MDEV-26778 row_start is not updated in current row for InnoDB

Update was skipped (need_update was false) because compare_record()
used HA_PARTIAL_COLUMN_READ branch and it skipped row_start check
has_explicit_value() was false. When we set bit for row_start in
has_value_set the row is updated with new row_start value.

The bug was caused by combination of MDEV-23446 and 3789692d. The
latter one says:

  ... But generated columns that are written to the table are always
  deterministic and cannot change unless normal non-generated columns
  were changed. ...

Since MDEV-23446 generated row_start can change while non-generated
columns are not changed.

Explicit value flag came from HAS_EXPLICIT_DEFAULT which was used to
distinguish default-generated value from user-supplied one.

mysql-test/suite/versioning/r/update.result

@@ -399,3 +399,14 @@ a	check_row(row_start, row_end)
 1	HISTORICAL ROW
 1	CURRENT ROW
 drop tables t1, t2, t3;
+#
+# MDEV-26778 row_start is not updated in current row for InnoDB
+#
+create or replace table t1 (x int) with system versioning;
+insert t1 values (1);
+update t1 set x= 1;
+select row_start from t1 into @r;
+select check_row_ts(row_start, row_end) from t1 for system_time all where row_start = @r;
+check_row_ts(row_start, row_end)
+CURRENT ROW
+drop table t1;

mysql-test/suite/versioning/t/update.test

@@ -326,4 +326,14 @@ select *, check_row(row_start, row_end) from t2 for system_time all order by row
 # cleanup
 drop tables t1, t2, t3;
 
+--echo #
+--echo # MDEV-26778 row_start is not updated in current row for InnoDB
+--echo #
+create or replace table t1 (x int) with system versioning;
+insert t1 values (1);
+update t1 set x= 1;
+select row_start from t1 into @r;
+select check_row_ts(row_start, row_end) from t1 for system_time all where row_start = @r;
+drop table t1;
+
 source suite/versioning/common_finish.inc;

sql/table.cc

@@ -8147,14 +8147,18 @@ void TABLE::vers_update_fields()
     return;
   }
 
-  if (versioned(VERS_TIMESTAMP) &&
-      vers_start_field()->store_timestamp(in_use->query_start(),
+  if (versioned(VERS_TIMESTAMP))
+  {
+    if (vers_start_field()->store_timestamp(in_use->query_start(),
                                           in_use->query_start_sec_part()))
     {
       DBUG_ASSERT(0);
     }
+    vers_start_field()->set_has_explicit_value();
+  }
 
   vers_end_field()->set_max();
+  vers_end_field()->set_has_explicit_value();
   bitmap_set_bit(read_set, vers_end_field()->field_index);
   file->column_bitmaps_signal();
   if (vfield)
@@ -8167,6 +8171,7 @@ void TABLE::vers_update_end()
   if (vers_end_field()->store_timestamp(in_use->query_start(),
                                         in_use->query_start_sec_part()))
     DBUG_ASSERT(0);
+  vers_end_field()->set_has_explicit_value();
 }
 
 /**

sql/table.h

@@ -1665,7 +1665,9 @@ struct TABLE
   bool vers_check_update(List<Item> &items);
 
   int delete_row();
+  /* Used in majority of DML (called from fill_record()) */
   void vers_update_fields();
+  /* Used in DELETE, DUP REPLACE and insert history row */
   void vers_update_end();
 
 /** Number of additional fields used in versioned tables */