fail over: Change the first server pick logic

The logic of selecting the server to fail over to was changed so
that we start from the server next to the one that didn't work the last
time. This is because the status of a server that failed last time might
get reset before we try another one. This can cause that we try to use
the nonworking server repeatedly, not giving a chance to other servers.

Fixes: #321

1 files changed, 26 insertions, 5 deletions

diff --git a/server/providers/fail_over.c b/server/providers/fail_over.c
index 5c91bfc6..9795ccec 100644
--- a/server/providers/fail_over.c
+++ b/server/providers/fail_over.c
@@ -56,6 +56,7 @@ struct fo_service {
     struct fo_ctx *ctx;
     char *name;
     struct fo_server *active_server;
+    struct fo_server *last_tried_server;
     struct fo_server *server_list;
 };
 
@@ -391,21 +392,41 @@ get_first_server_entity(struct fo_service *service, struct fo_server **_server)
     server = service->active_server;
     if (server != NULL) {
         if (service_works(server)) {
-            *_server = server;
-            return EOK;
+            goto done;
         }
         service->active_server = NULL;
     }
 
-    /* Otherwise iterate through the server list. */
+    /*
+     * Otherwise iterate through the server list.
+     */
+
+    /* First, try servers after the last one we tried. */
+    if (service->last_tried_server != NULL) {
+        DLIST_FOR_EACH(server, service->last_tried_server->next) {
+            if (service_works(server)) {
+                goto done;
+            }
+        }
+    }
+
+    /* If none were found, try at the start. */
     DLIST_FOR_EACH(server, service->server_list) {
         if (service_works(server)) {
-            *_server = server;
-            return EOK;
+            goto done;
+        }
+        if (server == service->last_tried_server) {
+            break;
         }
     }
 
+    service->last_tried_server = NULL;
     return ENOENT;
+
+done:
+    service->last_tried_server = server;
+    *_server = server;
+    return EOK;
 }
 
 static int