Re: ERROR: subtransaction logged without previous top-level txn record

From: Arseny Sher <a(dot)sher(at)postgrespro(dot)ru>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, "Hsu\, John" <hsuchen(at)amazon(dot)com>, "pgsql-bugs\(at)lists(dot)postgresql(dot)org" <pgsql-bugs(at)lists(dot)postgresql(dot)org>
Subject: Re: ERROR: subtransaction logged without previous top-level txn record
Date: 2020-02-10 13:04:46
Message-ID: 87wo8ulhjl.fsf@ars-thinkpad
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Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> writes:

>> I don't believe you can that without persisting additional
>> data. Basically, what we need is list of transactions who are running at
>> the point of snapshot serialization *and* already wrote something before
>> it -- those we hadn't seen in full and can't decode. We have no such
>> data currently. The closest thing we have is xl_running_xacts->nextXid,
>> but
>>
>> 1) issued xid doesn't necessarily means xact actually wrote something,
>> so we can't just skip xl_xact_assignment for xid < nextXid, it might
>> still be decoded
>> 2) snapshot might be serialized not at xl_running_xacts anyway
>>
>> Surely this thing doesn't deserve changing persisted data format.
>>
>
> I agree that it won't be a good idea to change the persisted data
> format, especially in back-branches. I don't see any fix which can
> avoid this without doing major changes in the code. Apart from this,
> we have to come up with a solution for point (3) discussed in the
> above email [1] which again could be change in design. I think we can
> first try to proceed with the patch
> 0002-Stop-demanding-that-top-xact-must-be-seen-before--v2 and then we
> can discuss the other patch. I can't see a way to write a test case
> for this, can you think of any way?

Yeah, let's finally get it.

Attached is raw version of isolation test triggering false
'subtransaction logged without...' (case (1)). However, frankly I don't
see much value in it, so I'm dubious whether it should be included in
the patch.

Attachment Content-Type Size
subxact_logged_without_top_test.patch text/x-diff 1.8 KB

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