Format
In ngx_log_error_core, the cached time, log level surrounded by [], pid#tid: and *$connection (connection serial number, if exists) are “printed” first. Then, the error message from the arguments is formatted and printed. The resulting string is then passed to log->handler to get the final message to log.
In ngx_http_request.c, c->log->handler is set to ngx_http_log_error, which appends while ... if there is a log->action, , client: ... (the connection’s client address), and passes the error message to r->log_handler.
r->log_handler is ngx_http_log_error_handler, also defined in ngx_http_request.c. It appends , server: $server_name, the request line, sub-request if exists, upstream information if exists, Host request header if exists, and Referrer request header if exists.
Therefore, the final format for an HTTP request is
1970/09/28 12:00:00 [$err_level] $pid#$tid: *$connection_serial_number $error___message while $action, client: $connection_addr, server: $server_name, request: "$request", subrequest: "$sr->uri", upstream: "schema://peer_name$upstream_uri", host: "$http_host", referrer: "$http_referer"