REPLCONF ACK command.

This special command is used by the slave to inform the master the
amount of replication stream it currently consumed.

it does not return anything so that we not need to consume additional
bandwidth needed by the master to reply something.

The master can do a number of things knowing the amount of stream
processed, such as understanding the "lag" in bytes of the slave, verify
if a given command was already processed by the slave, and so forth.

src/networking.c

@@ -88,6 +88,8 @@ redisClient *createClient(int fd) {
     c->authenticated = 0;
     c->replstate = REDIS_REPL_NONE;
     c->reploff = 0;
+    c->repl_ack_off = 0;
+    c->repl_ack_time = 0;
     c->slave_listening_port = 0;
     c->reply = listCreate();
     c->reply_bytes = 0;

src/redis.h

@@ -450,6 +450,8 @@ typedef struct redisClient {
     long repldboff;         /* replication DB file offset */
     off_t repldbsize;       /* replication DB file size */
     long long reploff;      /* replication offset if this is our master */
+    long long repl_ack_off; /* replication ack offset, if this is a slave */
+    long long repl_ack_time;/* replication ack time, if this is a slave */
     char replrunid[REDIS_RUN_ID_SIZE+1]; /* master run id if this is a master */
     int slave_listening_port; /* As configured with: SLAVECONF listening-port */
     multiState mstate;      /* MULTI/EXEC state */

src/replication.c

@@ -588,6 +588,19 @@ void replconfCommand(redisClient *c) {
                     &port,NULL) != REDIS_OK))
                 return;
             c->slave_listening_port = port;
+        } else if (!strcasecmp(c->argv[j]->ptr,"ack")) {
+            /* REPLCONF ACK is used by slave to inform the master the amount
+             * of replication stream that it processed so far. It is an
+             * internal only command that normal clients should never use. */
+            long long offset;
+
+            if (!(c->flags & REDIS_SLAVE)) return;
+            if ((getLongLongFromObject(c->argv[j+1], &offset) != REDIS_OK))
+                return;
+            if (offset > c->repl_ack_off)
+                c->repl_ack_off = offset;
+            c->repl_ack_time = server.unixtime;
+            /* Note: this command does not reply anything! */
         } else {
             addReplyErrorFormat(c,"Unrecognized REPLCONF option: %s",
                 (char*)c->argv[j]->ptr);