pgsql: Prohibit transaction commands in security definer procedures

Prohibit transaction commands in security definer procedures

Starting and aborting transactions in security definer procedures
doesn't work.  StartTransaction() insists that the security context
stack is empty, so this would currently cause a crash, and
AbortTransaction() resets it.  This could be made to work by
reorganizing the code, but right now we just prohibit it.

Reported-by: amul sul <sulamul@gmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CAAJ_b96Gupt_LFL7uNyy3c50-wbhA68NUjiK5%3DrF6_w%3Dpq_T%3DQ%40mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/3884072329bd1ad7d41bf7582c5d60e969365634

Modified Files
--------------
doc/src/sgml/ref/create_procedure.sgml              |  6 ++++++
src/backend/commands/functioncmds.c                 |  9 +++++++++
src/pl/plpgsql/src/expected/plpgsql_transaction.out | 12 ++++++++++++
src/pl/plpgsql/src/sql/plpgsql_transaction.sql      | 13 +++++++++++++
4 files changed, 40 insertions(+)