From: | Melanie Plageman <melanieplageman(at)gmail(dot)com> |
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To: | Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | FreezeLimit underflows in pg14 and 15 causing incorrect behavior in heap_prepare_freeze_tuple |
Date: | 2024-06-21 20:22:44 |
Message-ID: | CAAKRu_ZSegvRXo7bg+r9v4R33e60zaJ_9r-jLdAL0KuY12etMQ@mail.gmail.com |
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Lists: | pgsql-hackers |
While investigating a bug over on [1], I found that
vacuum_set_xid_limits() is calculating freezeLimit in an unsafe way on
at least Postgres 14 and 15.
limit = *oldestXmin - freezemin;
safeLimit = ReadNextTransactionId() - autovacuum_freeze_max_age;
if (TransactionIdPrecedes(limit, safeLimit))
limit = *oldestXmin;
*freezeLimit = limit;
All of these are unsigned, so it doesn't work very nicely when
freezemin (based on autovacuum_freeze_min_age) is larger than
oldestXmin and autovacuum_freeze_max_age is bigger than the next
transaction ID -- which is pretty common right after initdb, for
example.
I noticed the effect of this because FreezeLimit is saved in the
LVRelState and passed to heap_prepare_freeze_tuple() as cutoff_xid,
which is used to guard against freezing tuples that shouldn't be
frozen.
I didn't propose a fix because I just want to make sure I'm not
missing something first.
- Melanie
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