MXS-406: slave nodes are sorted before setting 'wsrep_sst_donor'

The slave nodes par od the cluster are sorted by wsrep_local_index or
server priority.

The sorted list od nodes is then used in ‘wsrep_sst_donor’ variable.

Note: candidate master servers are always at the end of the list

server/modules/monitor/galeramon/galeramon.c

@@ -55,6 +55,7 @@ static MONITOR_SERVERS *set_cluster_master(MONITOR_SERVERS *, MONITOR_SERVERS *,
 static void disableMasterFailback(void *, int);
 bool isGaleraEvent(monitor_event_t event);
 static void update_sst_donor_nodes(MONITOR*, int);
+static int compare_cluster_node_index (const void*, const void*, void *);
 
 /**
  * The module entry point routine. It is this routine that
@@ -614,7 +615,9 @@ static MONITOR_SERVERS *get_candidate_master(MONITOR* mon)
          * This means that we can't connect to the root node of the cluster.
          *
          * If the node is down, the cluster would recalculate the index values
-         * and we would find it. In this case, we just can't connect to it.  */
+         * and we would find it. In this case, we just can't connect to it.
+         */
+
         candidate_master = NULL;
     }
 
@@ -635,7 +638,8 @@ static MONITOR_SERVERS *get_candidate_master(MONITOR* mon)
  * @param   candidate_master The candidate master server accordingly to the selection rule
  * @return  The  master node pointer (could be NULL)
  */
-static MONITOR_SERVERS *set_cluster_master(MONITOR_SERVERS *current_master, MONITOR_SERVERS *candidate_master,
+static MONITOR_SERVERS *set_cluster_master(MONITOR_SERVERS *current_master,
+                                           MONITOR_SERVERS *candidate_master,
                                            int master_stickiness)
 {
     /*
@@ -687,12 +691,17 @@ static void update_sst_donor_nodes(MONITOR *mon, int is_cluster)
     MONITOR_SERVERS *ptr;
     MYSQL_ROW row;
     MYSQL_RES *result;
+    GALERA_MONITOR *handle = mon->handle;
+    bool ignore_priority = true;
+
     if (is_cluster == 1)
     {
         MXS_DEBUG("Only one server in the cluster: update_sst_donor_nodes is not performed");
         return;
     }
 
+    unsigned int found_slaves = 0;
+    MONITOR_SERVERS *node_list[is_cluster-1];
     /* Donor list size = DONOR_LIST_SET_VAR + n_hosts * max_host_len + n_hosts + 1 */
 
     char *donor_list = MXS_CALLOC(1, strlen(DONOR_LIST_SET_VAR) +
@@ -709,10 +718,43 @@ static void update_sst_donor_nodes(MONITOR *mon, int is_cluster)
 
     ptr = mon->databases;
 
+    /* Create an array of slave nodes */
     while (ptr)
     {
         if (SERVER_IS_JOINED(ptr->server) && SERVER_IS_SLAVE(ptr->server))
         {
+            node_list[found_slaves] = (MONITOR_SERVERS *)ptr;
+            found_slaves++;
+
+            /* Check the server parameter "priority"
+             * If no server has "priority" set, then
+             * the server list will be order by default method.
+             */
+            if (handle->use_priority &&
+                server_get_parameter(ptr->server, "priority"))
+            {
+                ignore_priority = false;
+            }
+        }
+        ptr = ptr->next;
+    }
+
+    if (ignore_priority)
+    {
+        MXS_DEBUG("Use priority is set but no server has priority parameter. "
+                    "Donor server list will be ordered by 'wsrep_local_index'");
+    }
+
+    /* Set order type */
+    int sort_order = (!ignore_priority) && (int)handle->use_priority;
+
+    /* Sort the array */
+    qsort_r(node_list, found_slaves, sizeof(MONITOR_SERVERS *), compare_cluster_node_index, (void *)&sort_order);
+
+    /* Select nodename from each server and append it to node_list */
+    for (int k=0; k < found_slaves; k++)
+    {
+        MONITOR_SERVERS *ptr = node_list[k];
 
         /* Get the Galera node name */
         if (mysql_query(ptr->con, "SHOW VARIABLES LIKE 'wsrep_node_name'") == 0
@@ -746,9 +788,6 @@ static void update_sst_donor_nodes(MONITOR *mon, int is_cluster)
         }
     }
 
-        ptr = ptr->next;
-    }
-
     int donor_list_size = strlen(donor_list);
     if (donor_list[donor_list_size - 1] == ',')
     {
@@ -761,12 +800,9 @@ static void update_sst_donor_nodes(MONITOR *mon, int is_cluster)
                donor_list);
 
     /* Set now rep_sst_donor in each slave node */
-    ptr = mon->databases;
+    for (int k=0; k < found_slaves; k++)
-    while (ptr)
     {
-        if (SERVER_IS_JOINED(ptr->server) && SERVER_IS_SLAVE(ptr->server))
+        MONITOR_SERVERS *ptr = node_list[k];
-        {
-
         /* Set the Galera SST donor node list */
         if (mysql_query(ptr->con, donor_list) == 0)
         {
@@ -781,8 +817,92 @@ static void update_sst_donor_nodes(MONITOR *mon, int is_cluster)
         }
     }
 
-        ptr = ptr->next;
-    }
-
     MXS_FREE(donor_list);
 }
+
+/**
+ * Compare routine for slave nodes sorting
+ *
+ * The default order is DESC.
+ *
+ * Some special cases, i.e: no give priority, or 0 value
+ * are handled.
+ *
+ * Note: the master selection algorithm is:
+ * 1) node with lowest  wsrep_local_index OR
+ * 2) lowest priority value and > 0
+ *
+ * the sorting order DESC, will add master candidates
+ * at the end of the list.
+ *
+ * @param   a        Pointer to array value
+ * @param   b        Pointer to array value
+ * @param   order    Ordering by wsrep_local_index or node priority
+ * @return  A number less than, threater than or equal to 0
+ */
+
+static int compare_cluster_node_index (const void *a, const void *b, void *order)
+{
+
+  const MONITOR_SERVERS *s_a = *(MONITOR_SERVERS * const *)a;
+  const MONITOR_SERVERS *s_b = *(MONITOR_SERVERS * const *)b;
+
+  int list_order = *(int *)order;
+
+  if (list_order)
+  {
+      const char *pri_a = server_get_parameter(s_a->server, "priority");
+      const char *pri_b = server_get_parameter(s_b->server, "priority");
+
+      /**
+       * Check priority parameter:
+       *
+       * Return a - b in case of issues
+       */
+      if (!pri_a && pri_b)
+      {
+          MXS_DEBUG("Server %s has no given priority. It will be at the beginning of the list",
+                    s_a->server->unique_name);
+          return -(INT_MAX - 1);
+      }
+      else if (pri_a && !pri_b)
+      {
+          MXS_DEBUG("Server %s has no given priority. It will be at the beginning of the list",
+                    s_b->server->unique_name);
+          return INT_MAX - 1;
+      }
+      else if (!pri_a && !pri_b)
+      {
+          MXS_DEBUG("Servers %s and %s have no given priority. They be at the beginning of the list",
+                    s_a->server->unique_name,
+                    s_b->server->unique_name);
+          return 0;
+      }
+
+      /* The given  priority is valid */
+      int pri_val_a = atoi(pri_a);
+      int pri_val_b = atoi(pri_b);
+
+      /* Return a - b in case of issues */
+      if ((pri_val_a < INT_MAX && pri_val_a > 0) && !(pri_val_b < INT_MAX && pri_val_b > 0))
+      {
+          return pri_val_a;
+      }
+      else if (!(pri_val_a < INT_MAX && pri_val_a > 0) && (pri_val_b < INT_MAX && pri_val_b > 0))
+      {
+          return -pri_val_b;
+      }
+      else if (!(pri_val_a < INT_MAX && pri_val_a > 0) && !(pri_val_b < INT_MAX && pri_val_b > 0))
+      {
+          return 0;
+      }
+
+      // The order is DESC: b -a
+      return pri_val_b - pri_val_a;
+  }
+  else
+  {
+      // Order is DESC: b - a
+      return s_b->server->node_id - s_a->server->node_id;
+  }
+}