Incorrect PONG issued on subscriber channel

[mgravell]

The PING command is one of only a few that are permitted once a connection has entered subscription mode; however, it has a different semantic:

If the client is subscribed to a channel or a pattern, it will instead return a multi-bulk with a "pong" in the first position and an empty bulk in the second position, unless an argument is provided in which case it returns a copy of the argument.

A typical reply might therefore be:

*2
$4
pong
$0

However, Garnet issues instead:

+PONG

To repro, try (looking in the obvious directory):

using StackExchange.Redis;
using StackExchange.Redis.Configuration;

var options = ConfigurationOptions.Parse("127.0.0.1:3278");
LoggingTunnel.LogToDirectory(options, @"c:\Code\RedisLog");
using var muxer = ConnectionMultiplexer.Connect(options);
var sub = muxer.GetSubscriber();
var ttl = sub.Ping();
Console.WriteLine(ttl);

This may impact clients that aren't expecting it; I think SE.Redis gracefully survives this, but I haven't checked for niche cases.

It may also be worth checking whether Garnet is responding to any other commands in subscriber mode; there are very few things it should allow while there are subscriptions assuming RESP2:

• PING (with different behavior)
• [P|S][UN]SUBSCRIBE
• RESET
• QUIT