[YSQL] Disallow changing from read-write to read-only transaction if using serializable isolation

[pkj415]

Jira Link: DB-11930

Description

Pg allows changing a read-write transaction to read-only even after executing a few writes in a transaction block. This is the case in YB too, and is okay for RC/ RR isolation levels. But for serializable isolation level, we face the following issue:

create table test (k int primary key, v int);
yugabyte=# begin transaction isolation level serializable;
BEGIN
yugabyte=# insert into test values (4, 4);
INSERT 0 1
yugabyte=# set transaction_read_only=true;
SET
yugabyte=# select * from test;
ERROR:  Attempt to change effective isolation from 2 to SNAPSHOT_ISOLATION in the middle of a transaction. Postgres-level isolation: SERIALIZABLE; read_only: 1.
yugabyte=#

Issue Type

kind/bug

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