Draft patch that fixes and a sketches test cases for:

Bug#20837 Apparent change of isolation level during transaction,
Bug#46527 COMMIT AND CHAIN RELEASE does not make sense,
Bug#53343 completion_type=1, COMMIT/ROLLBACK AND CHAIN don't 
preserve the isolation level
Bug#53346 completion_type has strange effect in a stored 
procedure/prepared statement

Make thd->tx_isolation mean strictly "current transaction 
isolation level"
Make thd->variables.tx_isolation mean "current session isolation
level".
The current transaction isolation level is now established
at transaction start. If there was a SET TRANSACTION
ISOLATION LEVEL statement, the value is taken from it.
Otherwise, the session value is used.
A change in a session value, made while a transaction is active,
whereas still allowed, no longer has any effect on the
current transaction isolation level. This is an incompatible
change.
A change in a session isolation level, made while there is
no active transaction, overrides SET TRANSACTION statement,
if there was any.
Changed the impelmentation to not look at @@session.completion_type
in the parser, and thus fixed Bug#53346.
Changed the parser to not allow AND NO CHAIN RELEASE,
and thus fixed Bug#46527.
Changed the transaction API to take the current transaction
isolation level into account:
- BEGIN/COMMIT now do preserve the current transaction
isolation level if chaining is on.
- implicit commit, XA COMMIT or XA ROLLBACK or autocommit don't.

1 files changed, 2 insertions, 2 deletions

diff --git a/sql/sql_class.cc b/sql/sql_class.cc
index 3144192a971..7db70c36d73 100644
--- a/sql/sql_class.cc
+++ b/sql/sql_class.cc
@@ -320,7 +320,7 @@ int thd_sql_command(const THD *thd)
 extern "C"
 int thd_tx_isolation(const THD *thd)
 {
-  return (int) thd->variables.tx_isolation;
+  return (int) thd->tx_isolation;
 }
 
 extern "C"
@@ -922,7 +922,7 @@ void THD::init(void)
   update_lock_default= (variables.low_priority_updates ?
 			TL_WRITE_LOW_PRIORITY :
 			TL_WRITE);
-  session_tx_isolation= (enum_tx_isolation) variables.tx_isolation;
+  tx_isolation= (enum_tx_isolation) variables.tx_isolation;
   update_charset();
   reset_current_stmt_binlog_format_row();
   bzero((char *) &status_var, sizeof(status_var));