Re: [v9.2] Add GUC sepgsql.client_label

Tom Lane wrote:

> More to the point, a GUC rollback transition *has to always
> succeed*. Period.

I was about to point out the exception of the transaction_read_only
GUC, which according to the standard must not be changed except at
the beginning of a transaction or a subtransaction, and must not be
changed from "on" to "off" in a subtransaction. Then I noticed that,
while we protect against an explicit change in a prohibited way, we
allow a RESET:

test=# begin transaction read only;
BEGIN
test=# select * from x;
x
---
1
(1 row)

test=# set transaction_read_only = off;
ERROR: transaction read-write mode must be set before any query
test=# rollback;
ROLLBACK
test=# begin transaction read only;
BEGIN
test=# select * from x;
x
---
1
(1 row)

test=# reset transaction_read_only ;
RESET
test=# insert into x VALUES (2);
INSERT 0 1
test=# commit;
COMMIT

I think that's a problem. It could allow back-door violations of
invariants enforced by triggers, and seems to violate the SQL
standard. I think this should be considered a bug, although I'm not
sure whether it's safe to back-patch, given the change to existing
behavior.

Whether such a (required) exception to what you assert above should
open the door to any others is another question.

-Kevin