Sentinel: send SLAVEOF with MULTI, CLIENT KILL, CONFIG REWRITE.

This implements the new Sentinel-Client protocol for the Sentinel part:
now instances are reconfigured using a transaction that ensures that the
config is rewritten in the target instance, and that clients lose the
connection with the instance, in order to be forced to: ask Sentinel,
reconnect to the instance, and verify the instance role with the new
ROLE command.

src/sentinel.c

@@ -3239,21 +3239,53 @@ int sentinelSendSlaveOf(sentinelRedisInstance *ri, char *host, int port) {
 
     ll2string(portstr,sizeof(portstr),port);
 
+    /* If host is NULL we send SLAVEOF NO ONE that will turn the instance
+     * into a master. */
     if (host == NULL) {
         host = "NO";
         memcpy(portstr,"ONE",4);
     }
 
+    /* In order to send SLAVEOF in a safe way, we send a transaction performing
+     * the following tasks:
+     * 1) Reconfigure the instance according to the specified host/port params.
+     * 2) Rewrite the configuraiton.
+     * 3) Disconnect all clients (but this one sending the commnad) in order
+     *    to trigger the ask-master-on-reconnection protocol for connected
+     *    clients.
+     *
+     * Note that we don't check the replies returned by commands, since we
+     * will observe instead the effects in the next INFO output. */
+    retval = redisAsyncCommand(ri->cc,
+        sentinelDiscardReplyCallback, NULL, "MULTI");
+    if (retval == REDIS_ERR) return retval;
+    ri->pending_commands++;
+
     retval = redisAsyncCommand(ri->cc,
         sentinelDiscardReplyCallback, NULL, "SLAVEOF %s %s", host, portstr);
     if (retval == REDIS_ERR) return retval;
+    ri->pending_commands++;
 
+    retval = redisAsyncCommand(ri->cc,
+        sentinelDiscardReplyCallback, NULL, "CONFIG REWRITE");
+    if (retval == REDIS_ERR) return retval;
     ri->pending_commands++;
-    if (redisAsyncCommand(ri->cc,
-        sentinelDiscardReplyCallback, NULL, "CONFIG REWRITE") == REDIS_OK)
-    {
-        ri->pending_commands++;
-    }
+
+    /* CLIENT KILL TYPE <type> is only supported starting from Redis 2.8.12,
+     * however sending it to an instance not understanding this command is not
+     * an issue because CLIENT is variadic command, so Redis will not
+     * recognized as a syntax error, and the transaction will not fail (but
+     * only the unsupported command will fail). */
+    retval = redisAsyncCommand(ri->cc,
+        sentinelDiscardReplyCallback, NULL, "CLIENT KILL TYPE normal");
+    if (retval == REDIS_ERR) return retval;
+    ri->pending_commands++;
+
+    retval = redisAsyncCommand(ri->cc,
+        sentinelDiscardReplyCallback, NULL, "EXEC");
+    if (retval == REDIS_ERR) return retval;
+    ri->pending_commands++;
+
     return REDIS_OK;
 }