commit 5fd8847bb194db6e87a7c10df349a12e66d17855
Author: Michael Osipov <michaelo@apache.org>
Date:   2019-08-05 12:39:28 +0200

    BZ 63626: mod_proxy_http uses incorrect status codes for timed out upstream requests
    
    If a status is known to time out at some point, use HTTP_GATEWAY_TIME_OUT
    rather than HTTP_BAD_GATEWAY because we know that the connection has been
    established and the upstream server has received our request.

diff --git a/modules/proxy/mod_proxy_http.c b/modules/proxy/mod_proxy_http.c
index c5e1777623..2aadd7c489 100644
--- a/modules/proxy/mod_proxy_http.c
+++ b/modules/proxy/mod_proxy_http.c
@@ -1307,7 +1307,9 @@ int ap_proxy_http_process_response(proxy_http_req_t *req)
                                    backend->hostname, backend->port);
             }
             proxy_run_detach_backend(r, backend);
-            return ap_proxyerror(r, HTTP_GATEWAY_TIME_OUT,
+            return ap_proxyerror(r, APR_STATUS_IS_TIMEUP(rc)
+                                        ? HTTP_GATEWAY_TIME_OUT
+                                        : HTTP_BAD_GATEWAY,
                                  "Error reading from remote server");
         }
         /* XXX: Is this a real headers length send from remote? */