Add persistence support for undelivered PubSub messages.

Refer to
http://abhinavsingh.com/customizing-redis-pubsub-for-message-persistence-part-2/
for details. Here is a simple demo of what happens to published messages on
channels with no active subscribers:

$ ./src/redis-cli
127.0.0.1:6379> publish persistent-channel this
(integer) 0
127.0.0.1:6379> publish persistent-channel is
(integer) 0
127.0.0.1:6379> publish persistent-channel gonna
(integer) 0
127.0.0.1:6379> publish persistent-channel be
(integer) 0
127.0.0.1:6379> publish persistent-channel awesome
(integer) 0
127.0.0.1:6379> lrange persistent-channel 0 -1
1) "this"
2) "is"
3) "gonna"
4) "be"
5) "awesome"
127.0.0.1:6379>

src/pubsub.c

@@ -274,6 +274,27 @@ int pubsubPublishMessage(robj *channel, robj *message) {
  * Pubsub commands implementation
  *----------------------------------------------------------------------------*/
 
+robj *llast(redisClient *c, robj *key) {
+	listTypeEntry entry;
+	robj *o, *value = NULL;
+	long llen;
+	o = lookupKeyRead(c->db, key);
+	if(o != NULL && o->encoding == REDIS_ENCODING_QUICKLIST) {
+		llen = listTypeLength(o);
+		listTypeIterator *iter = listTypeInitIterator(o, llen-1, REDIS_TAIL);
+		listTypeNext(iter, &entry);
+		quicklistEntry *qe = &entry.entry;
+		if (qe->value) {
+			value = createStringObject((char *)qe->value,
+									   qe->sz);
+		} else {
+			value = createStringObjectFromLongLong(qe->longval);
+		}
+		listTypeReleaseIterator(iter);
+	}
+	return value;
+}
+
 void subscribeCommand(redisClient *c) {
     int j;
 
@@ -282,14 +303,15 @@ void subscribeCommand(redisClient *c) {
     c->flags |= REDIS_PUBSUB;
 
     /* Send last received message on the subscribed channel(s) */
-    robj *o;
+    robj *value;
     for (j = 1; j < c->argc; j++) {
-    	o = lookupKeyRead(c->db, c->argv[j]);
-    	if(o != NULL) {
+    	value = llast(c, c->argv[j]);
+    	if (value != NULL) {
 			addReply(c,shared.mbulkhdr[3]);
 			addReply(c,shared.messagebulk);
 			addReplyBulk(c,c->argv[j]);
-			addReplyBulk(c,o);
+			addReplyBulk(c,value);
+			decrRefCount(value);
     	}
     }
 }
@@ -314,6 +336,7 @@ void psubscribeCommand(redisClient *c) {
     c->flags |= REDIS_PUBSUB;
 
     /* Send last received message on the channel(s) matching subscribed patterns */
+    robj *value;
     for (j = 1; j < c->argc; j++) {
     	robj *pat = c->argv[j];
     	dictIterator *di = dictGetIterator(server.pubsub_channels);
@@ -326,13 +349,14 @@ void psubscribeCommand(redisClient *c) {
 								(char*)channel,
 								sdslen(channel), 0))
 			{
-				robj *o = lookupKeyRead(c->db, cobj);
-				if(o != NULL) {
-	                addReply(c,shared.mbulkhdr[4]);
-	                addReply(c,shared.pmessagebulk);
-	                addReplyBulk(c,pat);
-	                addReplyBulk(c,cobj);
-	                addReplyBulk(c,o);
+				value = llast(c, cobj);
+				if (value != NULL) {
+					addReply(c,shared.mbulkhdr[4]);
+					addReply(c,shared.pmessagebulk);
+					addReplyBulk(c,pat);
+					addReplyBulk(c,cobj);
+					addReplyBulk(c,value);
+					decrRefCount(value);
 				}
 			}
 		}
@@ -345,7 +369,6 @@ void punsubscribeCommand(redisClient *c) {
         pubsubUnsubscribeAllPatterns(c,1);
     } else {
         int j;
-
         for (j = 1; j < c->argc; j++)
             pubsubUnsubscribePattern(c,c->argv[j],1);
     }
@@ -359,8 +382,27 @@ void publishCommand(redisClient *c) {
     else
         forceCommandPropagation(c,REDIS_PROPAGATE_REPL);
 
-    /* Persist last published message in channel specific key */
-    setKey(c->db, c->argv[1], c->argv[2]);
+    if (receivers == 0) {
+		int j, pushed = 0, where = REDIS_TAIL;
+		robj *lobj = lookupKeyWrite(c->db,c->argv[1]);
+		for (j = 2; j < c->argc; j++) {
+			c->argv[j] = tryObjectEncoding(c->argv[j]);
+			if (!lobj) {
+				lobj = createQuicklistObject();
+				quicklistSetOptions(lobj->ptr, server.list_max_ziplist_size,
+									server.list_compress_depth);
+				dbAdd(c->db,c->argv[1],lobj);
+			}
+			listTypePush(lobj,c->argv[j],where);
+			pushed++;
+		}
+		if (pushed) {
+			char *event = (where == REDIS_HEAD) ? "lpush" : "rpush";
+			signalModifiedKey(c->db,c->argv[1]);
+			notifyKeyspaceEvent(REDIS_NOTIFY_LIST,event,c->argv[1],c->db->id);
+		}
+		server.dirty += pushed;
+    }
 
     addReplyLongLong(c,receivers);
 }