MDEV-27746 Wrong comparision of BLOB's empty preffix with non-preffixed BLOB causes rows count mismatch for clustered and secondary indexes during non-locking read

row_sel_sec_rec_is_for_clust_rec() treats empty BLOB prefix field in
secondary index as a field equal to any external BLOB field in clustered
index. Row_sel_get_clust_rec_for_mysql::operator() doesn't zerro out
clustered record pointer in row_search_mvcc(), and row_search_mvcc()
thinks that delete-marked secondary index record has visible for
"CHECK TABLE"'s read view old-versioned clustered index record, and
row_scan_index_for_mysql() counts it as a row.

The fix is to execute row_sel_sec_rec_is_for_blob() in
row_sel_sec_rec_is_for_clust_rec() if clustered field contains BLOB's
reference.

Author: vlad-lesin

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

@@ -0,0 +1,19 @@
+connect prevent_purge,localhost,root,,;
+start transaction with consistent snapshot;
+connection default;
+SET @fill_amount = (@@innodb_page_size / 2 ) + 1;
+CREATE TABLE t1 (col_text TEXT NOT NULL, KEY (col_text(9))) ENGINE=InnoDB;
+INSERT INTO t1 (col_text) VALUES (REPEAT('x', @fill_amount));
+UPDATE t1 SET col_text='';
+UPDATE t1 SET col_text=REPEAT('y', @fill_amount);
+connect con1,localhost,root,,;
+SET @fill_amount = (@@innodb_page_size / 2 ) + 1;
+BEGIN;
+INSERT INTO t1 (col_text) VALUES (REPEAT('z', @fill_amount));
+connection default;
+CHECK TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	check	status	OK
+disconnect con1;
+disconnect prevent_purge;
+DROP TABLE t1;

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

@@ -0,0 +1,26 @@
+--source include/innodb_row_format.inc
+--source include/count_sessions.inc
+--connect(prevent_purge,localhost,root,,)
+start transaction with consistent snapshot;
+
+--connection default
+SET @fill_amount = (@@innodb_page_size / 2 ) + 1;
+CREATE TABLE t1 (col_text TEXT NOT NULL, KEY (col_text(9))) ENGINE=InnoDB;
+
+INSERT INTO t1 (col_text) VALUES (REPEAT('x', @fill_amount));
+UPDATE t1 SET col_text='';
+UPDATE t1 SET col_text=REPEAT('y', @fill_amount);
+
+--connect(con1,localhost,root,,)
+SET @fill_amount = (@@innodb_page_size / 2 ) + 1;
+BEGIN;
+INSERT INTO t1 (col_text) VALUES (REPEAT('z', @fill_amount));
+
+--connection default
+# If the bug is not fixed, CHECK TABLE will complain about wrong secondary index
+# rows count
+CHECK TABLE t1;
+--disconnect con1
+--disconnect prevent_purge
+DROP TABLE t1;
+--source include/wait_until_count_sessions.inc

storage/innobase/row/row0sel.cc

@@ -352,11 +352,16 @@ row_sel_sec_rec_is_for_clust_rec(
 			}
 
 			len = clust_len;
+			ulint prefix_len = ifield->prefix_len;
 			if (rec_offs_nth_extern(clust_offs, clust_pos)) {
+				/* BLOB can contain prefix. */
 				len -= BTR_EXTERN_FIELD_REF_SIZE;
+				if (!len) {
+					goto compare_blobs;
+				}
 			}
 
-			if (ulint prefix_len = ifield->prefix_len) {
+			if (prefix_len) {
 				len = dtype_get_at_most_n_mbchars(
 					col->prtype, col->mbminlen,
 					col->mbmaxlen, prefix_len, len,
@@ -369,6 +374,7 @@ row_sel_sec_rec_is_for_clust_rec(
 check_for_blob:
 				if (rec_offs_nth_extern(clust_offs,
 							clust_pos)) {
+compare_blobs:
 					if (!row_sel_sec_rec_is_for_blob(
 						    col->mtype, col->prtype,
 						    col->mbminlen,