MDEV-18722 Assertion `templ->mysql_null_bit_mask' failed in...

MDEV-18722 Assertion `templ->mysql_null_bit_mask' failed in row_sel_store_mysql_rec upon modifying indexed column into blob

don't assert that virtual columns are always nullable

mysql-test/main/long_unique_bugs.result

@@ -26,3 +26,10 @@ replace into t1 (pk) values (1);
 alter table t1 force;
 replace into t1 (pk) values (1);
 drop table t1;
+create table t1 (t time, unique(t)) engine=innodb;
+insert into t1 values (null),(null);
+alter ignore table t1 modify t text not null default '';
+Warnings:
+Warning	1265	Data truncated for column 't' at row 1
+Warning	1265	Data truncated for column 't' at row 2
+drop table t1;

mysql-test/main/long_unique_bugs.test

@@ -36,3 +36,11 @@ replace into t1 (pk) values (1);
 alter table t1 force;
 replace into t1 (pk) values (1);
 drop table t1;
+
+#
+# MDEV-18722 Assertion `templ->mysql_null_bit_mask' failed in row_sel_store_mysql_rec upon modifying indexed column into blob
+#
+create table t1 (t time, unique(t)) engine=innodb;
+insert into t1 values (null),(null);
+alter ignore table t1 modify t text not null default '';
+drop table t1;

storage/innobase/row/row0sel.cc

@@ -3057,9 +3057,6 @@ static bool row_sel_store_mysql_rec(
 		const mysql_row_templ_t*templ = &prebuilt->mysql_template[i];
 
 		if (templ->is_virtual && dict_index_is_clust(index)) {
-			/* Virtual columns are never declared NOT NULL. */
-			ut_ad(templ->mysql_null_bit_mask);
-
 			/* Skip virtual columns if it is not a covered
 			search or virtual key read is not requested. */
 			if (!rec_clust
@@ -3067,8 +3064,10 @@ static bool row_sel_store_mysql_rec(
 			    || (!prebuilt->read_just_key
 				&& !prebuilt->m_read_virtual_key)) {
 				/* Initialize the NULL bit. */
+				if (templ->mysql_null_bit_mask) {
 					mysql_rec[templ->mysql_null_byte_offset]
 						|= (byte) templ->mysql_null_bit_mask;
+				}
 				continue;
 			}