Sentinel: send hello messages ASAP after config change.

Eventual configuration convergence is guaranteed by our periodic hello
messages to all the instances, however when there are important notices
to share, better make a phone call. With this commit we force an hello
message to other Sentinal and Redis instances within the next 100
milliseconds of a config update, which is practically better than
waiting a few seconds.

src/sentinel.c

@@ -336,6 +336,7 @@ char *sentinelVoteLeader(sentinelRedisInstance *master, uint64_t req_epoch, char
 void sentinelFlushConfig(void);
 void sentinelGenerateInitialMonitorEvents(void);
 int sentinelSendPing(sentinelRedisInstance *ri);
+int sentinelForceHelloUpdateForMaster(sentinelRedisInstance *master);
 
 /* ========================= Dictionary types =============================== */
 
@@ -1921,6 +1922,7 @@ void sentinelRefreshInstanceInfo(sentinelRedisInstance *ri, const char *info) {
                 ri->master,"%@");
             sentinelCallClientReconfScript(ri->master,SENTINEL_LEADER,
                 "start",ri->master->addr,ri->addr);
+            sentinelForceHelloUpdateForMaster(ri->master);
         } else {
             /* A slave turned into a master. We want to force our view and
              * reconfigure as slave. Wait some time after the change before
@@ -2059,7 +2061,7 @@ void sentinelPublishReplyCallback(redisAsyncContext *c, void *reply, void *privd
     r = reply;
 
     /* Only update pub_time if we actually published our message. Otherwise
-     * we'll retry against in 100 milliseconds. */
+     * we'll retry again in 100 milliseconds. */
     if (r->type != REDIS_REPLY_ERROR)
         ri->last_pub_time = mstime();
 }
@@ -2225,6 +2227,38 @@ int sentinelSendHello(sentinelRedisInstance *ri) {
     return REDIS_OK;
 }
 
+/* Reset last_pub_time in all the instances in the specified dictionary
+ * in order to force the delivery of an Hello update ASAP. */
+void sentinelForceHelloUpdateDictOfRedisInstances(dict *instances) {
+    dictIterator *di;
+    dictEntry *de;
+
+    di = dictGetSafeIterator(instances);
+    while((de = dictNext(di)) != NULL) {
+        sentinelRedisInstance *ri = dictGetVal(de);
+        if (ri->last_pub_time >= (SENTINEL_PUBLISH_PERIOD+1))
+            ri->last_pub_time -= (SENTINEL_PUBLISH_PERIOD+1);
+    }
+    dictReleaseIterator(di);
+}
+
+/* This function forces the delivery of an "Hello" message (see
+ * sentinelSendHello() top comment for further information) to all the Redis
+ * and Sentinel instances related to the specified 'master'.
+ *
+ * It is technically not needed since we send an update to every instance
+ * with a period of SENTINEL_PUBLISH_PERIOD milliseconds, however when a
+ * Sentinel upgrades a configuration it is a good idea to deliever an update
+ * to the other Sentinels ASAP. */
+int sentinelForceHelloUpdateForMaster(sentinelRedisInstance *master) {
+    if (!(master->flags & SRI_MASTER)) return REDIS_ERR;
+    if (master->last_pub_time >= (SENTINEL_PUBLISH_PERIOD+1))
+        master->last_pub_time -= (SENTINEL_PUBLISH_PERIOD+1);
+    sentinelForceHelloUpdateDictOfRedisInstances(master->sentinels);
+    sentinelForceHelloUpdateDictOfRedisInstances(master->slaves);
+    return REDIS_OK;
+}
+
 /* Send a PING to the specified instance and refresh the last_ping_time
  * if it is zero (that is, if we received a pong for the previous ping).
  *