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, 16 insertions, 0 deletions

diff --git a/sql/sys_vars.h b/sql/sys_vars.h
index c4855ef4fd3..fbc48573487 100644
--- a/sql/sys_vars.h
+++ b/sql/sys_vars.h
@@ -1599,6 +1599,22 @@ public:
   { return type != STRING_RESULT; }
 };
 
+
+class Sys_var_tx_isolation: public Sys_var_enum
+{
+public:
+  Sys_var_tx_isolation(const char *name_arg,
+          const char *comment, int flag_args, ptrdiff_t off, size_t size,
+          CMD_LINE getopt,
+          const char *values[], uint def_val, PolyLock *lock,
+          enum binlog_status_enum binlog_status_arg,
+          on_check_function on_check_func)
+    :Sys_var_enum(name_arg, comment, flag_args, off, size, getopt,
+                  values, def_val, lock, binlog_status_arg, on_check_func)
+  {}
+  virtual bool session_update(THD *thd, set_var *var);
+};
+
 /****************************************************************************
   Used templates
 ****************************************************************************/