Cluster: FAILOVER_AUTH_REQUEST message type introduced.

This message is sent by a slave that is ready to failover its master to
other nodes to get the authorization from the majority of masters.

src/cluster.c

@@ -1248,6 +1248,23 @@ void clusterPropagatePublish(robj *channel, robj *message) {
  * SLAVE node specific functions
  * -------------------------------------------------------------------------- */
 
+/* This function sends a FAILOVE_AUTH_REQUEST message to every node in order to
+ * see if there is the quorum for this slave instance to failover its failing
+ * master.
+ *
+ * Note that we send the failover request to everybody, master and slave nodes,
+ * but only the masters are supposed to reply to our query. */
+void clusterRequestFailoverAuth(void) {
+    unsigned char buf[4096], *payload;
+    clusterMsg *hdr = (clusterMsg*) buf;
+    uint32_t totlen;
+
+    clusterBuildMessageHdr(hdr,CLUSTERMSG_TYPE_FAILOVER_AUTH_REQUEST);
+    totlen = sizeof(clusterMsg)-sizeof(union clusterMsgData);
+    hdr->totlen = htonl(totlen);
+    clusterBroadcastMessage(payload,totlen);
+}
+
 /* This function is called if we are a slave node and our master serving
  * a non-zero amount of hash slots is in PFAIL state.
  *

src/redis.h

@@ -594,6 +594,7 @@ typedef struct {
 #define CLUSTERMSG_TYPE_MEET 2          /* Meet "let's join" message */
 #define CLUSTERMSG_TYPE_FAIL 3          /* Mark node xxx as failing */
 #define CLUSTERMSG_TYPE_PUBLISH 4       /* Pub/Sub Publish propagation */
+#define CLUSTERMSG_TYPE_FAILOVER_AUTH_REQUEST 5 /* May I failover? */
 
 /* Initially we don't know our "name", but we'll find it once we connect
  * to the first node, using the getsockname() function. Then we'll use this