Bug #31145: ALTER TABLE DROP COLUMN, ADD COLUMN crashes (linux) or

  freezes (win) the server

The check for equality was assuming the field object is always 
created. If it's not it was de-referencing a NULL pointer.
Fixed to use the data in the create object instead.

mysql-test/r/alter_table.result

@@ -1338,4 +1338,11 @@ ALTER TABLE t1 CHANGE COLUMN f1 f1_no_real_change TIMESTAMP NULL DEFAULT NULL;
 affected rows: 0
 info: Records: 0  Duplicates: 0  Warnings: 0
 DROP TABLE t1;
+#
+# Bug #31145: ALTER TABLE DROP COLUMN, ADD COLUMN crashes (linux) 
+#   or freezes (win) the server
+#
+CREATE TABLE t1 (a TEXT, id INT, b INT);
+ALTER TABLE t1 DROP COLUMN a, ADD COLUMN c TEXT FIRST;
+DROP TABLE t1;
 End of 5.1 tests

mysql-test/t/alter_table.test

@@ -1061,4 +1061,16 @@ ALTER TABLE t1 CHANGE COLUMN f1 f1_no_real_change TIMESTAMP NULL DEFAULT NULL;
 --disable_info
 DROP TABLE t1;
 
+
+--echo #
+--echo # Bug #31145: ALTER TABLE DROP COLUMN, ADD COLUMN crashes (linux) 
+--echo #   or freezes (win) the server
+--echo #
+
+CREATE TABLE t1 (a TEXT, id INT, b INT);
+ALTER TABLE t1 DROP COLUMN a, ADD COLUMN c TEXT FIRST;
+
+DROP TABLE t1;
+
+
 --echo End of 5.1 tests

sql/field.cc

@@ -8282,8 +8282,7 @@ uint Field_blob::is_equal(Create_field *new_field)
 
   return ((new_field->sql_type == get_blob_type_from_length(max_data_length()))
           && new_field->charset == field_charset &&
-          ((Field_blob *)new_field->field)->max_data_length() ==
-          max_data_length());
+          new_field->pack_length == pack_length());
 }