BUG#53893: RBR: nullable unique key can lead to out-of-sync slave

When using Unique Keys with nullable parts in RBR, the slave can
choose the wrong row to update. This happens because a table with
an unique key containing nullable parts cannot strictly guarantee 
uniqueness. As stated in the manual, for all engines, a UNIQUE 
index allows multiple NULL values for columns that can contain 
NULL.

We fix this at the slave by extending the checks before assuming
that the row found through an unique index is is the correct
one. This means that when a record (R) is fetched from the storage
engine and a key that is not primary (K) is used, the server does 
the following: 

 - If K is unique and has no nullable parts, it returns R;
 - Otherwise, if any field in the before image that is part of K
   is null do an index scan;
 - If there is no NULL field in the BI part of K, then return R.

A side change: renamed the existing test case file and added a
test case covering the changes in this patch.

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

@@ -24,3 +24,15 @@ INSERT INTO t VALUES (1,2,4);
 INSERT INTO t VALUES (4,3,4);
 DELETE FROM t;
 DROP TABLE t;
+stop slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+reset master;
+reset slave;
+drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
+start slave;
+CREATE TABLE t1 (c1 INT NOT NULL, c2 INT NOT NULL, c3 INT, UNIQUE KEY(c1,c3), KEY(c2));
+INSERT INTO t1(c1,c2) VALUES(1,1);
+INSERT INTO t1(c1,c2) VALUES(1,2);
+UPDATE t1 SET c1=1000 WHERE c2=2;
+Comparing tables master:test.t1 and slave:test.t1
+DROP TABLE t1;

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

@@ -71,3 +71,34 @@ DELETE FROM t;
 DROP TABLE t;
 
 -- sync_slave_with_master
+
+#
+# BUG#53893: RBR: nullable unique key can lead to out-of-sync slave
+#
+
+#
+# We insert two rows. Both with part of UNIQUE KEY set to null.
+# Then we update the last row inserted. On master the correct
+# row is updated. On the slave the wrong row would be updated
+# because the engine would look it up by the NULL Unique KEY.
+# As a consquence, the wrong row would be updated.
+#
+
+-- connection master
+-- source include/master-slave-reset.inc
+-- connection master
+
+CREATE TABLE t1 (c1 INT NOT NULL, c2 INT NOT NULL, c3 INT, UNIQUE KEY(c1,c3), KEY(c2));
+INSERT INTO t1(c1,c2) VALUES(1,1);
+INSERT INTO t1(c1,c2) VALUES(1,2);
+UPDATE t1 SET c1=1000 WHERE c2=2;
+-- sync_slave_with_master
+
+-- let $diff_table_1=master:test.t1
+-- let $diff_table_2=slave:test.t1
+-- source include/diff_tables.inc
+
+-- connection master
+DROP TABLE t1;
+-- sync_slave_with_master
+

sql/log_event.cc

@@ -9015,8 +9015,38 @@ int Rows_log_event::find_row(const Relay_log_info *rli)
     */
     if (table->key_info->flags & HA_NOSAME)
     {
-      table->file->ha_index_end();
-      goto ok;
+      /* Unique does not have non nullable part */
+      if (!(table->key_info->flags & (HA_NULL_PART_KEY)))
+      {
+        table->file->ha_index_end();
+        goto ok;
+      }
+      else
+      {
+        KEY *keyinfo= table->key_info;
+        /*
+          Unique has nullable part. We need to check if there is any field in the
+          BI image that is null and part of UNNI.
+        */
+        bool null_found= FALSE;
+
+        for (uint i=0, fieldnr= keyinfo->key_part[i].fieldnr - 1 ;
+             (i < keyinfo->key_parts) && !null_found ;
+             i++, fieldnr= keyinfo->key_part[i].fieldnr - 1)
+        {
+          Field **f= table->field+fieldnr;
+          if ((*f)->is_null())
+            null_found= TRUE;
+        }
+
+        if (!null_found)
+        {
+          table->file->ha_index_end();
+          goto ok;
+        }
+
+        /* else fall through to index scan */
+      }
     }
 
     /*

sql/log_event_old.cc

@@ -2428,8 +2428,38 @@ int Old_rows_log_event::find_row(const Relay_log_info *rli)
     */
     if (table->key_info->flags & HA_NOSAME)
     {
-      table->file->ha_index_end();
-      DBUG_RETURN(0);
+      /* Unique does not have non nullable part */
+      if (!(table->key_info->flags & (HA_NULL_PART_KEY)))
+      {
+        table->file->ha_index_end();
+        DBUG_RETURN(0);
+      }
+      else
+      {
+        KEY *keyinfo= table->key_info;
+        /*
+          Unique has nullable part. We need to check if there is any field in the
+          BI image that is null and part of UNNI.
+        */
+        bool null_found= FALSE;
+
+        for (uint i=0, fieldnr= keyinfo->key_part[i].fieldnr - 1 ;
+             (i < keyinfo->key_parts) && !null_found ;
+             i++, fieldnr= keyinfo->key_part[i].fieldnr - 1)
+        {
+          Field **f= table->field+fieldnr;
+          if ((*f)->is_null())
+            null_found= TRUE;
+        }
+
+        if (!null_found)
+        {
+          table->file->ha_index_end();
+          DBUG_RETURN(0);
+        }
+
+        /* else fall through to index scan */
+      }
     }
 
     /*