pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts

Hi,

When upgrading a < 9.3 cluster pg_upgrade doesn't bother to keep the old
multixacts around because they won't be read after the upgrade (and
aren't compatible). It just resets the new cluster's nextMulti to the
old + 1.
Unfortunately that means that there'll be a offsets/0000 file created by
initdb around. Sounds harmless enough, but that'll actually cause
problems if the old cluster had a nextMulti that's bigger than that
page.

When vac_truncate_clog() calls TruncateMultiXact() that'll scan
pg_multixact/offsets to find the earliest existing segment. That'll be
0000. If the to-be-truncated data is older than the last existing
segment it returns. Then it'll try to determine the last required data
in members/ by accessing the oldest data in offsets/.

Unfortunately, due to the existing 0000/ segment, that means it'll
sometimes try to access a nonexistant offsets/ file. Preventing vacuum
et al from succeeding.

It seems to me the fix for this is to a) rmtree("pg_multixact/members",
false) in copy_clog_xlog_xid() in the oldcluster < 9.3 case b) add a
warning to the release notes that everyone that used pg_upgrade and has
a 0000 file lying around should report to the mailinglist.

b) is a bit unsatisfactory, but I don't want to suggest removing the
file. In too many situations it might actually still be needed.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services