Upstream: fixed "header already sent" alerts on backend errors. Following ad3f342f14ba046c (1.9.13), it is possible that a request where header was already sent will be finalized with NGX_HTTP_BAD_GATEWAY, triggering an attempt to return additional error response and the "header already sent" alert as a result. In particular, it is trivial to reproduce the problem with a HEAD request and caching enabled. With caching enabled nginx will change HEAD to GET and will set u->pipe->downstream_error to suppress sending the response body to the client. When a backend-related error occurs (for example, proxy_read_timeout expires), ngx_http_finalize_upstream_request() will be called with NGX_HTTP_BAD_GATEWAY. After ad3f342f14ba046c this will result in ngx_http_finalize_request(NGX_HTTP_BAD_GATEWAY). Fix is to move u->pipe->downstream_error handling to a later point, where all special response codes are changed to NGX_ERROR. Reported by Jan Prachar, http://mailman.nginx.org/pipermail/nginx-devel/2018-January/010737.html.