Introduce GUC_NO_RESET flag.

Previously, the transaction-property GUCs such as transaction_isolation
could be reset after starting a transaction, because we marked them
as GUC_NO_RESET_ALL but still allowed a targeted RESET.  That leads to
assertion failures or worse, because those properties aren't supposed
to change after we've acquired a transaction snapshot.

There are some NO_RESET_ALL variables for which RESET is okay, so
we can't just redefine the semantics of that flag.  Instead introduce
a separate GUC_NO_RESET flag.  Mark "seed", as well as the transaction
property GUCs, as GUC_NO_RESET.

We have to disallow GUC_ACTION_SAVE as well as straight RESET, because
otherwise a function having a "SET transaction_isolation" clause can
still break things: the end-of-function restore action is equivalent
to a RESET.

No back-patch, as it's conceivable that someone is doing something
this patch will forbid (like resetting one of these GUCs at transaction
start, or "CREATE FUNCTION ... SET transaction_read_only = 1") and not
running into problems with it today.  Given how long we've had this
issue and not noticed, the side effects in non-assert builds can't be
too serious.

Per bug #17385 from Andrew Bille.

Masahiko Sawada

Discussion: https://postgr.es/m/17385-9ee529fb091f0ce5@postgresql.org

2 files changed, 3 insertions, 5 deletions

diff --git a/src/pl/plpgsql/src/expected/plpgsql_transaction.out b/src/pl/plpgsql/src/expected/plpgsql_transaction.out
index 254e5b7a70..adff10fa6d 100644
--- a/src/pl/plpgsql/src/expected/plpgsql_transaction.out
+++ b/src/pl/plpgsql/src/expected/plpgsql_transaction.out
@@ -576,8 +576,7 @@ BEGIN
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;
-    SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
-    RESET TRANSACTION ISOLATION LEVEL;
+    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;
@@ -585,7 +584,7 @@ END;
 $$;
 INFO:  read committed
 INFO:  repeatable read
-INFO:  read committed
+INFO:  serializable
 -- error cases
 DO LANGUAGE plpgsql $$
 BEGIN
diff --git a/src/pl/plpgsql/src/sql/plpgsql_transaction.sql b/src/pl/plpgsql/src/sql/plpgsql_transaction.sql
index 8d76d00daa..c73fca7e03 100644
--- a/src/pl/plpgsql/src/sql/plpgsql_transaction.sql
+++ b/src/pl/plpgsql/src/sql/plpgsql_transaction.sql
@@ -481,8 +481,7 @@ BEGIN
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;
-    SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
-    RESET TRANSACTION ISOLATION LEVEL;
+    SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
     PERFORM 1;
     RAISE INFO '%', current_setting('transaction_isolation');
     COMMIT;