MDEV-15308 Assertion `ha_alter_info->alter_info->drop_list.elements

Problem was that handle_if_exists_options() didn't correct
alter_info->flags when things was removed from the list.

mysql-test/r/alter_table.result

@@ -2164,3 +2164,64 @@ t1	CREATE TABLE `t1` (
   `b` int(11) DEFAULT NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8
 DROP TABLE t1;
+#
+#
+# MDEV-15308
+# Assertion `ha_alter_info->alter_info->drop_list.elements > 0' failed
+# in ha_innodb::prepare_inplace_alter_table
+#
+CREATE TABLE t1 (a INT, b INT) ENGINE=InnoDB;
+ALTER TABLE t1 DROP FOREIGN KEY IF EXISTS fk, DROP COLUMN b;
+Warnings:
+Note	1091	Can't DROP 'fk'; check that column/key exists
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b INT) ENGINE=InnoDB;
+ALTER TABLE t1 DROP INDEX IF EXISTS fk, DROP COLUMN b;
+Warnings:
+Note	1091	Can't DROP 'fk'; check that column/key exists
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b INT, c INT, KEY(c)) ENGINE=InnoDB;
+ALTER TABLE t1 DROP FOREIGN KEY IF EXISTS fk, DROP COLUMN c;
+Warnings:
+Note	1091	Can't DROP 'fk'; check that column/key exists
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  `b` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b INT, c INT, KEY c1(c)) ENGINE=InnoDB;
+ALTER TABLE t1 DROP FOREIGN KEY IF EXISTS fk, DROP INDEX c1;
+Warnings:
+Note	1091	Can't DROP 'fk'; check that column/key exists
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  `b` int(11) DEFAULT NULL,
+  `c` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;
+CREATE TABLE t1 (a INT, b INT) ENGINE=InnoDB;
+ALTER TABLE t1 DROP INDEX IF EXISTS fk, DROP COLUMN IF EXISTS c;
+Warnings:
+Note	1091	Can't DROP 'fk'; check that column/key exists
+Note	1091	Can't DROP 'c'; check that column/key exists
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL,
+  `b` int(11) DEFAULT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=latin1
+DROP TABLE t1;

mysql-test/t/alter_table.test

@@ -1809,3 +1809,35 @@ SHOW CREATE TABLE t1;
 ALTER TABLE t1 CONVERT TO CHARACTER SET utf8;
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
+
+--echo #
+--echo #
+--echo # MDEV-15308
+--echo # Assertion `ha_alter_info->alter_info->drop_list.elements > 0' failed
+--echo # in ha_innodb::prepare_inplace_alter_table
+--echo #
+
+CREATE TABLE t1 (a INT, b INT) ENGINE=InnoDB;
+ALTER TABLE t1 DROP FOREIGN KEY IF EXISTS fk, DROP COLUMN b;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT, b INT) ENGINE=InnoDB;
+ALTER TABLE t1 DROP INDEX IF EXISTS fk, DROP COLUMN b;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT, b INT, c INT, KEY(c)) ENGINE=InnoDB;
+ALTER TABLE t1 DROP FOREIGN KEY IF EXISTS fk, DROP COLUMN c;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT, b INT, c INT, KEY c1(c)) ENGINE=InnoDB;
+ALTER TABLE t1 DROP FOREIGN KEY IF EXISTS fk, DROP INDEX c1;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;
+
+CREATE TABLE t1 (a INT, b INT) ENGINE=InnoDB;
+ALTER TABLE t1 DROP INDEX IF EXISTS fk, DROP COLUMN IF EXISTS c;
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

sql/sql_table.cc

@@ -5808,10 +5808,28 @@ drop_create_field:
     List_iterator<Alter_drop> drop_it(alter_info->drop_list);
     Alter_drop *drop;
     bool remove_drop;
+    ulonglong left_flags= 0;
     while ((drop= drop_it++))
     {
+      ulonglong cur_flag= 0;
+      switch (drop->type) {
+      case Alter_drop::COLUMN:
+        cur_flag= Alter_info::ALTER_DROP_COLUMN;
+        break;
+      case Alter_drop::FOREIGN_KEY:
+        cur_flag= Alter_info::DROP_FOREIGN_KEY;
+        break;
+      case Alter_drop::KEY:
+        cur_flag= Alter_info::ALTER_DROP_INDEX;
+        break;
+      default:
+        break;
+      }
       if (!drop->drop_if_exists)
+      {
+        left_flags|= cur_flag;
         continue;
+      }
       remove_drop= TRUE;
       if (drop->type == Alter_drop::COLUMN)
       {
@@ -5887,12 +5905,15 @@ drop_create_field:
             ER_CANT_DROP_FIELD_OR_KEY, ER(ER_CANT_DROP_FIELD_OR_KEY),
             drop->name);
         drop_it.remove();
-        if (alter_info->drop_list.is_empty())
+      }
+      else
+        left_flags|= cur_flag;
+    }
+    /* Reset state to what's left in drop list */
     alter_info->flags&= ~(Alter_info::ALTER_DROP_COLUMN |
                           Alter_info::ALTER_DROP_INDEX  |
                           Alter_info::DROP_FOREIGN_KEY);
-      }
+    alter_info->flags|= left_flags;
-    }
   }
 
   /* ALTER TABLE ADD KEY IF NOT EXISTS */