Add check for invalid offset at multixid truncation If a multixid with zero offset is left behind after a crash, and that multixid later becomes the oldest multixid, truncation might try to look up its offset and read the zero value. In the worst case, we might incorrectly use the zero offset to truncate valid SLRU segments that are still needed. I'm not sure if that can happen in practice, or if there are some other lower-level safeguards or incidental reasons that prevent the caller from passing an unwritten multixid as the oldest multi. But better safe than sorry, so let's add an explicit check for it.
In stable branches, we should perhaps do the same check for 'oldestOffset', i.e. the offset of the old oldest multixid (in master, 'oldestOffset' is gone). But if the old oldest multixid has an invalid offset, the damage has been done already, and we would never advance past that point. It's not clear what we should do in that case. The check that this commit adds will prevent such an multixid with invalid offset from becoming the oldest multixid in the first place, which seems enough for now. Reviewed-by: Andrey Borodin <[email protected]> Discussion: Discussion: https://www.postgresql.org/message-id/[email protected] Backpatch-through: 14 Branch ------ REL_17_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/d3ad4cef6ea90cc0be02ccee8fa8b3bd961c9c33 Modified Files -------------- src/backend/access/transam/multixact.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)
