Sentinel: SENTINEL MONITOR command implemented.

It allows to add new masters to monitor at runtime.

src/sentinel.c

@@ -2486,6 +2486,40 @@ void sentinelCommand(redisClient *c) {
 
         if (c->argc != 2) goto numargserr;
         sentinelPendingScriptsCommand(c);
+    } else if (!strcasecmp(c->argv[1]->ptr,"monitor")) {
+        /* SENTINEL MONITOR <name> <ip> <port> <quorum> */
+        long quorum, port;
+        char buf[32];
+
+        if (c->argc != 6) goto numargserr;
+        if (getLongFromObjectOrReply(c,c->argv[5],&quorum,"Invalid quorum")
+            != REDIS_OK) return;
+        if (getLongFromObjectOrReply(c,c->argv[4],&port,"Invalid port")
+            != REDIS_OK) return;
+        /* Make sure the IP field is actually a valid IP before passing it
+         * to createSentinelRedisInstance(), otherwise we may trigger a
+         * DNS lookup at runtime. */
+        if (anetResolveIP(NULL,c->argv[3]->ptr,buf,sizeof(buf)) == ANET_ERR) {
+            addReplyError(c,"Invalid IP address specified");
+            return;
+        }
+        if (createSentinelRedisInstance(c->argv[2]->ptr,SRI_MASTER,
+                c->argv[3]->ptr,port,quorum,NULL) == NULL)
+        {
+            switch(errno) {
+            case EBUSY:
+                addReplyError(c,"Duplicated master name");
+                break;
+            case EINVAL:
+                addReplyError(c,"Invalid port number");
+                break;
+            default:
+                addReplyError(c,"Unspecified error adding the instance");
+                break;
+            }
+        } else {
+            addReply(c,shared.ok);
+        }
     } else {
         addReplyErrorFormat(c,"Unknown sentinel subcommand '%s'",
                                (char*)c->argv[1]->ptr);