Re: Concurrency bug in UPDATE of partition-key

On 11 June 2018 at 15:29, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> On Mon, Jun 11, 2018 at 3:02 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> On Thu, Jun 7, 2018 at 1:53 PM, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com> wrote:
>>> On 7 June 2018 at 11:44, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>> On Tue, Jun 5, 2018 at 8:03 PM, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>
>>>> wrote:
>>>>
>>>> I think this will allow before row delete triggers to be fired more than
>>>> once. Normally, if the EvalPlanQual testing generates a new tuple, we don't
>>>> fire the triggers again.
>>>
>>> If there are BR delete triggers, the tuple will be locked using
>>> GetTupleForTrigger(). So the subsequent EvalPlanQual testing won't be
>>> run, since the tuple is already locked due to triggers having run.
>>>
>>> But that leads me to think : The same concurrency issue can occur in
>>> GetTupleForTrigger() also. Say, a concurrent session has already
>>> locked the tuple, and GetTupleForTrigger() would wait and then return
>>> the updated tuple in its last parameter newSlot. In that case, we need
>>> to pass this slot back through ExecBRDeleteTriggers(), and further
>>> through epqslot parameter of ExecDelete(). But yes, in this case, we
>>> should avoid calling this trigger function the second time.
>>>
>>> If you agree on the above, I will send an updated patch.
>>>
>>
>> Sounds reasonable to me.

Attached is v2 version of the patch. It contains the above
trigger-related issue fixed.

The updated tuple is passed back using the existing newslot parameter
of GetTupleForTrigger(). When ExecBRDeleteTriggers() is called using a
new epqslot parameter, it means caller wants to skip the trigger
execution, because the updated tuple needs to be again checked for
constraints. I have added comments of this behaviour in the
ExecBRDeleteTriggers() function header.

Because the trigger execution is skipped the first time ExecDelete()
is called, I didn't have to explicitly add any changes for preventing
trigger execution the second time.

> Try to add a test case which covers BR trigger code path where you are
> planning to update.

Added the test cases. Also added cases where the concurrent session
causes the EvalPlanQual() test to fail, so the partition key update
does not happen.

--
Thanks,
-Amit Khandekar
EnterpriseDB Corporation
The Postgres Database Company