Repeated crashes in GENERATED ... AS IDENTITY tests

woodlouse just showed a failure that looked a bit familiar:

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=woodlouse&dt=2018-04-17%2016%3A42%3A43

I trawled the buildfarm logs for similar failures, and there are several:

sysname | snapshot | stage | l
-----------+---------------------+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------
whelk | 2018-03-28 05:41:30 | Check | 2018-03-28 07:52:26.987 CEST [4928:6] DETAIL: Failed process was running: ALTER TABLE itest13 ADD COLUMN c int GENERATED BY DEFAULT AS IDENTITY;\r
conchuela | 2018-04-03 10:00:00 | Check | 2018-04-03 12:11:47.520 CEST [5ac35311.5248:5] DETAIL: Failed process was running: ALTER TABLE itest13 ADD COLUMN b int GENERATED BY DEFAULT AS IDENTITY;
gharial | 2018-04-10 00:32:08 | Check | 2018-04-09 18:59:39.075 MDT [5acc0b05.32ea:5] DETAIL: Failed process was running: ALTER TABLE itest13 ADD COLUMN b int GENERATED BY DEFAULT AS IDENTITY;
whelk | 2018-04-14 19:41:24 | Check | 2018-04-14 21:52:07.700 CEST [3760:6] DETAIL: Failed process was running: ALTER TABLE itest13 ADD COLUMN c int GENERATED BY DEFAULT AS IDENTITY;\r
woodlouse | 2018-04-17 16:42:43 | Check | 2018-04-17 18:54:04.311 CEST [864:6] DETAIL: Failed process was running: ALTER TABLE itest13 ADD COLUMN b int GENERATED BY DEFAULT AS IDENTITY;\r
(5 rows)

The test case that's failing, in identity.sql, has been there since
early February; the lack of any crashes till more recently suggests
that something committed in mid-to-late March broke it.

I have no idea what's going on there, but I think this is clearly
something we need to consider an open item for PG11. I'll put it
on the list.

regards, tom lane