A reimplementation of blocking operation internals.

Redis provides support for blocking operations such as BLPOP or BRPOP.
This operations are identical to normal LPOP and RPOP operations as long
as there are elements in the target list, but if the list is empty they
block waiting for new data to arrive to the list.

All the clients blocked waiting for th same list are served in a FIFO
way, so the first that blocked is the first to be served when there is
more data pushed by another client into the list.

The previous implementation of blocking operations was conceived to
serve clients in the context of push operations. For for instance:

1) There is a client "A" blocked on list "foo".
2) The client "B" performs `LPUSH foo somevalue`.
3) The client "A" is served in the context of the "B" LPUSH,
synchronously.

Processing things in a synchronous way was useful as if "A" pushes a
value that is served by "B", from the point of view of the database is a
NOP (no operation) thing, that is, nothing is replicated, nothing is
written in the AOF file, and so forth.

However later we implemented two things:

1) Variadic LPUSH that could add multiple values to a list in the
context of a single call.
2) BRPOPLPUSH that was a version of BRPOP that also provided a "PUSH"
side effect when receiving data.

This forced us to make the synchronous implementation more complex. If
client "B" is waiting for data, and "A" pushes three elemnents in a
single call, we needed to propagate an LPUSH with a missing argument
in the AOF and replication link. We also needed to make sure to
replicate the LPUSH side of BRPOPLPUSH, but only if in turn did not
happened to serve another blocking client into another list ;)

This were complex but with a few of mutually recursive functions
everything worked as expected... until one day we introduced scripting
in Redis.

Scripting + synchronous blocking operations = Issue #614.

Basically you can't "rewrite" a script to have just a partial effect on
the replicas and AOF file if the script happened to serve a few blocked
clients.

The solution to all this problems, implemented by this commit, is to
change the way we serve blocked clients. Instead of serving the blocked
clients synchronously, in the context of the command performing the PUSH
operation, it is now an asynchronous and iterative process:

1) If a key that has clients blocked waiting for data is the subject of
a list push operation, We simply mark keys as "ready" and put it into a
queue.
2) Every command pushing stuff on lists, as a variadic LPUSH, a script,
or whatever it is, is replicated verbatim without any rewriting.
3) Every time a Redis command, a MULTI/EXEC block, or a script,
completed its execution, we run the list of keys ready to serve blocked
clients (as more data arrived), and process this list serving the
blocked clients.
4) As a result of "3" maybe more keys are ready again for other clients
(as a result of BRPOPLPUSH we may have push operations), so we iterate
back to step "3" if it's needed.

The new code has a much simpler semantics, and a simpler to understand
implementation, with the disadvantage of not being able to "optmize out"
a PUSH+BPOP as a No OP.

This commit will be tested with care before the final merge, more tests
will be added likely.

src/redis.h

@@ -295,6 +295,7 @@ typedef struct redisDb {
     dict *dict;                 /* The keyspace for this DB */
     dict *expires;              /* Timeout of keys with a timeout set */
     dict *blocking_keys;        /* Keys with clients waiting for data (BLPOP) */
+    dict *ready_keys;           /* Blocked keys that received a PUSH */
     dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */
     int id;
 } redisDb;
@@ -321,6 +322,22 @@ typedef struct blockingState {
                              * for BRPOPLPUSH. */
 } blockingState;
 
+/* The following structure represents a node in the server.ready_keys list,
+ * where we accumulate all the keys that had clients blocked with a blocking
+ * operation such as B[LR]POP, but received new data in the context of the
+ * last executed command.
+ *
+ * After the execution of every command or script, we run this list to check
+ * if as a result we should serve data to clients blocked, unblocking them.
+ * Note that server.ready_keys will not have duplicates as there dictionary
+ * also called ready_keys in every structure representing a Redis database,
+ * where we make sure to remember if a given key was already added in the
+ * server.ready_keys list. */
+typedef struct readyList {
+    redisDb *db;
+    robj *key;
+} readyList;
+
 /* With multiplexing we need to take per-clinet state.
  * Clients are taken in a liked list. */
 typedef struct redisClient {
@@ -375,6 +392,7 @@ struct sharedObjectsStruct {
     *masterdownerr, *roslaveerr,
     *oomerr, *plus, *messagebulk, *pmessagebulk, *subscribebulk,
     *unsubscribebulk, *psubscribebulk, *punsubscribebulk, *del, *rpop, *lpop,
+    *lpush,
     *select[REDIS_SHARED_SELECT_CMDS],
     *integers[REDIS_SHARED_INTEGERS],
     *mbulkhdr[REDIS_SHARED_BULKHDR_LEN], /* "*<value>\r\n" */
@@ -469,7 +487,8 @@ struct redisServer {
     off_t loading_loaded_bytes;
     time_t loading_start_time;
     /* Fast pointers to often looked up command */
-    struct redisCommand *delCommand, *multiCommand, *lpushCommand;
+    struct redisCommand *delCommand, *multiCommand, *lpushCommand, *lpopCommand,
+                        *rpopCommand;
     /* Fields used only for stats */
     time_t stat_starttime;          /* Server start time */
     long long stat_numcommands;     /* Number of processed commands */
@@ -568,6 +587,7 @@ struct redisServer {
     /* Blocked clients */
     unsigned int bpop_blocked_clients; /* Number of clients blocked by lists */
     list *unblocked_clients; /* list of clients to unblock before next loop */
+    list *ready_keys;        /* List of readyList structures for BLPOP & co */
     /* Sort parameters - qsort_r() is only available under BSD so we
      * have to take this state global, in order to pass it to sortCompare() */
     int sort_desc;
@@ -777,7 +797,7 @@ int listTypeEqual(listTypeEntry *entry, robj *o);
 void listTypeDelete(listTypeEntry *entry);
 void listTypeConvert(robj *subject, int enc);
 void unblockClientWaitingData(redisClient *c);
-int handleClientsWaitingListPush(redisClient *c, robj *key, robj *ele);
+void handleClientsBlockedOnLists(void);
 void popGenericCommand(redisClient *c, int where);
 
 /* MULTI/EXEC/WATCH... */