Cluster: force AUTH ACK on manual failover.

When a slave requests masters vote for a manual failover, the
REQUEST_AUTH message is flagged in a special way in order to force the
masters to give the authorization even if the master is not marked as
failing.

src/cluster.c

@@ -1896,6 +1896,10 @@ void clusterRequestFailoverAuth(void) {
     uint32_t totlen;
 
     clusterBuildMessageHdr(hdr,CLUSTERMSG_TYPE_FAILOVER_AUTH_REQUEST);
+    /* If this is a manual failover, set the CLUSTERMSG_FLAG0_FORCEACK bit
+     * in the header to communicate the nodes receiving the message that
+     * they should authorized the failover even if the master is working. */
+    if (server.cluster->mf_end) hdr->mflags[0] |= CLUSTERMSG_FLAG0_FORCEACK;
     totlen = sizeof(clusterMsg)-sizeof(union clusterMsgData);
     hdr->totlen = htonl(totlen);
     clusterBroadcastMessage(buf,totlen);
@@ -1933,6 +1937,7 @@ void clusterSendFailoverAuthIfNeeded(clusterNode *node, clusterMsg *request) {
     uint64_t requestCurrentEpoch = ntohu64(request->currentEpoch);
     uint64_t requestConfigEpoch = ntohu64(request->configEpoch);
     unsigned char *claimed_slots = request->myslots;
+    int force_ack = request->mflags[0] & CLUSTERMSG_FLAG0_FORCEACK;
     int j;
 
     /* IF we are not a master serving at least 1 slot, we don't have the
@@ -1947,8 +1952,11 @@ void clusterSendFailoverAuthIfNeeded(clusterNode *node, clusterMsg *request) {
     /* I already voted for this epoch? Return ASAP. */
     if (server.cluster->last_vote_epoch == server.cluster->currentEpoch) return;
 
-    /* Node must be a slave and its master down. */
-    if (nodeIsMaster(node) || master == NULL || !nodeFailed(master)) return;
+    /* Node must be a slave and its master down.
+     * The master can be non failing if the request is flagged
+     * with CLUSTERMSG_FLAG0_FORCEACK (manual failover). */
+    if (nodeIsMaster(node) || master == NULL ||
+        (!nodeFailed(master) && !force_ack)) return;
 
     /* We did not voted for a slave about this master for two
      * times the node timeout. This is not strictly needed for correctness

src/cluster.h

@@ -220,6 +220,8 @@ typedef struct {
 /* Message flags better specify the packet content or are used to
  * provide some information about the node state. */
 #define CLUSTERMSG_FLAG0_PAUSED (1<<0) /* Master paused for manual failover. */
+#define CLUSTERMSG_FLAG0_FORCEACK (1<<1) /* Give ACK to AUTH_REQUEST even if
+                                            master is up. */
 
 /* ---------------------- API exported outside cluster.c -------------------- */
 clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **argv, int argc, int *hashslot, int *ask);