MDEV-26577 InnoDB: Failing assertion: dict_tf2_is_valid(flags, flags2) during ADD COLUMN

prepare_inplace_alter_table_dict(): If the table will not be rebuilt,
preserve all of the original ROW_FORMAT, including the compressed
page size flags related to ROW_FORMAT=COMPRESSED.

Author: dr-m

mysql-test/suite/innodb/r/instant_alter_upgrade.result

@@ -0,0 +1,9 @@
+#
+# MDEV-26577 InnoDB: Failing assertion: dict_tf2_is_valid(flags, flags2)
+# during ADD COLUMN
+#
+CREATE TABLE t1(a INT PRIMARY KEY) ENGINE=InnoDB ROW_FORMAT=COMPRESSED;
+CREATE TABLE t2(a INT PRIMARY KEY) ENGINE=InnoDB;
+FLUSH TABLES;
+ALTER TABLE t2 ADD COLUMN b INT;
+DROP TABLE t1,t2;

mysql-test/suite/innodb/t/instant_alter_upgrade.test

@@ -0,0 +1,16 @@
+--source include/have_innodb.inc
+--source include/innodb_row_format.inc
+
+--echo #
+--echo # MDEV-26577 InnoDB: Failing assertion: dict_tf2_is_valid(flags, flags2)
+--echo # during ADD COLUMN
+--echo #
+
+CREATE TABLE t1(a INT PRIMARY KEY) ENGINE=InnoDB ROW_FORMAT=COMPRESSED;
+CREATE TABLE t2(a INT PRIMARY KEY) ENGINE=InnoDB;
+FLUSH TABLES;
+--let $datadir=`select @@datadir`
+--remove_file $datadir/test/t2.frm
+--copy_file $datadir/test/t1.frm $datadir/test/t2.frm
+ALTER TABLE t2 ADD COLUMN b INT;
+DROP TABLE t1,t2;

storage/innobase/handler/handler0alter.cc

@@ -5012,8 +5012,10 @@ prepare_inplace_alter_table_dict(
 		/* If we promised ALGORITHM=NOCOPY or ALGORITHM=INSTANT,
 		we must retain the original ROW_FORMAT of the table. */
 		flags = (user_table->flags & (DICT_TF_MASK_COMPACT
+					      | DICT_TF_MASK_ZIP_SSIZE
 					      | DICT_TF_MASK_ATOMIC_BLOBS))
 			| (flags & ~(DICT_TF_MASK_COMPACT
+				     | DICT_TF_MASK_ZIP_SSIZE
 				     | DICT_TF_MASK_ATOMIC_BLOBS));
 	}