BUG#14005409 - 64624

Problem: After the fix for Bug#12589870, a new field that
stores the length of db name was added in the buffer that
stores the query to be executed. Unlike for the plain user
session, the replication execution did not allocate the
necessary chunk in Query-event constructor. This caused an
invalid read while accessing this field.
      
Solution: We fix this problem by allocating a necessary chunk
in the buffer created in the Query_log_event::Query_log_event()
and store the length of database name.

sql/log_event.cc

@@ -2835,24 +2835,34 @@ Query_log_event::Query_log_event(const char* buf, uint event_len,
       pos= (const uchar*) end;                         // Break loop
     }
   }
-  
+
+  /**
+    Layout for the data buffer is as follows
+    +--------+-----------+------+------+---------+----+-------+
+    | catlog | time_zone | user | host | db name | \0 | Query |
+    +--------+-----------+------+------+---------+----+-------+
+
+    To support the query cache we append the following buffer to the above
+    +-------+----------------------------------------+-------+
+    |db len | uninitiatlized space of size of db len | FLAGS |
+    +-------+----------------------------------------+-------+
+
+    The area of buffer starting from Query field all the way to the end belongs
+    to the Query buffer and its structure is described in alloc_query() in
+    sql_parse.cc
+    */
+
+  if (!(start= data_buf = (Log_event::Byte*) my_malloc(catalog_len + 1
+                                                    +  time_zone_len + 1
+                                                    +  user.length + 1
+                                                    +  host.length + 1
+                                                    +  data_len + 1
 #if !defined(MYSQL_CLIENT) && defined(HAVE_QUERY_CACHE)
-  if (!(start= data_buf = (Log_event::Byte*) my_malloc(catalog_len + 1 +
-                                              time_zone_len + 1 +
-                                              data_len + 1 +
-                                              QUERY_CACHE_FLAGS_SIZE +
-                                              user.length + 1 +
-                                              host.length + 1 +
-                                              db_len + 1,
-                                              MYF(MY_WME))))
-#else
-  if (!(start= data_buf = (Log_event::Byte*) my_malloc(catalog_len + 1 +
-                                             time_zone_len + 1 +
-                                             data_len + 1 +
-                                             user.length + 1 +
-                                             host.length + 1,
-                                             MYF(MY_WME))))
+                                                    + sizeof(size_t)//for db_len
+                                                    + db_len + 1
+                                                    + QUERY_CACHE_FLAGS_SIZE
 #endif
+                                                    , MYF(MY_WME))))
       DBUG_VOID_RETURN;
   if (catalog_len)                                  // If catalog is given
   {
@@ -2891,6 +2901,14 @@ Query_log_event::Query_log_event(const char* buf, uint event_len,
   db= (char *)start;
   query= (char *)(start + db_len + 1);
   q_len= data_len - db_len -1;
+  /**
+    Append the db length at the end of the buffer. This will be used by
+    Query_cache::send_result_to_client() in case the query cache is On.
+   */
+#if !defined(MYSQL_CLIENT) && defined(HAVE_QUERY_CACHE)
+  size_t db_length= (size_t)db_len;
+  memcpy(start + data_len + 1, &db_length, sizeof(size_t));
+#endif
   DBUG_VOID_RETURN;
 }