Insufficient locking for ALTER DEFAULT PRIVILEGES

I came across the following bug this week:

Session 0:
begin;
create schema bug;
alter default privileges in schema bug grant all on tables to postgres;
commit;

Session 1:
begin;
alter default privileges in schema bug grant all on tables to postgres;

Session 2:
alter default privileges in schema bug grant all on tables to postgres;
<hangs>

Session 1:
commit;

Session 2:
ERROR:  tuple concurrently updated
-- 
Vik Fearing                                          +33 6 46 75 15 36
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