Re: pgsql: Fix deadlock hazard in CREATE INDEX CONCURRENTLY

Hi,

On 2018-01-03 02:20:14 +0000, Alvaro Herrera wrote:
> Fix deadlock hazard in CREATE INDEX CONCURRENTLY
> 
> Multiple sessions doing CREATE INDEX CONCURRENTLY simultaneously are
> supposed to be able to work in parallel, as evidenced by fixes in commit
> c3d09b3bd23f specifically to support this case.  In reality, one of the
> sessions would be aborted by a misterious "deadlock detected" error.
> 
> Jeff Janes diagnosed that this is because of leftover snapshots used for
> system catalog scans -- this was broken by 8aa3e47510b9 keeping track of
> (registering) the catalog snapshot.  To fix the deadlocks, it's enough
> to de-register that snapshot prior to waiting.
> 
> Backpatch to 9.4, which introduced MVCC catalog scans.
> 
> Include an isolationtester spec that 8 out of 10 times reproduces the
> deadlock with the unpatched code for me (Álvaro).

The isolation test doesn't appear to be fully stable:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=baiji&dt=2018-01-03%2003%3A00%3A01

Greetings,

Andres Freund