Cluster: added stub for verifyClusterConfigWithData().

See the top-comment for the function in this commit for details about
what the function is supposed to do.

src/cluster.c

@@ -1276,6 +1276,32 @@ void clusterUpdateState(void) {
     }
 }
 
+/* This function is called after the node startup in order to verify that data
+ * loaded from disk is in agreement with the cluster configuration:
+ *
+ * 1) If we find keys about hash slots we have no responsibility for, the
+ *    following happens:
+ *    A) If no other node is in charge according to the current cluster
+ *       configuration, we add these slots to our node.
+ *    B) If according to our config other nodes are already in charge for
+ *       this lots, we set the slots as IMPORTING from our point of view
+ *       in order to justify we have those slots, and in order to make
+ *       redis-trib aware of the issue, so that it can try to fix it.
+ * 2) If we find data in a DB different than DB0 we return REDIS_ERR to
+ *    signal the caller it should quit the server with an error message
+ *    or take other actions.
+ *
+ * The function always returns REDIS_OK even if it will try to correct
+ * the error described in "1". However if data is found in DB different
+ * from DB0, REDIS_ERR is returned.
+ *
+ * The function also uses the logging facility in order to warn the user
+ * about desynchronizations between the data we have in memory and the
+ * cluster configuration. */
+int verifyClusterConfigWithData(void) {
+    return REDIS_OK;
+}
+
 /* -----------------------------------------------------------------------------
  * CLUSTER command
  * -------------------------------------------------------------------------- */

src/redis.c

@@ -2741,6 +2741,14 @@ int main(int argc, char **argv) {
         linuxOvercommitMemoryWarning();
     #endif
         loadDataFromDisk();
+        if (server.cluster_enabled) {
+            if (verifyClusterConfigWithData() == REDIS_ERR) {
+                redisLog(REDIS_WARNING,
+                    "You can't have keys in a DB different than DB 0 when in "
+                    "Cluster mode. Exiting.");
+                exit(1);
+            }
+        }
         if (server.ipfd > 0)
             redisLog(REDIS_NOTICE,"The server is now ready to accept connections on port %d", server.port);
         if (server.sofd > 0)

src/redis.h

@@ -1227,6 +1227,7 @@ int selectDb(redisClient *c, int id);
 void signalModifiedKey(redisDb *db, robj *key);
 void signalFlushedDb(int dbid);
 unsigned int GetKeysInSlot(unsigned int hashslot, robj **keys, unsigned int count);
+int verifyClusterConfigWithData(void);
 
 /* API to get key arguments from commands */
 #define REDIS_GETKEYS_ALL 0