MDEV-33049 Assertion `marked_for_write_or_computed()' failed in bool

           Field_new_decimal::store_value(const my_decimal*, int*)

Analysis
========
When rpl applier is unpacking a before row image, Field::reset() will be
called before setting a field to null if null bit of the field is set in
the row image. For Field_new_decimal::reset(), it calls
Field_new_decimal::store_value() to reset the value. store_value() asserts
that the field is in the write_set bitmap since it thinks the field is
updating.

But that is not true for the row image generated in FULL_NODUP
mode. In the mode, the before image includes all fields and the after
image includes only updated fields.

Fix
===
In the case unpacking binlog row images, the assertion is meaningless.
So the unpacking field is marked in write_set temporarily to avoid the
assertion failure.

mysql-test/suite/rpl/r/rpl_row_img_eng_full_nodup.result

@@ -4752,6 +4752,21 @@ include/diff_tables.inc [server_1:test.t, server_2:test.t, server_3:test.t]
 connection server_1;
 DROP TABLE t;
 include/rpl_sync.inc
+CREATE TABLE t1(c1 INT NOT NULL PRIMARY KEY, c2 varchar(10) DEFAULT NULL,
+c3 decimal(12,4) DEFAULT NULL);
+INSERT INTO t1(c1) VALUES (1);
+include/save_master_pos.inc
+UPDATE t1 SET c2 = 'c2';
+include/rpl_sync.inc
+FLUSH BINARY LOGS;
+UPDATE t1 SET c2 = "c2_new";
+# Use 'BINLOG' statement to apply the first update row event
+# BINLOG statement is same to slave applier, it should work well.
+# After 'BINLOG' statement c2's value should be "c2"
+SELECT * FROM t1;
+c1	c2	c3
+1	c2	NULL
+DROP TABLE t1;
 CON: 'server_1', IMG: 'FULL', RESTART SLAVE: 'N'
 connection server_1;
 SET SESSION binlog_row_image= 'FULL';

mysql-test/suite/rpl/t/rpl_row_img_eng_full_nodup.test

@@ -27,8 +27,41 @@
 -- let $row_img_test_script= include/rpl_row_img.test
 -- source include/rpl_row_img_general_loop.inc
 
+################################################################################
+# MDEV-33049 Assertion `marked_for_write_or_computed()' failed in bool
+#            Field_new_decimal::store_value(const my_decimal*, int*)
+#
+# In FULL_NODUP mode, the before image has all fields and the after image has
+# only updated fields. Crash happened when slave was unpacking a row event
+# if the new decimal field is null in the before image.
+################################################################################
+CREATE TABLE t1(c1 INT NOT NULL PRIMARY KEY, c2 varchar(10) DEFAULT NULL,
+                c3 decimal(12,4) DEFAULT NULL);
+INSERT INTO t1(c1) VALUES (1);
+--source include/save_master_pos.inc
+--let $datadir= `SELECT @@datadir`
+
+# It will generate a row event that c3 is only in before image and it is null.
+UPDATE t1 SET c2 = 'c2';
+
+# the UPDATE will crash the slave without this fix.
+--source include/rpl_sync.inc
+
+FLUSH BINARY LOGS;
+UPDATE t1 SET c2 = "c2_new";
+
+--echo # Use 'BINLOG' statement to apply the first update row event
+--echo # BINLOG statement is same to slave applier, it should work well.
+--exec $MYSQL_BINLOG --start-position=$_master_pos $datadir$_master_file > $MYSQLTEST_VARDIR/tmp/binlog.sql
+--exec $MYSQL < $MYSQLTEST_VARDIR/tmp/binlog.sql
+
+--echo # After 'BINLOG' statement c2's value should be "c2"
+SELECT * FROM t1;
+
+--remove_file $MYSQLTEST_VARDIR/tmp/binlog.sql
+DROP TABLE t1;
+
 -- let $row_img_set=server_1:FULL:N,server_2:FULL:Y,server_3:FULL:Y
 -- source include/rpl_row_img_set.inc
 
-
 --source include/rpl_end.inc

sql/rpl_record.cc

@@ -201,7 +201,29 @@ static bool unpack_field(const table_def *tabledef, Field *f,
               the assertions triggered need to be
               addressed/revisited.
        */
+
+#ifndef DBUG_OFF
+      /*
+        f->reset() may call store_value() to reset the value, for example
+        Field_new_decimal. store_value() has below assertion:
+
+        DBUG_ASSERT(marked_for_write_or_computed());
+
+        It asserts write bitmap must be set. That caused an assertion
+        failure for row images generated by FULL_NODUP mode.
+        The assertion is meaningless for unpacking a row image, so
+        the field is marked in write_set temporarily to avoid the
+        assertion failure.
+      */
+      bool was_not_set = !bitmap_is_set(f->table->write_set, f->field_index);
+      if (was_not_set)
+        bitmap_set_bit(f->table->write_set, f->field_index);
+#endif
       f->reset();
+#ifndef DBUG_OFF
+      if (was_not_set)
+        bitmap_clear_bit(f->table->write_set, f->field_index);
+#endif
       f->set_null();
     }
     else