Additionally, make sure to check for errors from ngx_http_parse_header_line() call after joining saved parts. There shouldn't be any errors, though check may help to catch bugs like missing f->split_parts reset. Reported by Lucas Molas.
If a request is finalized in the first call to the ngx_http_upstream_process_upgraded() function, e.g., because upstream server closed the connection for some reason, in the second call the u->peer.connection pointer will be null, resulting in segmentation fault. Fix is to avoid second direct call, and post event instead. This ensures that ngx_http_upstream_process_upgraded() won't be called again if a request is finalized.