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From "Plüm, Rüdiger, VF-Group" <ruediger.pl...@vodafone.com>
Subject Behaviour of mod_proxy_ajp if CPING/CPONG fails
Date Fri, 05 Sep 2008 14:21:49 GMT
IMHO the current behaviour of mod_proxy_ajp is not correct if a CPING/CPONG fails
on a backend connection. In this case the status is set to HTTP_SERVICE_UNAVAILABLE
and the scheme handler returns. In the case of an unbalanced backend this results
in a HTTP_SERVICE_UNAVAILABLE (503) to be returned to the client. But a failing
CPING/CPONG can be caused by a faulty AJP connection that was closed by the backend
or a race condition in sending the CPING and closing the connection on the backend.
So after the first failed CPING/CPONG we should try again with a new TCP connection
and should only return HTTP_SERVICE_UNAVAILABLE if this fails as well. The following
(and attached patch) does exactly that. Thoughs to the above and/or to the patch
before I commit?


Index: modules/proxy/mod_proxy_ajp.c
===================================================================
--- modules/proxy/mod_proxy_ajp.c       (revision 692409)
+++ modules/proxy/mod_proxy_ajp.c       (working copy)
@@ -554,6 +554,7 @@
     conn_rec *origin = NULL;
     proxy_conn_rec *backend = NULL;
     const char *scheme = "AJP";
+    int retry;
     proxy_dir_conf *dconf = ap_get_module_config(r->per_dir_config,
                                                  &proxy_module);

@@ -597,43 +598,53 @@
     backend->is_ssl = 0;
     backend->close = 0;

-    /* Step One: Determine Who To Connect To */
-    status = ap_proxy_determine_connection(p, r, conf, worker, backend,
-                                           uri, &url, proxyname, proxyport,
-                                           server_portstr,
-                                           sizeof(server_portstr));
+    retry = 0;
+    while (retry < 2) {
+        /* Step One: Determine Who To Connect To */
+        status = ap_proxy_determine_connection(p, r, conf, worker, backend,
+                                               uri, &url, proxyname, proxyport,
+                                               server_portstr,
+                                               sizeof(server_portstr));

-    if (status != OK)
-        goto cleanup;
+        if (status != OK)
+            break;

-    /* Step Two: Make the Connection */
-    if (ap_proxy_connect_backend(scheme, backend, worker, r->server)) {
-        ap_log_error(APLOG_MARK, APLOG_ERR, 0, r->server,
-                     "proxy: AJP: failed to make connection to backend: %s",
-                     backend->hostname);
-        status = HTTP_SERVICE_UNAVAILABLE;
-        goto cleanup;
-    }
-
-    /* Handle CPING/CPONG */
-    if (worker->ping_timeout_set) {
-        status = ajp_handle_cping_cpong(backend->sock, r,
-                                        worker->ping_timeout);
-        if (status != APR_SUCCESS) {
-            backend->close++;
-            ap_log_error(APLOG_MARK, APLOG_ERR, status, r->server,
-                         "proxy: AJP: cping/cpong failed to %pI (%s)",
-                         worker->cp->addr,
-                         worker->hostname);
+        /* Step Two: Make the Connection */
+        if (ap_proxy_connect_backend(scheme, backend, worker, r->server)) {
+            ap_log_error(APLOG_MARK, APLOG_ERR, 0, r->server,
+                         "proxy: AJP: failed to make connection to backend: %s",
+                         backend->hostname);
             status = HTTP_SERVICE_UNAVAILABLE;
-            goto cleanup;
+            break;
         }
+
+        /* Handle CPING/CPONG */
+        if (worker->ping_timeout_set) {
+            status = ajp_handle_cping_cpong(backend->sock, r,
+                                            worker->ping_timeout);
+            /*
+             * In case the CPING / CPONG failed for the first time we might be
+             * just out of luck and got a faulty backend connection, but the
+             * backend might be healthy nevertheless. So ensure that the backend
+             * TCP connection gets closed and try it once again.
+             */
+            if (status != APR_SUCCESS) {
+                backend->close++;
+                ap_log_error(APLOG_MARK, APLOG_ERR, status, r->server,
+                             "proxy: AJP: cping/cpong failed to %pI (%s)",
+                             worker->cp->addr,
+                             worker->hostname);
+                status = HTTP_SERVICE_UNAVAILABLE;
+                retry++;
+                continue;
+            }
+        }
+        /* Step Three: Process the Request */
+        status = ap_proxy_ajp_request(p, r, backend, origin, dconf, uri, url,
+                                      server_portstr);
+        break;
     }
-    /* Step Three: Process the Request */
-    status = ap_proxy_ajp_request(p, r, backend, origin, dconf, uri, url,
-                                  server_portstr);

-cleanup:
     /* Do not close the socket */
     ap_proxy_release_connection(scheme, backend, r->server);
     return status;

Regards

Rüdiger

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