MDEV-30024 InnoDB: tried to purge non-delete-marked of a virtual column prefix

row_vers_vc_matches_cluster(): Invoke dtype_get_at_most_n_mbchars()
to extract the correct number of bytes corresponding to the number
of characters in a virtual column prefix index, just like we do in
row_sel_sec_rec_is_for_clust_rec().

The test case would occasionally reproduce the failure when this
fix is not present.

mysql-test/suite/gcol/r/innodb_virtual_purge.result

@@ -159,4 +159,18 @@ UNIQUE(pk),
 KEY(e)
 ) ENGINE=InnoDB;
 DROP TABLE t1, t2;
+#
+# MDEV-30024 InnoDB: tried to purge non-delete-marked record
+#            of an index on a virtual column prefix
+#
+CREATE TABLE t(a BINARY(8), b CHAR(8) AS (a) VIRTUAL, KEY(b(4)))
+CHARACTER SET utf8 ENGINE=InnoDB;
+INSERT INTO t (a) VALUES (''),('');
+UPDATE t SET a = 'x';
+UPDATE t SET a = '';
+SET GLOBAL innodb_max_purge_lag_wait=0;
+CHECK TABLE t EXTENDED;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+DROP TABLE t;
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;

mysql-test/suite/gcol/t/innodb_virtual_purge.test

@@ -172,5 +172,19 @@ dec $n;
 
 DROP TABLE t1, t2;
 
+--echo #
+--echo # MDEV-30024 InnoDB: tried to purge non-delete-marked record
+--echo #            of an index on a virtual column prefix
+--echo #
+
+CREATE TABLE t(a BINARY(8), b CHAR(8) AS (a) VIRTUAL, KEY(b(4)))
+CHARACTER SET utf8 ENGINE=InnoDB;
+INSERT INTO t (a) VALUES (''),('');
+UPDATE t SET a = 'x';
+UPDATE t SET a = '';
+SET GLOBAL innodb_max_purge_lag_wait=0;
+CHECK TABLE t EXTENDED;
+DROP TABLE t;
+
 --source include/wait_until_count_sessions.inc
 SET GLOBAL innodb_purge_rseg_truncate_frequency = @saved_frequency;

storage/innobase/row/row0vers.cc

@@ -739,9 +739,16 @@ row_vers_vc_matches_cluster(
 			    && (!compare[v_col->v_pos])) {
 
 				if (ind_field->prefix_len != 0
-				    && !dfield_is_null(field2)
-				    && field2->len > ind_field->prefix_len) {
-					field2->len = ind_field->prefix_len;
+				    && !dfield_is_null(field2)) {
+					field2->len = unsigned(
+						dtype_get_at_most_n_mbchars(
+							field2->type.prtype,
+							field2->type.mbminlen,
+							field2->type.mbmaxlen,
+							ind_field->prefix_len,
+							field2->len,
+							static_cast<char*>
+							(field2->data)));
 				}
 
 				/* The index field mismatch */