Bug#31662: 'null' is shown as type of fields for view with bad definer, breaks mysqldump

SHOW FIELDS FROM a view with no valid definer was possible (since fix
for Bug#26817), but gave NULL as a field-type. This led to mysqldump-ing
of such views being successful, but loading such a dump with the client
failing. Patch allows SHOW FIELDS to give data-type of field in underlying
table.


mysql-test/r/information_schema_db.result:
  Fix test results: SHOW FIELDS FROM a view with no valid DEFINER
  gives us the field-type of the underlying table now rather than NULL.
sql/sql_base.cc:
  In the case of SHOW FIELDS FROM <view>, do not require a valid
  DEFINER for determining underlying data-type like we usually do.
  This is needed for mysqldump.

mysql-test/r/information_schema_db.result

@@ -130,7 +130,7 @@ Warnings:
 Warning	1356	View 'testdb_1.v7' references invalid table(s) or column(s) or function(s) or definer/invoker of view lack rights to use them
 show fields from testdb_1.v7;
 Field	Type	Null	Key	Default	Extra
-f1	null	YES		NULL	
+f1	char(4)	YES		NULL	
 Warnings:
 Note	1449	There is no 'no_such_user'@'no_such_host' registered
 create table t3 (f1 char(4), f2 char(4));
@@ -150,7 +150,7 @@ View	Create View
 v6	CREATE ALGORITHM=UNDEFINED DEFINER=`root`@`localhost` SQL SECURITY DEFINER VIEW `testdb_1`.`v6` AS select `testdb_1`.`t1`.`f1` AS `f1` from `testdb_1`.`t1`
 show fields from testdb_1.v7;
 Field	Type	Null	Key	Default	Extra
-f1	null	YES		NULL	
+f1	char(4)	YES		NULL	
 Warnings:
 Note	1449	There is no 'no_such_user'@'no_such_host' registered
 show create view testdb_1.v7;
@@ -178,7 +178,7 @@ show create view v4;
 ERROR HY000: EXPLAIN/SHOW can not be issued; lacking privileges for underlying table
 show fields from v4;
 Field	Type	Null	Key	Default	Extra
-f1	null	YES		NULL	
+f1	char(4)	YES		NULL	
 f2	char(4)	YES		NULL	
 show fields from v2;
 Field	Type	Null	Key	Default	Extra

sql/sql_base.cc

@@ -3958,7 +3958,9 @@ find_field_in_tables(THD *thd, Item_ident *item,
   {
     Field *cur_field= find_field_in_table_ref(thd, cur_table, name, length,
                                               item->name, db, table_name, ref,
-                                              check_privileges,
+                                              (thd->lex->sql_command ==
+                                               SQLCOM_SHOW_FIELDS)
+                                              ? false : check_privileges,
                                               allow_rowid,
                                               &(item->cached_field_index),
                                               register_tree_change,