Bug #27041543: ASSERTION `MAX_LENGTH >= LENGTH' FAILED.

In Item_func_set_user_var::resolve_type(), collation is changed but
max_length is not accordingly. Fix by changing max_length if collation is
changed.

Change-Id: Iffe50a6b100a2bf5fe4bdd07651b3e11232db0de

Author: Xing Zhang

mysql-test/r/filesort_debug.result

@@ -142,3 +142,29 @@ HEX(WEIGHT_STRING(a COLLATE utf8mb4_0900_ai_ci, 4, 3, 0xC0))
 1C3E1C3D
 1C3F1C3D
 DROP TABLE t1;
+#
+# Bug #27041543: ASSERTION `MAX_LENGTH >= LENGTH' FAILED.
+#
+SET NAMES utf8,@@character_set_results=NULL;
+CREATE TEMPORARY TABLE t1(a INT);
+INSERT INTO t1 VALUES(1);
+SELECT 1 FROM t1 ORDER BY @x:=makedate(a,a);
+1
+1
+DROP TABLE t1;
+CREATE TABLE t1 AS SELECT 1 AS a WHERE false;
+CREATE TABLE t2 AS SELECT @x:=makedate(a,a) FROM t1;
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `@x:=makedate(a,a)` varchar(10) CHARACTER SET utf8 DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
+DROP TABLE t2;
+CREATE TABLE t2 AS SELECT @a:=@b:=@x:=makedate(a,a) FROM t1;
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `@a:=@b:=@x:=makedate(a,a)` varchar(10) CHARACTER SET utf8 DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
+DROP TABLE t2;
+DROP TABLE t1;

mysql-test/t/filesort_debug.test

@@ -214,3 +214,20 @@ SELECT HEX(WEIGHT_STRING(a COLLATE utf8mb4_0900_ai_ci, 3, 3, 0xC0)) FROM t1;
 # and get even number's weight.
 SELECT HEX(WEIGHT_STRING(a COLLATE utf8mb4_0900_ai_ci, 4, 3, 0xC0)) FROM t1;
 DROP TABLE t1;
+--echo #
+--echo # Bug #27041543: ASSERTION `MAX_LENGTH >= LENGTH' FAILED.
+--echo #
+SET NAMES utf8,@@character_set_results=NULL;
+CREATE TEMPORARY TABLE t1(a INT);
+INSERT INTO t1 VALUES(1);
+SELECT 1 FROM t1 ORDER BY @x:=makedate(a,a);
+DROP TABLE t1;
+
+CREATE TABLE t1 AS SELECT 1 AS a WHERE false;
+CREATE TABLE t2 AS SELECT @x:=makedate(a,a) FROM t1;
+SHOW CREATE TABLE t2;
+DROP TABLE t2;
+CREATE TABLE t2 AS SELECT @a:=@b:=@x:=makedate(a,a) FROM t1;
+SHOW CREATE TABLE t2;
+DROP TABLE t2;
+DROP TABLE t1;

sql/item.h

@@ -738,11 +738,12 @@ class Item : public Parse_tree_node
     Provide data type for a user or system variable, based on the type of
     the item that is assigned to the variable.
 
+    @note MYSQL_TYPE_VARCHAR is returned for all string types, but must be
+          further adjusted based on maximum string length by the caller.
+
     @param src_type  Source type that variable's type is derived from
-    @param max_bytes Maximum string size in bytes, used for string types
   */
-  static enum_field_types type_for_variable(enum_field_types src_type,
-                                            uint32 max_bytes)
+  static enum_field_types type_for_variable(enum_field_types src_type)
   {
     switch (src_type)
     {
@@ -778,12 +779,11 @@ class Item : public Parse_tree_node
       case MYSQL_TYPE_SET:
       case MYSQL_TYPE_GEOMETRY:
       case MYSQL_TYPE_NULL:
-        return MYSQL_TYPE_VARCHAR;
       case MYSQL_TYPE_TINY_BLOB:
       case MYSQL_TYPE_BLOB:
       case MYSQL_TYPE_MEDIUM_BLOB:
       case MYSQL_TYPE_LONG_BLOB:
-        return string_field_type(max_bytes);
+        return MYSQL_TYPE_VARCHAR;
       default:
         DBUG_ASSERT(false);
         return MYSQL_TYPE_NULL;

sql/item_func.cc

@@ -6009,10 +6009,31 @@ bool Item_func_set_user_var::resolve_type(THD*)
   else
     collation.collation= args[0]->collation.collation;
 
-  set_data_type(Item::type_for_variable(args[0]->data_type(),
-                                        args[0]->max_length));
-  max_length= args[0]->max_length;
+  enum_field_types data_type= Item::type_for_variable(args[0]->data_type());
+  switch (data_type)
+  {
+  case MYSQL_TYPE_LONGLONG:
+    set_data_type_longlong();
+    max_length= args[0]->max_length; // Preserves "length" of integer constants
+    break;
+  case MYSQL_TYPE_NEWDECIMAL:
+    set_data_type_decimal(args[0]->decimal_precision(), args[0]->decimals);
+    break;
+  case MYSQL_TYPE_DOUBLE:
+    set_data_type_double();
+    break;
+  case MYSQL_TYPE_VARCHAR:
+    set_data_type_string(args[0]->max_char_length());
+    break;
+  case MYSQL_TYPE_NULL:
+  default:
+    DBUG_ASSERT(false);
+    set_data_type(MYSQL_TYPE_NULL);
+    break;
+  }
+
   unsigned_flag= args[0]->unsigned_flag;
+
   return false;
 }