Cluster: new command flag forcing implicit ASKING.

Also using this new flag the RESTORE-ASKING command was implemented that
will be used by MIGRATE.

src/cluster.c

@@ -2007,7 +2007,7 @@ clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **arg
      * it is assigned to a different node, but only if the client
      * issued an ASKING command before. */
     if (server.cluster->importing_slots_from[slot] != NULL &&
-        c->flags & REDIS_ASKING) {
+        (c->flags & REDIS_ASKING || cmd->flags & REDIS_CMD_ASKING)) {
         return server.cluster->myself;
     }
     /* It's not a -ASK case. Base case: just return the right node. */

src/redis.c

@@ -111,6 +111,8 @@ struct redisCommand *commandTable;
  *    server this data. Normally no command is accepted in this condition
  *    but just a few.
  * M: Do not automatically propagate the command on MONITOR.
+ * k: Perform an implicit ASKING for this command, so the command will be
+ *    accepted in cluster mode if the slot is marked as 'importing'.
  */
 struct redisCommand redisCommandTable[] = {
     {"get",getCommand,2,"r",0,NULL,1,1,1,0,0},
@@ -242,6 +244,7 @@ struct redisCommand redisCommandTable[] = {
     {"unwatch",unwatchCommand,1,"rs",0,NULL,0,0,0,0,0},
     {"cluster",clusterCommand,-2,"ar",0,NULL,0,0,0,0,0},
     {"restore",restoreCommand,-4,"awm",0,NULL,1,1,1,0,0},
+    {"restore-asking",restoreCommand,-4,"awmk",0,NULL,1,1,1,0,0},
     {"migrate",migrateCommand,-6,"aw",0,NULL,0,0,0,0,0},
     {"asking",askingCommand,1,"r",0,NULL,0,0,0,0,0},
     {"dump",dumpCommand,2,"ar",0,NULL,1,1,1,0,0},
@@ -1453,6 +1456,7 @@ void populateCommandTable(void) {
             case 'l': c->flags |= REDIS_CMD_LOADING; break;
             case 't': c->flags |= REDIS_CMD_STALE; break;
             case 'M': c->flags |= REDIS_CMD_SKIP_MONITOR; break;
+            case 'k': c->flags |= REDIS_CMD_ASKING; break;
             default: redisPanic("Unsupported command flag"); break;
             }
             f++;

src/redis.h

@@ -122,6 +122,7 @@
 #define REDIS_CMD_LOADING 512               /* "l" flag */
 #define REDIS_CMD_STALE 1024                /* "t" flag */
 #define REDIS_CMD_SKIP_MONITOR 2048         /* "M" flag */
+#define REDIS_CMD_ASKING 4096               /* "k" flag */
 
 /* Object types */
 #define REDIS_STRING 0