Variadic PING with support for Pub/Sub.

PING can now be called with an additional arugment, behaving exactly
like the ECHO command. PING can now also be called in Pub/Sub mode (with
one more more subscriptions to channels / patterns) in order to trigger
the delivery of an asynchronous pong message with the optional payload.

This fixes issue #420.

src/redis.c

@@ -224,7 +224,7 @@ struct redisCommand redisCommandTable[] = {
     {"scan",scanCommand,-2,"rR",0,NULL,0,0,0,0,0},
     {"dbsize",dbsizeCommand,1,"rF",0,NULL,0,0,0,0,0},
     {"auth",authCommand,2,"rsltF",0,NULL,0,0,0,0,0},
-    {"ping",pingCommand,1,"rtF",0,NULL,0,0,0,0,0},
+    {"ping",pingCommand,-1,"rtF",0,NULL,0,0,0,0,0},
     {"echo",echoCommand,2,"rF",0,NULL,0,0,0,0,0},
     {"save",saveCommand,1,"ars",0,NULL,0,0,0,0,0},
     {"bgsave",bgsaveCommand,1,"ar",0,NULL,0,0,0,0,0},
@@ -2398,8 +2398,28 @@ void authCommand(redisClient *c) {
     }
 }
 
+/* The PING command. It works in a different way if the client is in
+ * in Pub/Sub mode. */
 void pingCommand(redisClient *c) {
-    addReply(c,shared.pong);
+    /* The command takes zero or one arguments. */
+    if (c->argc > 2) {
+        addReply(c,shared.syntaxerr);
+        return;
+    }
+
+    if (c->flags & REDIS_PUBSUB) {
+        addReply(c,shared.mbulkhdr[2]);
+        addReplyBulkCBuffer(c,"pong",4);
+        if (c->argc == 1)
+            addReplyBulkCBuffer(c,"",0);
+        else
+            addReplyBulk(c,c->argv[1]);
+    } else {
+        if (c->argc == 1)
+            addReply(c,shared.pong);
+        else
+            addReplyBulk(c,c->argv[1]);
+    }
 }
 
 void echoCommand(redisClient *c) {