X-Git-Url: https://git.saurik.com/redis.git/blobdiff_plain/d38d2fdfdcfc2390fd9b02f1016e5571faf1fc6e..6fdc635447b1a5dd0a4f7b13c15fdd6c108dabee:/src/cluster.c?ds=sidebyside diff --git a/src/cluster.c b/src/cluster.c index 603a9557..57243132 100644 --- a/src/cluster.c +++ b/src/cluster.c @@ -1,8 +1,10 @@ #include "redis.h" +#include "endianconv.h" #include #include #include +#include void clusterAcceptHandler(aeEventLoop *el, int fd, void *privdata, int mask); void clusterReadHandler(aeEventLoop *el, int fd, void *privdata, int mask); @@ -19,20 +21,6 @@ int clusterAddSlot(clusterNode *n, int slot); * Initialization * -------------------------------------------------------------------------- */ -void clusterGetRandomName(char *p) { - FILE *fp = fopen("/dev/urandom","r"); - char *charset = "0123456789abcdef"; - int j; - - if (fp == NULL || fread(p,REDIS_CLUSTER_NAMELEN,1,fp) == 0) { - for (j = 0; j < REDIS_CLUSTER_NAMELEN; j++) - p[j] = rand(); - } - for (j = 0; j < REDIS_CLUSTER_NAMELEN; j++) - p[j] = charset[p[j] & 0x0F]; - fclose(fp); -} - int clusterLoadConfig(char *filename) { FILE *fp = fopen(filename,"r"); char *line; @@ -222,7 +210,7 @@ void clusterInit(void) { exit(1); } if (aeCreateFileEvent(server.el, server.cfd, AE_READABLE, - clusterAcceptHandler, NULL) == AE_ERR) oom("creating file event"); + clusterAcceptHandler, NULL) == AE_ERR) redisPanic("Unrecoverable error creating Redis Cluster file event."); server.cluster.slots_to_keys = zslCreate(); } @@ -304,7 +292,7 @@ clusterNode *createClusterNode(char *nodename, int flags) { if (nodename) memcpy(node->name, nodename, REDIS_CLUSTER_NAMELEN); else - clusterGetRandomName(node->name); + getRandomHexChars(node->name, REDIS_CLUSTER_NAMELEN); node->flags = flags; memset(node->slots,0,sizeof(node->slots)); node->numslaves = 0; @@ -377,7 +365,7 @@ clusterNode *clusterLookupNode(char *name) { de = dictFind(server.cluster.nodes,s); sdsfree(s); if (de == NULL) return NULL; - return dictGetEntryVal(de); + return dictGetVal(de); } /* This is only used after the handshake. When we connect a given IP/PORT @@ -439,7 +427,7 @@ void clusterProcessGossipSection(clusterMsg *hdr, clusterLink *link) { * time PONG figure if it is newer than our figure. * Note that it's not a problem if we have a PING already * in progress against this node. */ - if (node->pong_received < ntohl(g->pong_received)) { + if (node->pong_received < (signed) ntohl(g->pong_received)) { redisLog(REDIS_DEBUG,"Node pong_received updated by gossip"); node->pong_received = ntohl(g->pong_received); } @@ -493,6 +481,7 @@ void nodeIp2String(char *buf, clusterLink *link) { /* Update the node address to the IP address that can be extracted * from link->fd, and at the specified port. */ void nodeUpdateAddress(clusterNode *node, clusterLink *link, int port) { + /* TODO */ } /* When this function is called, there is a packet to process starting @@ -510,8 +499,10 @@ int clusterProcessPacket(clusterLink *link) { uint16_t type = ntohs(hdr->type); clusterNode *sender; - redisLog(REDIS_DEBUG,"--- packet to process %lu bytes (%lu) ---", - (unsigned long) totlen, sdslen(link->rcvbuf)); + redisLog(REDIS_DEBUG,"--- Processing packet of type %d, %lu bytes", + type, (unsigned long) totlen); + + /* Perform sanity checks */ if (totlen < 8) return 1; if (totlen > sdslen(link->rcvbuf)) return 1; if (type == CLUSTERMSG_TYPE_PING || type == CLUSTERMSG_TYPE_PONG || @@ -530,7 +521,16 @@ int clusterProcessPacket(clusterLink *link) { explen += sizeof(clusterMsgDataFail); if (totlen != explen) return 1; } + if (type == CLUSTERMSG_TYPE_PUBLISH) { + uint32_t explen = sizeof(clusterMsg)-sizeof(union clusterMsgData); + + explen += sizeof(clusterMsgDataPublish) + + ntohl(hdr->data.publish.msg.channel_len) + + ntohl(hdr->data.publish.msg.message_len); + if (totlen != explen) return 1; + } + /* Ready to process the packet. Dispatch by type. */ sender = clusterLookupNode(hdr->sender); if (type == CLUSTERMSG_TYPE_PING || type == CLUSTERMSG_TYPE_MEET) { int update_config = 0; @@ -601,7 +601,7 @@ int clusterProcessPacket(clusterLink *link) { } } /* Update our info about the node */ - link->node->pong_received = time(NULL); + if (link->node) link->node->pong_received = time(NULL); /* Update master/slave info */ if (sender) { @@ -664,8 +664,24 @@ int clusterProcessPacket(clusterLink *link) { clusterUpdateState(); clusterSaveConfigOrDie(); } + } else if (type == CLUSTERMSG_TYPE_PUBLISH) { + robj *channel, *message; + uint32_t channel_len, message_len; + + /* Don't bother creating useless objects if there are no Pub/Sub subscribers. */ + if (dictSize(server.pubsub_channels) || listLength(server.pubsub_patterns)) { + channel_len = ntohl(hdr->data.publish.msg.channel_len); + message_len = ntohl(hdr->data.publish.msg.message_len); + channel = createStringObject( + (char*)hdr->data.publish.msg.bulk_data,channel_len); + message = createStringObject( + (char*)hdr->data.publish.msg.bulk_data+channel_len, message_len); + pubsubPublishMessage(channel,message); + decrRefCount(channel); + decrRefCount(message); + } } else { - redisLog(REDIS_NOTICE,"Received unknown packet type: %d", type); + redisLog(REDIS_WARNING,"Received unknown packet type: %d", type); } return 1; } @@ -758,9 +774,25 @@ void clusterSendMessage(clusterLink *link, unsigned char *msg, size_t msglen) { link->sndbuf = sdscatlen(link->sndbuf, msg, msglen); } +/* Send a message to all the nodes with a reliable link */ +void clusterBroadcastMessage(void *buf, size_t len) { + dictIterator *di; + dictEntry *de; + + di = dictGetIterator(server.cluster.nodes); + while((de = dictNext(di)) != NULL) { + clusterNode *node = dictGetVal(de); + + if (!node->link) continue; + if (node->flags & (REDIS_NODE_MYSELF|REDIS_NODE_NOADDR)) continue; + clusterSendMessage(node->link,buf,len); + } + dictReleaseIterator(di); +} + /* Build the message header */ void clusterBuildMessageHdr(clusterMsg *hdr, int type) { - int totlen; + int totlen = 0; memset(hdr,0,sizeof(*hdr)); hdr->type = htons(type); @@ -805,7 +837,7 @@ void clusterSendPing(clusterLink *link, int type) { /* Populate the gossip fields */ while(freshnodes > 0 && gossipcount < 3) { struct dictEntry *de = dictGetRandomKey(server.cluster.nodes); - clusterNode *this = dictGetEntryVal(de); + clusterNode *this = dictGetVal(de); clusterMsgDataGossip *gossip; int j; @@ -842,20 +874,48 @@ void clusterSendPing(clusterLink *link, int type) { clusterSendMessage(link,buf,totlen); } -/* Send a message to all the nodes with a reliable link */ -void clusterBroadcastMessage(void *buf, size_t len) { - dictIterator *di; - dictEntry *de; +/* Send a PUBLISH message. + * + * If link is NULL, then the message is broadcasted to the whole cluster. */ +void clusterSendPublish(clusterLink *link, robj *channel, robj *message) { + unsigned char buf[4096], *payload; + clusterMsg *hdr = (clusterMsg*) buf; + uint32_t totlen; + uint32_t channel_len, message_len; - di = dictGetIterator(server.cluster.nodes); - while((de = dictNext(di)) != NULL) { - clusterNode *node = dictGetEntryVal(de); + channel = getDecodedObject(channel); + message = getDecodedObject(message); + channel_len = sdslen(channel->ptr); + message_len = sdslen(message->ptr); - if (!node->link) continue; - if (node->flags & (REDIS_NODE_MYSELF|REDIS_NODE_NOADDR)) continue; - clusterSendMessage(node->link,buf,len); + clusterBuildMessageHdr(hdr,CLUSTERMSG_TYPE_PUBLISH); + totlen = sizeof(clusterMsg)-sizeof(union clusterMsgData); + totlen += sizeof(clusterMsgDataPublish) + channel_len + message_len; + + hdr->data.publish.msg.channel_len = htonl(channel_len); + hdr->data.publish.msg.message_len = htonl(message_len); + hdr->totlen = htonl(totlen); + + /* Try to use the local buffer if possible */ + if (totlen < sizeof(buf)) { + payload = buf; + } else { + payload = zmalloc(totlen); + hdr = (clusterMsg*) payload; + memcpy(payload,hdr,sizeof(*hdr)); } - dictReleaseIterator(di); + memcpy(hdr->data.publish.msg.bulk_data,channel->ptr,sdslen(channel->ptr)); + memcpy(hdr->data.publish.msg.bulk_data+sdslen(channel->ptr), + message->ptr,sdslen(message->ptr)); + + if (link) + clusterSendMessage(link,payload,totlen); + else + clusterBroadcastMessage(payload,totlen); + + decrRefCount(channel); + decrRefCount(message); + if (payload != buf) zfree(payload); } /* Send a FAIL message to all the nodes we are able to contact. @@ -872,6 +932,17 @@ void clusterSendFail(char *nodename) { clusterBroadcastMessage(buf,ntohl(hdr->totlen)); } +/* ----------------------------------------------------------------------------- + * CLUSTER Pub/Sub support + * + * For now we do very little, just propagating PUBLISH messages across the whole + * cluster. In the future we'll try to get smarter and avoiding propagating those + * messages to hosts without receives for a given channel. + * -------------------------------------------------------------------------- */ +void clusterPropagatePublish(robj *channel, robj *message) { + clusterSendPublish(NULL, channel, message); +} + /* ----------------------------------------------------------------------------- * CLUSTER cron job * -------------------------------------------------------------------------- */ @@ -887,7 +958,7 @@ void clusterCron(void) { /* Check if we have disconnected nodes and reestablish the connection. */ di = dictGetIterator(server.cluster.nodes); while((de = dictNext(di)) != NULL) { - clusterNode *node = dictGetEntryVal(de); + clusterNode *node = dictGetVal(de); if (node->flags & (REDIS_NODE_MYSELF|REDIS_NODE_NOADDR)) continue; if (node->link == NULL) { @@ -922,7 +993,7 @@ void clusterCron(void) { * the oldest ping_sent time */ for (j = 0; j < 5; j++) { de = dictGetRandomKey(server.cluster.nodes); - clusterNode *this = dictGetEntryVal(de); + clusterNode *this = dictGetVal(de); if (this->link == NULL) continue; if (this->flags & (REDIS_NODE_MYSELF|REDIS_NODE_HANDSHAKE)) continue; @@ -939,7 +1010,7 @@ void clusterCron(void) { /* Iterate nodes to check if we need to flag something as failing */ di = dictGetIterator(server.cluster.nodes); while((de = dictNext(di)) != NULL) { - clusterNode *node = dictGetEntryVal(de); + clusterNode *node = dictGetVal(de); int delay; if (node->flags & @@ -1070,7 +1141,7 @@ sds clusterGenNodesDescription(void) { di = dictGetIterator(server.cluster.nodes); while((de = dictNext(di)) != NULL) { - clusterNode *node = dictGetEntryVal(de); + clusterNode *node = dictGetVal(de); /* Node coordinates */ ci = sdscatprintf(ci,"%.40s %s:%d ", @@ -1192,7 +1263,10 @@ void clusterCommand(redisClient *c) { addReplyBulk(c,o); decrRefCount(o); } else if ((!strcasecmp(c->argv[1]->ptr,"addslots") || - !strcasecmp(c->argv[1]->ptr,"delslots")) && c->argc >= 3) { + !strcasecmp(c->argv[1]->ptr,"delslots")) && c->argc >= 3) + { + /* CLUSTER ADDSLOTS [slot] ... */ + /* CLUSTER DELSLOTS [slot] ... */ int j, slot; unsigned char *slots = zmalloc(REDIS_CLUSTER_SLOTS); int del = !strcasecmp(c->argv[1]->ptr,"delslots"); @@ -1232,7 +1306,7 @@ void clusterCommand(redisClient *c) { retval = del ? clusterDelSlot(j) : clusterAddSlot(server.cluster.myself,j); - redisAssert(retval == REDIS_OK); + redisAssertWithInfo(c,NULL,retval == REDIS_OK); } } zfree(slots); @@ -1385,9 +1459,86 @@ void clusterCommand(redisClient *c) { } /* ----------------------------------------------------------------------------- - * RESTORE and MIGRATE commands + * DUMP, RESTORE and MIGRATE commands * -------------------------------------------------------------------------- */ +/* Generates a DUMP-format representation of the object 'o', adding it to the + * io stream pointed by 'rio'. This function can't fail. */ +void createDumpPayload(rio *payload, robj *o) { + unsigned char buf[2]; + uint64_t crc; + + /* Serialize the object in a RDB-like format. It consist of an object type + * byte followed by the serialized object. This is understood by RESTORE. */ + rioInitWithBuffer(payload,sdsempty()); + redisAssert(rdbSaveObjectType(payload,o)); + redisAssert(rdbSaveObject(payload,o)); + + /* Write the footer, this is how it looks like: + * ----------------+---------------------+---------------+ + * ... RDB payload | 2 bytes RDB version | 8 bytes CRC64 | + * ----------------+---------------------+---------------+ + * RDB version and CRC are both in little endian. + */ + + /* RDB version */ + buf[0] = REDIS_RDB_VERSION & 0xff; + buf[1] = (REDIS_RDB_VERSION >> 8) & 0xff; + payload->io.buffer.ptr = sdscatlen(payload->io.buffer.ptr,buf,2); + + /* CRC64 */ + crc = crc64(0,(unsigned char*)payload->io.buffer.ptr, + sdslen(payload->io.buffer.ptr)); + memrev64ifbe(&crc); + payload->io.buffer.ptr = sdscatlen(payload->io.buffer.ptr,&crc,8); +} + +/* Verify that the RDB version of the dump payload matches the one of this Redis + * instance and that the checksum is ok. + * If the DUMP payload looks valid REDIS_OK is returned, otherwise REDIS_ERR + * is returned. */ +int verifyDumpPayload(unsigned char *p, size_t len) { + unsigned char *footer; + uint16_t rdbver; + uint64_t crc; + + /* At least 2 bytes of RDB version and 8 of CRC64 should be present. */ + if (len < 10) return REDIS_ERR; + footer = p+(len-10); + + /* Verify RDB version */ + rdbver = (footer[1] << 8) | footer[0]; + if (rdbver != REDIS_RDB_VERSION) return REDIS_ERR; + + /* Verify CRC64 */ + crc = crc64(0,p,len-8); + memrev64ifbe(&crc); + return (memcmp(&crc,footer+2,8) == 0) ? REDIS_OK : REDIS_ERR; +} + +/* DUMP keyname + * DUMP is actually not used by Redis Cluster but it is the obvious + * complement of RESTORE and can be useful for different applications. */ +void dumpCommand(redisClient *c) { + robj *o, *dumpobj; + rio payload; + + /* Check if the key is here. */ + if ((o = lookupKeyRead(c->db,c->argv[1])) == NULL) { + addReply(c,shared.nullbulk); + return; + } + + /* Create the DUMP encoded representation. */ + createDumpPayload(&payload,o); + + /* Transfer to the client */ + dumpobj = createObject(REDIS_STRING,payload.io.buffer.ptr); + addReplyBulk(c,dumpobj); + decrRefCount(dumpobj); + return; +} + /* RESTORE key ttl serialized-value */ void restoreCommand(redisClient *c) { long ttl; @@ -1409,6 +1560,12 @@ void restoreCommand(redisClient *c) { return; } + /* Verify RDB version and data checksum. */ + if (verifyDumpPayload(c->argv[3]->ptr,sdslen(c->argv[3]->ptr)) == REDIS_ERR) { + addReplyError(c,"DUMP payload version or checksum are wrong"); + return; + } + rioInitWithBuffer(&payload,c->argv[3]->ptr); if (((type = rdbLoadObjectType(&payload)) == -1) || ((obj = rdbLoadObject(type,&payload)) == NULL)) @@ -1419,8 +1576,10 @@ void restoreCommand(redisClient *c) { /* Create the key and set the TTL if any */ dbAdd(c->db,c->argv[1],obj); - if (ttl) setExpire(c->db,c->argv[1],time(NULL)+ttl); + if (ttl) setExpire(c->db,c->argv[1],mstime()+ttl); + signalModifiedKey(c->db,c->argv[1]); addReply(c,shared.ok); + server.dirty++; } /* MIGRATE host port key dbid timeout */ @@ -1428,7 +1587,7 @@ void migrateCommand(redisClient *c) { int fd; long timeout; long dbid; - time_t ttl; + long long ttl = 0, expireat; robj *o; rio cmd, payload; @@ -1443,7 +1602,7 @@ void migrateCommand(redisClient *c) { * nothing to migrate (for instance the key expired in the meantime), but * we include such information in the reply string. */ if ((o = lookupKeyRead(c->db,c->argv[3])) == NULL) { - addReplySds(c,sdsnew("+NOKEY")); + addReplySds(c,sdsnew("+NOKEY\r\n")); return; } @@ -1456,28 +1615,32 @@ void migrateCommand(redisClient *c) { return; } if ((aeWait(fd,AE_WRITABLE,timeout*1000) & AE_WRITABLE) == 0) { - addReplyError(c,"Timeout connecting to the client"); + addReplySds(c,sdsnew("-IOERR error or timeout connecting to the client\r\n")); return; } + /* Create RESTORE payload and generate the protocol to call the command. */ rioInitWithBuffer(&cmd,sdsempty()); - redisAssert(rioWriteBulkCount(&cmd,'*',2)); - redisAssert(rioWriteBulkString(&cmd,"SELECT",6)); - redisAssert(rioWriteBulkLongLong(&cmd,dbid)); - - ttl = getExpire(c->db,c->argv[3]); - redisAssert(rioWriteBulkCount(&cmd,'*',4)); - redisAssert(rioWriteBulkString(&cmd,"RESTORE",7)); - redisAssert(c->argv[3]->encoding == REDIS_ENCODING_RAW); - redisAssert(rioWriteBulkString(&cmd,c->argv[3]->ptr,sdslen(c->argv[3]->ptr))); - redisAssert(rioWriteBulkLongLong(&cmd,(ttl == -1) ? 0 : ttl)); + redisAssertWithInfo(c,NULL,rioWriteBulkCount(&cmd,'*',2)); + redisAssertWithInfo(c,NULL,rioWriteBulkString(&cmd,"SELECT",6)); + redisAssertWithInfo(c,NULL,rioWriteBulkLongLong(&cmd,dbid)); + + expireat = getExpire(c->db,c->argv[3]); + if (expireat != -1) { + ttl = expireat-mstime(); + if (ttl < 1) ttl = 1; + } + redisAssertWithInfo(c,NULL,rioWriteBulkCount(&cmd,'*',4)); + redisAssertWithInfo(c,NULL,rioWriteBulkString(&cmd,"RESTORE",7)); + redisAssertWithInfo(c,NULL,c->argv[3]->encoding == REDIS_ENCODING_RAW); + redisAssertWithInfo(c,NULL,rioWriteBulkString(&cmd,c->argv[3]->ptr,sdslen(c->argv[3]->ptr))); + redisAssertWithInfo(c,NULL,rioWriteBulkLongLong(&cmd,ttl)); /* Finally the last argument that is the serailized object payload - * in the form: . */ - rioInitWithBuffer(&payload,sdsempty()); - redisAssert(rdbSaveObjectType(&payload,o)); - redisAssert(rdbSaveObject(&payload,o) != -1); - redisAssert(rioWriteBulkString(&cmd,payload.io.buffer.ptr,sdslen(payload.io.buffer.ptr))); + * in the DUMP format. */ + createDumpPayload(&payload,o); + redisAssertWithInfo(c,NULL,rioWriteBulkString(&cmd,payload.io.buffer.ptr, + sdslen(payload.io.buffer.ptr))); sdsfree(payload.io.buffer.ptr); /* Tranfer the query to the other node in 64K chunks. */ @@ -1488,7 +1651,7 @@ void migrateCommand(redisClient *c) { while ((towrite = sdslen(buf)-pos) > 0) { towrite = (towrite > (64*1024) ? (64*1024) : towrite); - nwritten = syncWrite(fd,buf+nwritten,towrite,timeout); + nwritten = syncWrite(fd,buf+pos,towrite,timeout); if (nwritten != (signed)towrite) goto socket_wr_err; pos += nwritten; } @@ -1508,8 +1671,17 @@ void migrateCommand(redisClient *c) { addReplyErrorFormat(c,"Target instance replied with error: %s", (buf1[0] == '-') ? buf1+1 : buf2+1); } else { + robj *aux; + dbDelete(c->db,c->argv[3]); + signalModifiedKey(c->db,c->argv[3]); addReply(c,shared.ok); + server.dirty++; + + /* Translate MIGRATE as DEL for replication/AOF. */ + aux = createStringObject("DEL",3); + rewriteClientCommandVector(c,2,aux,c->argv[3]); + decrRefCount(aux); } } @@ -1518,48 +1690,29 @@ void migrateCommand(redisClient *c) { return; socket_wr_err: - redisLog(REDIS_NOTICE,"Can't write to target node for MIGRATE: %s", - strerror(errno)); - addReplyErrorFormat(c,"MIGRATE failed, writing to target node: %s.", - strerror(errno)); + addReplySds(c,sdsnew("-IOERR error or timeout writing to target instance\r\n")); sdsfree(cmd.io.buffer.ptr); close(fd); return; socket_rd_err: - redisLog(REDIS_NOTICE,"Can't read from target node for MIGRATE: %s", - strerror(errno)); - addReplyErrorFormat(c,"MIGRATE failed, reading from target node: %s.", - strerror(errno)); + addReplySds(c,sdsnew("-IOERR error or timeout reading from target node\r\n")); sdsfree(cmd.io.buffer.ptr); close(fd); return; } -/* DUMP keyname - * DUMP is actually not used by Redis Cluster but it is the obvious - * complement of RESTORE and can be useful for different applications. */ -void dumpCommand(redisClient *c) { - robj *o, *dumpobj; - rio payload; - - /* Check if the key is here. */ - if ((o = lookupKeyRead(c->db,c->argv[1])) == NULL) { - addReply(c,shared.nullbulk); +/* The ASKING command is required after a -ASK redirection. + * The client should issue ASKING before to actualy send the command to + * the target instance. See the Redis Cluster specification for more + * information. */ +void askingCommand(redisClient *c) { + if (server.cluster_enabled == 0) { + addReplyError(c,"This instance has cluster support disabled"); return; } - - /* Serialize the object in a RDB-like format. It consist of an object type - * byte followed by the serialized object. This is understood by RESTORE. */ - rioInitWithBuffer(&payload,sdsempty()); - redisAssert(rdbSaveObjectType(&payload,o)); - redisAssert(rdbSaveObject(&payload,o)); - - /* Transfer to the client */ - dumpobj = createObject(REDIS_STRING,payload.io.buffer.ptr); - addReplyBulk(c,dumpobj); - decrRefCount(dumpobj); - return; + c->flags |= REDIS_ASKING; + addReply(c,shared.ok); } /* ----------------------------------------------------------------------------- @@ -1623,7 +1776,7 @@ clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **arg slot = keyHashSlot((char*)firstkey->ptr, sdslen(firstkey->ptr)); n = server.cluster.slots[slot]; - redisAssert(n != NULL); + redisAssertWithInfo(c,firstkey,n != NULL); } else { /* If it is not the first key, make sure it is exactly * the same key as the first we saw. */ @@ -1655,9 +1808,12 @@ clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **arg } /* Handle the case in which we are receiving this hash slot from * another instance, so we'll accept the query even if in the table - * it is assigned to a different node. */ - if (server.cluster.importing_slots_from[slot] != NULL) + * 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) { return server.cluster.myself; + } /* It's not a -ASK case. Base case: just return the right node. */ return n; }