Bug #16494: Updates that set a column to NULL fail sometimes

 When building the UPDATE query to send to the remote server, the
 federated storage engine built the query incorrectly if it was updating
 a field to be NULL.

 Thanks to Bjšrn Steinbrink for an initial patch for the problem.

mysql-test/r/federated.result

@@ -1601,6 +1601,22 @@ fld_cid	fld_name	fld_parentid	fld_delt
 5	Torkel	0	0
 DROP TABLE federated.t1;
 DROP TABLE federated.bug_17377_table;
+create table t1 (id int not null auto_increment primary key, val int);
+create table t1
+(id int not null auto_increment primary key, val int) engine=federated
+connection='mysql://root@127.0.0.1:9308/test/t1';
+insert into t1 values (1,0),(2,0);
+update t1 set val = NULL where id = 1;
+select * from t1;
+id	val
+1	NULL
+2	0
+select * from t1;
+id	val
+1	NULL
+2	0
+drop table t1;
+drop table t1;
 DROP TABLE IF EXISTS federated.t1;
 DROP DATABASE IF EXISTS federated;
 DROP TABLE IF EXISTS federated.t1;

mysql-test/t/federated.test

@@ -1309,5 +1309,22 @@ DROP TABLE federated.t1;
 connection slave;
 DROP TABLE federated.bug_17377_table;
 
+#
+# Bug #16494: Updates that set a column to NULL fail sometimes
+#
+connection slave;
+create table t1 (id int not null auto_increment primary key, val int);
+connection master;
+eval create table t1
+  (id int not null auto_increment primary key, val int) engine=federated
+  connection='mysql://root@127.0.0.1:$SLAVE_MYPORT/test/t1';
+insert into t1 values (1,0),(2,0);
+update t1 set val = NULL where id = 1;
+select * from t1;
+connection slave;
+select * from t1;
+drop table t1;
+connection master;
+drop table t1;
 
 source include/federated_cleanup.inc;

sql/ha_federated.cc

@@ -1857,8 +1857,8 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
   In this loop, we want to match column names to values being inserted
   (while building INSERT statement).
 
-  Iterate through table->field (new data) and share->old_filed (old_data)
-  using the same index to created an SQL UPDATE statement, new data is
+  Iterate through table->field (new data) and share->old_field (old_data)
+  using the same index to create an SQL UPDATE statement. New data is
   used to create SET field=value and old data is used to create WHERE
   field=oldvalue
  */
@@ -1870,30 +1870,28 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
     update_string.append(FEDERATED_EQ);
 
     if ((*field)->is_null())
-      new_field_value.append(FEDERATED_NULL);
+      update_string.append(FEDERATED_NULL);
     else
     {
       /* otherwise = */
       (*field)->val_str(&new_field_value);
       (*field)->quote_data(&new_field_value);
-
-      if (!field_in_record_is_null(table, *field, (char*) old_data))
-        where_string.append(FEDERATED_EQ);
+      update_string.append(new_field_value);
+      new_field_value.length(0);
     }
 
     if (field_in_record_is_null(table, *field, (char*) old_data))
       where_string.append(FEDERATED_ISNULL);
     else
     {
+      where_string.append(FEDERATED_EQ);
       (*field)->val_str(&old_field_value,
                         (char*) (old_data + (*field)->offset()));
       (*field)->quote_data(&old_field_value);
       where_string.append(old_field_value);
+      old_field_value.length(0);
     }
 
-    update_string.append(new_field_value);
-    new_field_value.length(0);
-
     /*
       Only append conjunctions if we have another field in which
       to iterate
@@ -1903,7 +1901,6 @@ int ha_federated::update_row(const byte *old_data, byte *new_data)
       update_string.append(FEDERATED_COMMA);
       where_string.append(FEDERATED_AND);
     }
-    old_field_value.length(0);
   }
   update_string.append(FEDERATED_WHERE);
   update_string.append(where_string);