Sentinel: Sentinel-side support for slave priority.

The slave priority that is now published by Redis in INFO output is
now used by Sentinel in order to select the slave with minimum priority
for promotion, and in order to consider slaves with priority set to 0 as
not able to play the role of master (they will never be promoted by
Sentinel).

The "slave-priority" field is now one of the fileds that Sentinel
publishes when describing an instance via the SENTINEL commands such as
"SENTINEL slaves mastername".

src/sentinel.c

@@ -1410,6 +1410,10 @@ void sentinelRefreshInstanceInfo(sentinelRedisInstance *ri, const char *info) {
                     SENTINEL_MASTER_LINK_STATUS_UP :
                     SENTINEL_MASTER_LINK_STATUS_DOWN;
             }
+
+            /* slave_priority:<priority> */
+            if (sdslen(l) >= 15 && !memcmp(l,"slave_priority:",15))
+                ri->slave_priority = atoi(l+15);
         }
     }
     ri->info_refresh = mstime();
@@ -1882,6 +1886,10 @@ void addReplySentinelRedisInstance(redisClient *c, sentinelRedisInstance *ri) {
         addReplyBulkCString(c,"master-port");
         addReplyBulkLongLong(c,ri->slave_master_port);
         fields++;
+
+        addReplyBulkCString(c,"slave-priority");
+        addReplyBulkLongLong(c,ri->slave_priority);
+        fields++;
     }
 
     /* Only sentinels */
@@ -2439,6 +2447,7 @@ void sentinelStartFailoverIfNeeded(sentinelRedisInstance *master) {
  * 3) info_refresh more recent than SENTINEL_INFO_VALIDITY_TIME.
  * 4) master_link_down_time no more than:
  *     (now - master->s_down_since_time) + (master->down_after_period * 10).
+ * 5) Slave priority can't be zero, otherwise the slave is discareded.
  *
  * Among all the slaves matching the above conditions we select the slave
  * with lower slave_priority. If priority is the same we select the slave
@@ -2476,6 +2485,7 @@ sentinelRedisInstance *sentinelSelectSlave(sentinelRedisInstance *master) {
 
         if (slave->flags & (SRI_S_DOWN|SRI_O_DOWN|SRI_DISCONNECTED)) continue;
         if (slave->last_avail_time < info_validity_time) continue;
+        if (slave->slave_priority == 0) continue;
 
         /* If the master is in SDOWN state we get INFO for slaves every second.
          * Otherwise we get it with the usual period so we need to account for