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.

--- a/src/http/ngx_http_upstream.c
+++ b/src/http/ngx_http_upstream.c
@@ -4374,8 +4374,7 @@ ngx_http_upstream_finalize_request(ngx_h
 
     if (!u->header_sent
         || rc == NGX_HTTP_REQUEST_TIME_OUT
-        || rc == NGX_HTTP_CLIENT_CLOSED_REQUEST
-        || (u->pipe && u->pipe->downstream_error))
+        || rc == NGX_HTTP_CLIENT_CLOSED_REQUEST)
     {
         ngx_http_finalize_request(r, rc);
         return;
@@ -4388,7 +4387,9 @@ ngx_http_upstream_finalize_request(ngx_h
         flush = 1;
     }
 
-    if (r->header_only) {
+    if (r->header_only
+        || (u->pipe && u->pipe->downstream_error))
+    {
         ngx_http_finalize_request(r, rc);
         return;
     }