bugfix: stored column depends on virtual depends on updated

TABLE::mark_virtual_col() was polluting table->vcol_set
and that confused the following mark_virtual_col()

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

+create table t1 (a int, b int as (a+1), c int as (b+1) stored);
+insert t1 set a=1;
+select * from t1;
+a	b	c
+1	2	3
+update t1 set a=2;
+select * from t1;
+a	b	c
+2	3	4
+drop table t1;

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

+#
+# Test how UPDATE detects what columns need to be read (or generated) in a row
+#
+# stored column depends on virtual column depends on updated column.
+# this tests TABLE::mark_virtual_columns_for_write()
+#
+create table t1 (a int, b int as (a+1), c int as (b+1) stored);
+insert t1 set a=1;
+select * from t1;
+update t1 set a=2;
+select * from t1;
+drop table t1;

sql/table.cc

@@ -6537,7 +6537,7 @@ bool TABLE::mark_virtual_col(Field *field)
 bool TABLE::mark_virtual_columns_for_write(bool insert_fl)
 {
   Field **vfield_ptr, *tmp_vfield;
-  bool bitmap_updated= FALSE;
+  bool bitmap_updated= false;
 
   for (vfield_ptr= vfield; *vfield_ptr; vfield_ptr++)
   {
@@ -6546,25 +6546,30 @@ bool TABLE::mark_virtual_columns_for_write(bool insert_fl)
       bitmap_updated= mark_virtual_col(tmp_vfield);
     else if (tmp_vfield->vcol_info->stored_in_db)
     {
-      bool mark_fl= insert_fl;
-      if (!mark_fl)
+      if (insert_fl)
       {
-        MY_BITMAP *save_read_set;
+        bitmap_set_bit(write_set, tmp_vfield->field_index);
+        mark_virtual_col(tmp_vfield);
+        bitmap_updated= true;
+      }
+      else
+      {
+        MY_BITMAP *save_read_set= read_set, *save_vcol_set= vcol_set;
         Item *vcol_item= tmp_vfield->vcol_info->expr_item;
         DBUG_ASSERT(vcol_item);
         bitmap_clear_all(&tmp_set);
-        save_read_set= read_set;
-        read_set= &tmp_set;
+        read_set= vcol_set= &tmp_set;
         vcol_item->walk(&Item::register_field_in_read_map, 1, 0);
         read_set= save_read_set;
-        bitmap_intersect(&tmp_set, write_set);
-        mark_fl= !bitmap_is_clear_all(&tmp_set);
-      }
-      if (mark_fl)
+        vcol_set= save_vcol_set;
+        if (bitmap_is_overlapping(&tmp_set, write_set))
         {
           bitmap_set_bit(write_set, tmp_vfield->field_index);
-        mark_virtual_col(tmp_vfield);
-        bitmap_updated= TRUE;
+          bitmap_set_bit(vcol_set, tmp_vfield->field_index);
+          bitmap_union(read_set, &tmp_set);
+          bitmap_union(vcol_set, &tmp_set);
+          bitmap_updated= true;
+        }
       }
     }
   }