Bug#22351 - handler::index_next_same() call to key_cmp_if_same() uses

            the wrong buffer

handler::index_next_same() did not take into account that the
internally called function key_cmp_if_same() uses the fixed
buffer table->record[0] for key comparison instead of the
buffer provided by the caller of handler::index_next_same().

Added code to temporarily redirect table->record[0] and the fields
used for the key to the record buffer provided by the caller of
handler::index_next_same().

The test case is in partition.test already.

sql/handler.cc

@@ -2522,15 +2522,56 @@ int ha_enable_transaction(THD *thd, bool on)
 int handler::index_next_same(uchar *buf, const uchar *key, uint keylen)
 {
   int error;
+  DBUG_ENTER("index_next_same");
   if (!(error=index_next(buf)))
   {
+    my_ptrdiff_t ptrdiff= buf - table->record[0];
+    uchar *save_record_0;
+    KEY *key_info;
+    KEY_PART_INFO *key_part;
+    KEY_PART_INFO *key_part_end;
+    LINT_INIT(save_record_0);
+    LINT_INIT(key_info);
+    LINT_INIT(key_part);
+    LINT_INIT(key_part_end);
+
+    /*
+      key_cmp_if_same() compares table->record[0] against 'key'.
+      In parts it uses table->record[0] directly, in parts it uses
+      field objects with their local pointers into table->record[0].
+      If 'buf' is distinct from table->record[0], we need to move
+      all record references. This is table->record[0] itself and
+      the field pointers of the fields used in this key.
+    */
+    if (ptrdiff)
+    {
+      save_record_0= table->record[0];
+      table->record[0]= buf;
+      key_info= table->key_info + active_index;
+      key_part= key_info->key_part;
+      key_part_end= key_part + key_info->key_parts;
+      for (; key_part < key_part_end; key_part++)
+      {
+        DBUG_ASSERT(key_part->field);
+        key_part->field->move_field_offset(ptrdiff);
+      }
+    }
+
     if (key_cmp_if_same(table, key, active_index, keylen))
     {
       table->status=STATUS_NOT_FOUND;
       error=HA_ERR_END_OF_FILE;
     }
+
+    /* Move back if necessary. */
+    if (ptrdiff)
+    {
+      table->record[0]= save_record_0;
+      for (key_part= key_info->key_part; key_part < key_part_end; key_part++)
+        key_part->field->move_field_offset(-ptrdiff);
+    }
   }
-  return error;
+  DBUG_RETURN(error);
 }