patch for preventing the specification of conflicting transaction read/write options

Hi,

This is regarding the TODO item:
"Prevent the specification of conflicting transaction read/write options"

listed at:
http://wiki.postgresql.org/wiki/Todo

The issue is :

SET TRANSACTION read only read write read only;

The fix involved iteration over transaction_mode_list and checking for
duplicate entries.
The patch is based on suggestions mentioned in message:
http://archives.postgresql.org/pgsql-hackers/2009-01/msg00692.php

As per this, the patch does not throw any error for the first test case
mentioned above.
It throws error only in case of conflicting modes.

For ex:
postgres=# SET TRANSACTION read only read only;
SET

postgres=# SET TRANSACTION read only read write;
ERROR: conflicting options
LINE 1: SET TRANSACTION read only read write;
^

Below are basic unit test logs:

postgres=# SET TRANSACTION ISOLATION LEVEL serializable ISOLATION LEVEL
serializable;
SET
postgres=# SET TRANSACTION ISOLATION LEVEL serializable ISOLATION LEVEL
repeatable read;
ERROR: conflicting options
LINE 1: SET TRANSACTION ISOLATION LEVEL serializable ISOLATION LEVEL...

postgres=# SET TRANSACTION DEFERRABLE DEFERRABLE;
SET
postgres=# SET TRANSACTION DEFERRABLE NOT DEFERRABLE;
ERROR: conflicting options
LINE 1: SET TRANSACTION DEFERRABLE NOT DEFERRABLE;
^
postgres=# START TRANSACTION read only, read only;
START TRANSACTION
postgres=# rollback;
ROLLBACK
postgres=# START TRANSACTION read only, read write;
ERROR: conflicting options
LINE 1: START TRANSACTION read only, read write;
postgres=# rollback;
ROLLBACK ^
postgres=# BEGIN TRANSACTION ISOLATION LEVEL serializable, ISOLATION LEVEL
serializable;
BEGIN
postgres=# rollback;
ROLLBACK
postgres=# BEGIN TRANSACTION ISOLATION LEVEL serializable, ISOLATION LEVEL
repeatable read;
ERROR: conflicting options
LINE 1: BEGIN TRANSACTION ISOLATION LEVEL serializable, ISOLATION LE...
^

Please pass on any inputs on the patch.

Regards,
Chetan

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