MDEV-17148 DROP DATABASE throw "Directory not empty" after changed lower_case_table_names.

No need to lowercase table names on case-sensitive file systems, as the
cache won't contain the 'lowercased' table anyway. And it prevents the
UPPERCASE.frm from being deleted.

mysql-test/r/lowercase_table.result

@@ -127,3 +127,7 @@ Database (mysql_TE%)
 mysql_test
 drop database mysql_TEST;
 End of 10.0 tests
+create database db1;
+create table t1 (a int);
+drop database db1;
+drop table t1;

mysql-test/t/lowercase_table.test

@@ -118,3 +118,15 @@ show databases like "mysql_TE%";
 drop database mysql_TEST;
 
 --echo End of 10.0 tests
+
+#
+# MDEV-17148 DROP DATABASE throw "Directory not empty" after changed lower_case_table_names.
+#
+
+let $datadir=`select @@datadir`;
+create database db1;
+create table t1 (a int);
+copy_file $datadir/test/t1.frm $datadir/db1/T1.frm;
+drop database db1;
+drop table t1;
+

sql/sql_db.cc

@@ -1107,8 +1107,12 @@ static bool find_db_tables_and_rm_known_files(THD *thd, MY_DIR *dirp,
     table_list->table_name_length= table->length;
     table_list->open_type= OT_BASE_ONLY;
 
-    /* To be able to correctly look up the table in the table cache. */
-    if (lower_case_table_names)
+    /*
+      On the case-insensitive file systems table is opened
+      with the lowercased file name. So we should lowercase
+      as well to look up the cache properly.
+    */
+    if (lower_case_file_system)
       table_list->table_name_length= my_casedn_str(files_charset_info,
                                                    table_list->table_name);