PING not allowed while subscribing

[michael-grunder]

This is in reference to redis/redis#420, specifically redis/redis#420 (comment)

The reason hiredis silently ignores the ping response is because it stores subscribe and psubscribe callbacks in separate dictionaries in order to deliver messages to the correct handler.

It looks for a redis publish message, which can take the following forms (with pong being new):

SUBSCRIBE	PSUBSCRIBE	PONG
1) "message"	1) pmessage	1) pong
2) "foo"	2) "foo*"	2) pongarg
3) "bar"	3) "foo"
	4) "bar"

Hiredis is only accounting for message or pmessage, and specifically just checks the first character of the first multibulk reply string to determine which one it is, which is done here. Given that pong starts with p it looks in the psubscribe dict.

It uses the second argument (which for subscribe or psubscribe will be the channel/pattern) to search, but in the case of pong can now be literally anything.

You can actually "trick" hiredis into executing your callback by doing something like this:

int main (int argc, char **argv) {
    signal(SIGPIPE, SIG_IGN);

    redisAsyncContext *c = redisAsyncConnect("127.0.0.1", 6379);

    redisLibevAttach(EV_DEFAULT_ c);
    redisAsyncSetConnectCallback(c,connectCallback);
    redisAsyncSetDisconnectCallback(c,disconnectCallback);

    redisAsyncCommand(c, subscribeCallback, NULL, "PSUBSCRIBE pong*");

    ev_loop(EV_DEFAULT_ 0);
    return 0;
}

void subscribeCallback(redisAsyncContext *c, void *r, void *privdata) {
    printf("In subscribeCallback...\n");

    redisReply *reply = r;
    int i;

    if (reply == NULL) return;

    for (i = 0; i < reply->elements; i++) {
        if (reply->element[i]->type == REDIS_REPLY_STRING) {
            printf("%i: %s\n", i, reply->element[i]->str);
        }
    }
    redisAsyncCommand(c, subscribeCallback, NULL, "PING pong*");
}

This is obviously not a solution of any kind but is what's going on. Hiredis could be made to handle the new message types by detecting "pong" specifically, and using the argument as either the channel name or pattern. This would mean that you couldn't add any other kind of context to the ping, but would allow you to use it as a mechanism to detect the connection was still live.

[echoma]

This works but a little buggy.
When the connection closed, subscribeCallback() was called many many times with r=NULL. It seems that hiredis remembers all "PING pong*" command and called them when close. I am afraid that hiredis will allocate more memory for the remember work.

[echoma]

I tried "PSUBSCRIBE pong_" instead of "PING pong_", it works fine in subscribe mode.
And in fact, you can subscribe anything as PING, I use tcpdump and can see network data when subsctibe again and again.