Re: Connections hang indefinitely while taking a gin index's LWLockbuffer_content lock

On Thu, Mar 21, 2019 at 9:26 PM Simon Riggs <simon@2ndquadrant.com> wrote:
> On Thu, 13 Dec 2018 at 14:48, Alexander Korotkov <aekorotkov@gmail.com> wrote:
>> On Thu, Dec 13, 2018 at 10:46 PM Andres Freund <andres@anarazel.de> wrote:
>> > On 2018-12-13 22:40:59 +0300, Alexander Korotkov wrote:
>> > > It doesn't mater, because we release all locks on every buffer at one
>> > > time.  The unlock order can have effect on what waiter will acquire
>> > > the lock next after ginRedoDeletePage().  However, I don't see why one
>> > > unlock order is better than another.  Thus, I just used the rule of
>> > > thumb to not change code when it's not necessary for bug fix.
>> >
>> > I think it's right to not change unlock order at the same time as a
>> > bugfix here.  More generally I think it can often be useful to default
>> > to release locks in the inverse order they've been acquired - if there's
>> > any likelihood that somebody will acquire them in the same order, that
>> > ensures that such a party would only need to wait for a lock once,
>> > instead of being woken up for one lock, and then immediately having to
>> > wait for the next one.
>>
>> Good point, thank you!
>
>
> It's been pointed out to me that 52ac6cd2d0cd70e01291e0ac4ee6d068b69bc478
> introduced a WAL incompatibility that has not been flagged.
>
> In ginRedoDeletePage() we use the struct directly to read the WAL record, so if a WAL record was written prior to
52ac6cd2d0cd70e01291e0ac4ee6d068b69bc478,yet read by code at 52ac6cd2d0cd70e01291e0ac4ee6d068b69bc478 or later then we
willhave problems, since deleteXid will not be set correctly. 
>
> It seems this should not have been backpatched.
>
> Please give your assessment.

Oh, right.  This is my fault.

However, I think this still can be backpatched correctly.  We can
determine whether xlog record data contains deleteXid by its size.
See the attached patch.  I haven't test this yet.  I'm going to test
it.  If OK, then push.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company