 0001 - patch to avoid dropping last label from an element
> 0002 - patch to test concurrent ALTER PROPERTY GRAPH
> 0003 - patch from [1] which would conflict with 0001, if not included here.
> 
> [1] https://www.postgresql.org/message-id/E3D6552E-DB26-47BC-9C7C-0F9CB936BF5B@gmail.com

I have committed 0001 and 0003.  You are right that the lock prevents 
concurrent modifications.  This is pretty straightforward, so I have 
omitted the patch 0002 with the new tests.  I have added a brief comment 
where the lock is taken.