votequorum: add auto_tie_breaker support (default: off)

this flag (0|1) can be configured via quorum.auto_tie_breaker and when
enabled, support for perfect even split is on.

In case of a 50% of votes loss in one single transition, the partition
with the node that has the lowest node id will remain quorate.

Reviewed-by: Steven Dake <sdake@redhat.com>
Signed-off-by: Fabio M. Di Nitto <fdinitto@redhat.com>

exec/coroparse.c

@@ -369,6 +369,7 @@ static int main_config_parser_cb(const char *path,
 			if ((strcmp(path, "quorum.disallowed") == 0) ||
 			    (strcmp(path, "quorum.two_node") == 0) ||
 			    (strcmp(path, "quorum.wait_for_all") == 0) ||
+			    (strcmp(path, "quorum.auto_tie_breaker") == 0) ||
 			    (strcmp(path, "quorum.quorate") == 0)) {
 				i = atoi(value);
 				icmap_set_uint8(path, i);

services/votequorum.c

@@ -125,6 +125,8 @@ static int first_trans = 1;
 static unsigned int quorumdev_poll = DEFAULT_QDEV_POLL;
 static unsigned int leaving_timeout = DEFAULT_LEAVE_TMO;
 static uint8_t wait_for_all = 0;
+static uint8_t auto_tie_breaker = 0;
+static int lowest_node_id = -1;
 
 static struct cluster_node *us;
 static struct cluster_node *quorum_device = NULL;
@@ -387,6 +389,16 @@ static void votequorum_init(struct corosync_api_v1 *api,
 	set_quorum = report;
 
 	icmap_get_uint8("quorum.wait_for_all", &wait_for_all);
+	icmap_get_uint8("quorum.auto_tie_breaker", &auto_tie_breaker);
+
+	/*
+	 * TODO: we need to know the lowest node-id in the cluster
+	 * current lack of node list with node-id's requires us to see all nodes
+	 * to determine which is the lowest.
+	 */
+	if (auto_tie_breaker) {
+		wait_for_all = 1;
+	}
 
 	/* Load the library-servicing part of this module */
 	api->service_link_and_init(api, "corosync_votequorum_iface", 0);
@@ -606,6 +618,39 @@ static void send_expectedvotes_notification(void)
 	}
 }
 
+static void get_lowest_node_id(void)
+{
+	struct cluster_node *node = NULL;
+	struct list_head *tmp;
+
+	lowest_node_id = us->node_id;
+
+	list_iterate(tmp, &cluster_members_list) {
+		node = list_entry(tmp, struct cluster_node, list);
+		if (node->node_id < lowest_node_id)
+			lowest_node_id = node->node_id;
+	}
+	log_printf(LOGSYS_LEVEL_DEBUG, "lowest node id: %d us: %d\n", lowest_node_id, us->node_id);
+}
+
+static int check_low_node_id_partition(void)
+{
+	struct cluster_node *node = NULL;
+	struct list_head *tmp;
+	int found = 0;
+
+	list_iterate(tmp, &cluster_members_list) {
+		node = list_entry(tmp, struct cluster_node, list);
+		if (node->state == NODESTATE_MEMBER) {
+			if (node->node_id == lowest_node_id) {
+				found = 1;
+			}
+		}
+	}
+
+	return found;
+}
+
 static void set_quorate(int total_votes)
 {
 	int quorate;
@@ -626,6 +671,7 @@ static void set_quorate(int total_votes)
 			return;
 		}
 		wait_for_all = 0;
+		get_lowest_node_id();
 	}
 
 	if (quorum > total_votes) {
@@ -635,6 +681,12 @@ static void set_quorate(int total_votes)
 		quorate = 1;
 	}
 
+	if ((auto_tie_breaker) &&
+	    (total_votes == (us->expected_votes / 2)) &&
+	    (check_low_node_id_partition() == 1)) {
+		quorate = 1;
+	}
+
 	if (cluster_is_quorate && !quorate)
 		log_printf(LOGSYS_LEVEL_INFO, "quorum lost, blocking activity\n");
 	if (!cluster_is_quorate && quorate)