Re: shared-memory based stats collector

Hello.

At Fri, 22 Feb 2019 17:19:56 +0900 (Tokyo Standard Time), Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote in <20190222(dot)171956(dot)98584931(dot)horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
> > It seems to me that an autovacuum process terminates because of
> > segfault.
> >
> > Segfault occurs within get_pgstat_tabentry_relid(). If I'm not
> > mistaken, somehow 'dbentry' hasn't valid pointer anymore.

do_autovacuum does the followings:

dbentry = pgstat_fetch_stat_dbentry() -- create cached dbentry
StartTransactionCommand() -- starts transaction
pgstat_vacuum_stat() -- blows away the cached dbentry.
shared = pgstat_fetch_stat_dbentry()

It was harmless previously, but pgstat_* functions blow away
local cache at the first call after transaction start. As the
result dbentry becomes invalid. The reason I didin't see the same
crash is the second pgstat_fetch_stat_dbentry() accidentially
zeroes-out the once invalidated dbentry.

It is fixed by moving StartTransactionCommand to before the first
pgstat_f_s_dbentry(), which looks better not having this problem.

me> I found another problem commit_ts test reliably fails by dshash
me> corruption in startup process. I've not found why and will
me> investigate it, too.

It is rather stupid, pgstat_reset_all() releases an entry within
the sequential scan loop, which contradicts the protocol of
dshash_seq_next.

The two aboves are fixed in the attached v17.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center