Client side caching: RESP2 support.

src/server.h

@@ -1946,6 +1946,7 @@ int pubsubUnsubscribeAllPatterns(client *c, int notify);
 void freePubsubPattern(void *p);
 int listMatchPubsubPattern(void *a, void *b);
 int pubsubPublishMessage(robj *channel, robj *message);
+void addReplyPubsubMessage(client *c, robj *channel, robj *msg);
 
 /* Keyspace events notification */
 void notifyKeyspaceEvent(int type, char *event, robj *key, int dbid);

src/tracking.c

@@ -60,6 +60,7 @@
  * use the most significant bits instead of the full 24 bits. */
 #define TRACKING_TABLE_SIZE (1<<24)
 rax **TrackingTable = NULL;
+robj *TrackingChannelName;
 
 /* Remove the tracking state from the client 'c'. Note that there is not much
  * to do for us here, if not to decrement the counter of the clients in
@@ -87,8 +88,10 @@ void enableTracking(client *c, uint64_t redirect_to) {
     c->flags &= ~CLIENT_TRACKING_BROKEN_REDIR;
     c->client_tracking_redirection = redirect_to;
     server.tracking_clients++;
-    if (TrackingTable == NULL)
+    if (TrackingTable == NULL) {
         TrackingTable = zcalloc(sizeof(rax*) * TRACKING_TABLE_SIZE);
+        TrackingChannelName = createStringObject("__redis__:invalidate",20);
+    }
 }
 
 /* This function is called after the excution of a readonly command in the
@@ -130,6 +133,7 @@ void trackingInvalidateKey(robj *keyobj) {
         uint64_t id;
         memcpy(&id,ri.key,ri.key_len);
         client *c = lookupClientByID(id);
+        int using_redirection = 0;
         if (c->client_tracking_redirection) {
             client *redir = lookupClientByID(c->client_tracking_redirection);
             if (!redir) {
@@ -144,13 +148,21 @@ void trackingInvalidateKey(robj *keyobj) {
                 continue;
             }
             c = redir;
+            using_redirection = 1;
         }
 
-        /* Only send such info for clients in RESP version 3 or more. */
+        /* Only send such info for clients in RESP version 3 or more. However
+         * if redirection is active, and the connection we redirect to is
+         * in Pub/Sub mode, we can support the feature with RESP 2 as well,
+         * by sending Pub/Sub messages in the __redis__:invalidate channel. */
         if (c->resp > 2) {
             addReplyPushLen(c,2);
             addReplyBulkCBuffer(c,"invalidate",10);
             addReplyLongLong(c,hash);
+        } else if (using_redirection && c->flags & CLIENT_PUBSUB) {
+            robj *msg = createStringObjectFromLongLong(hash);
+            addReplyPubsubMessage(c,TrackingChannelName,msg);
+            decrRefCount(msg);
         }
     }
     raxStop(&ri);