Re: [HACKERS] 9.4.1 -> 9.4.2 problem: could not access status of transaction 1

Just wanted to report that I rolled back my VM to where it was with 9.4.2
installed and it wouldn't start. I installed 9.4.4 and now it starts up
just fine:

> 2015-06-12 16:05:58 PDT [6453]: [1-1] LOG: database system was shut down
at 2015-05-27 13:12:55 PDT
> 2015-06-12 16:05:58 PDT [6453]: [2-1] LOG: MultiXact member wraparound
protections are disabled because oldest checkpointed MultiXact 1 does not
exist on disk
> 2015-06-12 16:05:58 PDT [6457]: [1-1] LOG: autovacuum launcher started
> 2015-06-12 16:05:58 PDT [6452]: [1-1] LOG: database system is ready to
accept connections
> done
> server started

And this is showing up in my serverlog periodically as the emergency
autovacuums are running:

> 2015-06-12 16:13:44 PDT [6454]: [1-1] LOG: MultiXact member wraparound
protections are disabled because oldest checkpointed MultiXact 1 does not
exist on disk

**Thank you Robert and all involved for the resolution to this.**

> With the fixes introduced in this release, such a situation will result
in immediate emergency autovacuuming until a correct oldestMultiXid value
can be determined

Okay, I notice these vacuums are of the "to prevent wraparound" type (like
VACUUM FREEZE), that do hold locks preventing ALTER TABLEs and such. Good
to know, we'll plan our software updates accordingly.

Is there any risk until these autovacuums finish?