Bug #20837 Apparent change of isolation level during transaction

      
SET TRANSACTION ISOLATION LEVEL is used to temporarily set
the trans.iso.level for the next transaction.  After the
transaction, the iso.level is (re-)set to value of the 
session variable 'tx_isolation'.

The bug is caused by setting the thd->variables.tx_isolation 
field to the value of the session variable upon each
statement commit.  It should only be set at the end of the
full transaction.

The fix has been to remove the setting of the variable in
ha_autocommit_or_rollback if we're in a transaction, as it 
will be correctly set in  either ha_rollback or 
ha_commit_one_phase.  

If, on the other hand, we're in  autocommit mode, tx_isolation 
will be explicitly set here.



mysql-test/t/innodb_mysql.test:
  "SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED" sets
  the trans.isolation for the next transaction.  We test
  that @@tx_isolation keeps this value during the transaction,
  and is only reset back to the session value at the end
  of the transaction.

mysql-test/r/innodb_mysql.result

@@ -2273,4 +2273,35 @@ END|
 DROP PROCEDURE p1;
 DROP VIEW v1;
 DROP TABLE t1,t2;
+#
+# Bug #20837 Apparent change of isolation 
+#     level during transaction
+#
+SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+CREATE TABLE t1 (s1 INT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (1),(2);
+COMMIT;
+SET @@autocommit = 0;
+COMMIT;
+SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
+START TRANSACTION;
+SELECT * FROM t1;
+s1
+1
+2
+Should be READ UNCOMMITTED
+SELECT @@tx_isolation;
+@@tx_isolation
+READ-UNCOMMITTED
+INSERT INTO t1 VALUES (-1);
+Should be READ UNCOMMITTED
+SELECT @@tx_isolation;
+@@tx_isolation
+READ-UNCOMMITTED
+COMMIT;
+Should now be REPEATABLE READ
+SELECT @@tx_isolation;
+@@tx_isolation
+REPEATABLE-READ
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/innodb_mysql.test

@@ -536,4 +536,30 @@ DROP PROCEDURE p1;
 DROP VIEW v1;
 DROP TABLE t1,t2;
 
+
+--echo #
+--echo # Bug #20837 Apparent change of isolation 
+--echo #     level during transaction
+--echo #
+
+SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ;
+CREATE TABLE t1 (s1 INT) ENGINE=InnoDB;
+INSERT INTO t1 VALUES (1),(2);
+COMMIT;
+
+SET @@autocommit = 0;
+COMMIT;
+SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
+START TRANSACTION;
+SELECT * FROM t1;
+--echo Should be READ UNCOMMITTED
+SELECT @@tx_isolation;
+INSERT INTO t1 VALUES (-1);
+--echo Should be READ UNCOMMITTED
+SELECT @@tx_isolation;
+COMMIT;
+--echo Should now be REPEATABLE READ
+SELECT @@tx_isolation;
+DROP TABLE t1;
+
 --echo End of 5.1 tests

sql/handler.cc

@@ -1369,8 +1369,14 @@ int ha_autocommit_or_rollback(THD *thd, int error)
       if (thd->transaction_rollback_request && !thd->in_sub_stmt)
         (void) ha_rollback(thd);
     }
-
-    thd->variables.tx_isolation=thd->session_tx_isolation;
+  } 
+  else if (!(thd->options & (OPTION_NOT_AUTOCOMMIT | OPTION_BEGIN)))
+  {
+    /* 
+       If we're in autocommit mode, reset tx_isolation
+       to the default value
+    */
+    thd->variables.tx_isolation= thd->session_tx_isolation;
   }
 #endif
   DBUG_RETURN(error);