Cluster: slaves start failover with a small delay.

Redis Cluster can cope with a minority of nodes not informed about the
failure of a master in time for some reason (netsplit or node not
functioning properly, blocked, ...) however to wait a few seconds before
to start the failover will make most "normal" failovers simpler as the
FAIL message will propagate before the slave election happens.

src/cluster.c

@@ -1565,10 +1565,16 @@ void clusterCron(void) {
     }
 
     /* If we are a slave and our master is down, but is serving slots,
-     * call the function that handles the failover. */
+     * call the function that handles the failover.
+     * This function is called with a small delay in order to let the
+     * FAIL message to propagate after failure detection, this is not
+     * strictly required but makes 99.99% of failovers mechanically
+     * simpler. */
     if (server.cluster->myself->flags & REDIS_NODE_SLAVE &&
         server.cluster->myself->slaveof &&
         server.cluster->myself->slaveof->flags & REDIS_NODE_FAIL &&
+        (server.unixtime - server.cluster->myself->slaveof->fail_time) >
+         REDIS_CLUSTER_FAILOVER_DELAY &&
         server.cluster->myself->slaveof->numslots != 0)
     {
         clusterHandleSlaveFailover();

src/redis.h

@@ -521,6 +521,7 @@ typedef struct redisOpArray {
 #define REDIS_CLUSTER_FAIL 1        /* The cluster can't work */
 #define REDIS_CLUSTER_NAMELEN 40    /* sha1 hex length */
 #define REDIS_CLUSTER_PORT_INCR 10000 /* Cluster port = baseport + PORT_INCR */
+#define REDIS_CLUSTER_FAILOVER_DELAY 5 /* Seconds */
 
 struct clusterNode;