MDEV-18790 Server crash in fields_in_hash_keyinfo after unsuccessful...

attempt to drop BLOB with long index

Restore long table->key_info, So that in the case of unsuccessful table
alter table->key_info should have a correct value. In the case of successful
table alter old table is flushed so that is why we don't see this error in
the case of successful alter.

mysql-test/main/long_unique_bugs.result

@@ -57,3 +57,9 @@ create table t (b blob, unique(b)) engine=myisam;
 insert into t values ('foo');
 replace into t values ('foo');
 drop table t;
+CREATE TABLE t1 (f INT, x BLOB, UNIQUE (x));
+INSERT INTO t1 VALUES (1,'foo');
+ALTER TABLE t1 DROP x, ALGORITHM=INPLACE;
+ERROR 0A000: ALGORITHM=INPLACE is not supported for this operation. Try ALGORITHM=COPY
+UPDATE t1 SET x = 'bar';
+DROP TABLE t1;

mysql-test/main/long_unique_bugs.test

@@ -72,3 +72,13 @@ create table t (b blob, unique(b)) engine=myisam;
 insert into t values ('foo');
 replace into t values ('foo');
 drop table t;
+
+#
+# MDEV-18790 Server crash in fields_in_hash_keyinfo after unsuccessful attempt to drop BLOB with long index
+#
+CREATE TABLE t1 (f INT, x BLOB, UNIQUE (x));
+INSERT INTO t1 VALUES (1,'foo');
+--error ER_ALTER_OPERATION_NOT_SUPPORTED
+ALTER TABLE t1 DROP x, ALGORITHM=INPLACE;
+UPDATE t1 SET x = 'bar';
+DROP TABLE t1;

sql/sql_table.cc

@@ -8476,6 +8476,8 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
                    &key_parts, key_info->option_list, DDL_options());
       new_key_list.push_back(key, thd->mem_root);
     }
+    if (key_info->algorithm == HA_KEY_ALG_LONG_HASH)
+      re_setup_keyinfo_hash(key_info);
   }
   {
     Key *key;