Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may corrupt

   definition at engine

If a single ALTER TABLE contains both DROP INDEX and ADD INDEX using 
the same index name (a.k.a. index modification) we need to disable 
in-place alter table because we can't ask the storage engine to have 
two copies of the index with the same name even temporarily (if we 
first do the ADD INDEX and then DROP INDEX) and we can't modify 
indexes that are needed by e.g. foreign keys if we first do 
DROP INDEX and then ADD INDEX.
Fixed the problem by disabling in-place ALTER TABLE for these cases.

mysql-test/r/innodb_mysql.result

@@ -2317,4 +2317,37 @@ ref	NULL
 rows	10
 Extra	Using index
 DROP TABLE t1;
+#
+# Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may
+#   corrupt definition at engine
+#
+CREATE TABLE t1 (a INT NOT NULL, b INT NOT NULL, KEY k (a,b)) 
+ENGINE=InnoDB;
+ALTER TABLE t1 DROP INDEX k, ADD UNIQUE INDEX k (a,b);
+SHOW INDEXES FROM t1;;
+Table	t1
+Non_unique	0
+Key_name	k
+Seq_in_index	1
+Column_name	a
+Collation	A
+Cardinality	0
+Sub_part	NULL
+Packed	NULL
+Null	
+Index_type	BTREE
+Comment	
+Table	t1
+Non_unique	0
+Key_name	k
+Seq_in_index	2
+Column_name	b
+Collation	A
+Cardinality	0
+Sub_part	NULL
+Packed	NULL
+Null	
+Index_type	BTREE
+Comment	
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/innodb_mysql.test

@@ -575,5 +575,18 @@ INSERT INTO t1 VALUES (1,1,1,1,1,1), (2,2,2,2,2,2), (3,3,3,3,3,3),
 
 DROP TABLE t1;
 
+--echo #
+--echo # Bug #49838: DROP INDEX and ADD UNIQUE INDEX for same index may
+--echo #   corrupt definition at engine
+--echo #
+
+CREATE TABLE t1 (a INT NOT NULL, b INT NOT NULL, KEY k (a,b)) 
+  ENGINE=InnoDB;
+
+ALTER TABLE t1 DROP INDEX k, ADD UNIQUE INDEX k (a,b);
+
+--query_vertical SHOW INDEXES FROM t1;
+
+DROP TABLE t1;
 
 --echo End of 5.1 tests

sql/sql_table.cc

@@ -5566,6 +5566,45 @@ mysql_discard_or_import_tablespace(THD *thd,
   DBUG_RETURN(-1);
 }
 
+/**
+  @brief Check if both DROP and CREATE are present for an index in ALTER TABLE
+ 
+  @details Checks if any index is being modified (present as both DROP INDEX 
+    and ADD INDEX) in the current ALTER TABLE statement. Needed for disabling 
+    online ALTER TABLE.
+  
+  @param table       The table being altered
+  @param alter_info  The ALTER TABLE structure
+  @return presence of index being altered  
+  @retval FALSE  No such index
+  @retval TRUE   Have at least 1 index modified
+*/
+
+static bool
+is_index_maintenance_unique (TABLE *table, Alter_info *alter_info)
+{
+  List_iterator<Key> key_it(alter_info->key_list);
+  List_iterator<Alter_drop> drop_it(alter_info->drop_list);
+  Key *key;
+
+  while ((key= key_it++))
+  {
+    if (key->name)
+    {
+      Alter_drop *drop;
+
+      drop_it.rewind();
+      while ((drop= drop_it++))
+      {
+        if (drop->type == Alter_drop::KEY &&
+            !my_strcasecmp(system_charset_info, key->name, drop->name))
+          return TRUE;
+      }
+    }
+  }
+  return FALSE;
+}
+
 
 /*
   SYNOPSIS
@@ -5654,6 +5693,7 @@ compare_tables(TABLE *table,
   */
   Alter_info tmp_alter_info(*alter_info, thd->mem_root);
   uint db_options= 0; /* not used */
+
   /* Create the prepared information. */
   if (mysql_prepare_create_table(thd, create_info,
                                  &tmp_alter_info,
@@ -6851,9 +6891,13 @@ bool mysql_alter_table(THD *thd,char *new_db, char *new_name,
   */
   new_db_type= create_info->db_type;
 
+  if (is_index_maintenance_unique (table, alter_info))
+    need_copy_table= ALTER_TABLE_DATA_CHANGED;
+
   if (mysql_prepare_alter_table(thd, table, create_info, alter_info))
     goto err;
   
+  if (need_copy_table == ALTER_TABLE_METADATA_ONLY)
     need_copy_table= alter_info->change_level;
 
   set_table_default_charset(thd, create_info, db);