BUG#23509275 :DBUG_PRINT in THD::decide_logging_format prints incorrectly, access out-of-bound

Problem:
In debug builds, there is a chance that an out-of-bounds
read is performed when tables are locked in
LTM_PRELOCKED_UNDER_LOCK_TABLES mode. It can happen because
the debug code uses enum values as index for an array of
mode descriptions, but it only takes into consideration 3
out of 4 of the enum values.

Fix:
This patch fixes it by implementing a getter for the enum which
returns a string representation of the enum,
effectively removing the out-of-bounds read.

Moreover, it also fixes the lock mode descriptions that
would be print out in debug builds.

sql/sql_class.cc

 /*
-   Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -4246,6 +4246,25 @@ has_write_table_auto_increment_not_first_in_pk(TABLE_LIST *tables)
   return 0;
 }
 
+#ifndef DBUG_OFF
+const char * get_locked_tables_mode_name(enum_locked_tables_mode locked_tables_mode)
+{
+   switch (locked_tables_mode)
+   {
+   case LTM_NONE:
+     return "LTM_NONE";
+   case LTM_LOCK_TABLES:
+     return "LTM_LOCK_TABLES";
+   case LTM_PRELOCKED:
+     return "LTM_PRELOCKED";
+   case LTM_PRELOCKED_UNDER_LOCK_TABLES:
+     return "LTM_PRELOCKED_UNDER_LOCK_TABLES";
+   default:
+     return "Unknown table lock mode";
+   }
+}
+#endif
+
 /**
   Decide on logging format to use for the statement and issue errors
   or warnings as needed.  The decision depends on the following
@@ -4397,15 +4416,8 @@ int THD::decide_logging_format(TABLE_LIST *tables)
     TABLE* prev_access_table= NULL;
 
 #ifndef DBUG_OFF
-    {
-      static const char *prelocked_mode_name[] = {
-        "NON_PRELOCKED",
-        "PRELOCKED",
-        "PRELOCKED_UNDER_LOCK_TABLES",
-      };
-      DBUG_PRINT("debug", ("prelocked_mode: %s",
-                           prelocked_mode_name[locked_tables_mode]));
-    }
+    DBUG_PRINT("debug", ("prelocked_mode: %s",
+                          get_locked_tables_mode_name(locked_tables_mode)));
 #endif
 
     if (variables.binlog_format != BINLOG_FORMAT_ROW && tables)

sql/sql_class.h

@@ -963,6 +963,8 @@ typedef I_List<Item_change_record> Item_change_list;
 /**
   Type of locked tables mode.
   See comment for THD::locked_tables_mode for complete description.
+  While adding new enum values add them to the getter method for this enum
+  declared below and defined in sql_class.cc as well.
 */
 
 enum enum_locked_tables_mode
@@ -973,6 +975,15 @@ enum enum_locked_tables_mode
   LTM_PRELOCKED_UNDER_LOCK_TABLES
 };
 
+#ifndef DBUG_OFF
+/**
+  Getter for the enum enum_locked_tables_mode
+  @param locked_tables_mode enum for types of locked tables mode
+
+  @return The string represantation of that enum value
+*/
+const char * get_locked_tables_mode_name(enum_locked_tables_mode locked_tables_mode);
+#endif
 
 /**
   Class that holds information about tables which were opened and locked