Bug #35449266 - An ALTER TABLE query corrupted the data dictionary so

mysqld crashes

Background: Column names are case insensitive in MySQL. When "ALTER
TABLE .. ADD COLUMN .. ALGORITHM=INSTANT" is triggered, the new row
version is created for this record. This version is stored on disk
with record and the column's metadata information is updated in
storage engine (se_private_data). And when "ALTER TABLE .. CHANGE
COLUMN" is triggered, we check if the column is renamed and the old
column's metadata is copied to new column.

Issue: While checking if the column is renamed, we compare the name of
the column in the create table info with that of in the alter table
info. But here the comparison is case sensitive which causes c1 to
differ from C1. Thus returning a nullptr when get_renamed_col() is
called, i.e., doesn't detect column rename and further leads to
skipping the updation of physical position of newly renamed column.
Which is why it hits the assertion failure in debug build where a
renamed column should have the physical position updated already.

Fix: The solution is to compare the column names while ignoring the
case sensitivity.

Change-Id: Ic30b183666da466a553cea45eb7028be3c2a36bb

Author: Vidhya Sree Rajuladevi

storage/innobase/handler/handler0alter.cc

@@ -475,7 +475,7 @@ static dd::Column *get_renamed_col(const Alter_inplace_info *ha_alter_info,
   Create_field *cf;
   while ((cf = cf_it++) != nullptr) {
     if (cf->field && cf->field->is_flag_set(FIELD_IS_RENAMED) &&
-        strcmp(cf->change, old_dd_column->name().c_str()) == 0) {
+        innobase_strcasecmp(cf->change, old_dd_column->name().c_str()) == 0) {
       /* This column is being renamed */
       return (const_cast<dd::Column *>(
           dd_find_column(&new_dd_tab->table(), cf->field_name)));