MDEV-18799 Long unique does not work after failed alter table

Restore table->key_info after calling setup_keyinfo_hash in
mysql_prepare_alter_table.

mysql-test/main/long_unique_bugs.result

@@ -63,3 +63,24 @@ 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;
+create table t1(a blob unique , b blob);
+insert into t1 values(1,1),(2,1);
+alter table t1 add unique(b);
+ERROR 23000: Duplicate entry '1' for key 'b'
+show keys from t1;;
+Table	t1
+Non_unique	0
+Key_name	a
+Seq_in_index	1
+Column_name	a
+Collation	A
+Cardinality	NULL
+Sub_part	NULL
+Packed	NULL
+Null	YES
+Index_type	HASH
+Comment	
+Index_comment	
+insert into t1 values(1,1);
+ERROR 23000: Duplicate entry '1' for key 'a'
+DROP TABLE t1;

mysql-test/main/long_unique_bugs.test

@@ -82,3 +82,15 @@ INSERT INTO t1 VALUES (1,'foo');
 ALTER TABLE t1 DROP x, ALGORITHM=INPLACE;
 UPDATE t1 SET x = 'bar';
 DROP TABLE t1;
+
+#
+# MDEV-18799 Long unique does not work after failed alter table
+#
+create table t1(a blob unique , b blob);
+insert into t1 values(1,1),(2,1);
+--error ER_DUP_ENTRY
+alter table t1 add unique(b);
+--query_vertical show keys from t1;
+--error ER_DUP_ENTRY
+insert into t1 values(1,1);
+DROP TABLE t1;

sql/sql_table.cc

@@ -8299,10 +8299,14 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   */
   for (uint i=0 ; i < table->s->keys ; i++,key_info++)
   {
+    bool long_hash_key= false;
     if (key_info->flags & HA_INVISIBLE_KEY)
       continue;
     if (key_info->algorithm == HA_KEY_ALG_LONG_HASH)
+    {
       setup_keyinfo_hash(key_info);
+      long_hash_key= true;
+    }
     const char *key_name= key_info->name.str;
     Alter_drop *drop;
     drop_it.rewind();
@@ -8427,10 +8431,7 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
       LEX_CSTRING tmp_name;
       bzero((char*) &key_create_info, sizeof(key_create_info));
       if (key_info->algorithm == HA_KEY_ALG_LONG_HASH)
-      {
-        key_info->flags|= HA_NOSAME;
         key_info->algorithm= HA_KEY_ALG_UNDEF;
-      }
       key_create_info.algorithm= key_info->algorithm;
       /*
         We copy block size directly as some engines, like Area, sets this
@@ -8476,9 +8477,12 @@ 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)
+    if (long_hash_key)
+    {
+      key_info->algorithm= HA_KEY_ALG_LONG_HASH;
       re_setup_keyinfo_hash(key_info);
     }
+  }
   {
     Key *key;
     while ((key=key_it++))			// Add new keys

sql/table.cc

@@ -8983,6 +8983,7 @@ void setup_keyinfo_hash(KEY *key_info)
   key_info->key_part-= no_of_keyparts;
   key_info->user_defined_key_parts= key_info->usable_key_parts=
                key_info->ext_key_parts= no_of_keyparts;
+  key_info->flags|= HA_NOSAME;
 }
 /*
   re_setup_keyinfo_hash reverts th setup_keyinfo_hash and this type of
@@ -8997,6 +8998,7 @@ void re_setup_keyinfo_hash(KEY *key_info)
     key_info->key_part++;
   key_info->user_defined_key_parts= key_info->usable_key_parts=
                key_info->ext_key_parts= 1;
+  key_info->flags&= ~HA_NOSAME;
 }
 /**
   @brief clone of current handler.