Re: Incorrect assumption in heap_prepare_freeze_tuple

From: Andres Freund <andres(at)anarazel(dot)de>
To: Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Incorrect assumption in heap_prepare_freeze_tuple
Date: 2020-10-02 18:34:58
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On 2020-10-02 23:26:05 +0530, Kuntal Ghosh wrote:
> In heap_prepare_freeze_tuple, we make the following assumption:
> * It is assumed that the caller has checked the tuple with
> * HeapTupleSatisfiesVacuum() and determined that it is not HEAPTUPLE_DEAD
> * (else we should be removing the tuple, not freezing it).
> Thus, when we see a committed xmax that precedes the cutoff_xid, we throw
> the following data corruption error:
> errmsg_internal("cannot freeze committed xmax %u", xid)
> However, in the caller (lazy_scan_heap), HeapTupleSatisfiesVacuum may
> return HEAPTUPLE_DEAD for an updated/deleted tuple that got modified by a
> transaction older than OldestXmin. And, if the tuple is HOT-updated, it
> should only be removed by a hot-chain prune operation. So, we treat the
> tuple as RECENTLY_DEAD and don't remove the tuple.

This code is so terrible :(

We really should just merge the HOT pruning and lazy_scan_heap()
removal/freeze operations. That'd avoid this corner case and *also*
would significantly reduce the WAL volume of VACUUM. And safe a good bit
of CPU time.

> So, it may lead to an incorrect data corruption error. IIUC, following will
> be the exact scenario where the error may happen,
> An updated/deleted tuple whose xamx is in between cutoff_xid and
> OldestXmin. Since cutoff_xid depends on vacuum_freeze_min_age and
> autovacuum_freeze_max_age, it'll not be encountered easily. But, I think
> it can be reproduced with some xid burner patch.

I don't think this case is possible (*). By definition, there cannot be any
transactions needing tuples from before OldestXmin. Which means that the
heap_page_prune() earlier in lazy_scan_heap() would have pruned away a
DEAD tuple version that is part of a hot chain.

The HEAPTUPLE_DEAD branch you're referring to really can only be hit for
tuples that are *newer* than OldestXmin but become DEAD (instead of
RECENTLY_DEAD) because the inserting transaction aborted.

(*) with the exception of temp tables due to some recent changes, I am
currently working on a fix for that.

> I think the fix should be something like following:
> if (!HEAP_XMAX_IS_LOCKED_ONLY(tuple->t_infomask) &&
> - TransactionIdDidCommit(xid))
> + TransactionIdDidCommit(xid) &&
> + !HeapTupleHeaderIsHotUpdated(tuple))
> ereport(ERROR,
> errmsg_internal("cannot freeze committed xmax %u",
> xid)));
> - freeze_xmax = true;
> +
> + freeze_xmax = HeapTupleHeaderIsHotUpdated(tuple) ? false : true;

I don't think that would be correct - we'd end up with an xmax that's
older than cutoff_xid left in the table. Breaking relfrozenxid /
creating wraparound and clog lookup dangers. This branch is only entered
when xmax precedes cutoff_xid - which is what we may set relfrozenxid

What made you look at this? Did you hit the error?


Andres Freund

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