server: Add support for KEYS cmd to list topics

[papanikge]

This commit adds a new cmd in the likes of the redis KEYS command.
It is used to list all the topics of the cluster in their fully qualified names.

Note: Normal redis KEYS command takes one argument which matches
it as a glob against all keyspace. We do indeed accept an argument for
the sake of compatibility but we ignore it.

Usage example:

localhost:6380> keys topics:
1) "topics:greetings"
2) "topics:__consumer_offsets"
3) "topics:another-topic"

[dtheodor]

Looks fine

[agis]

For the sake of semantics, I believe we should only accept topics: as the argument. We do the same for DEL.

[papanikge]

Do you mean performing the command only in the case the first arg is topics:? ok sounds good, since we do it elsewhere too

[agis]

Any reason this isn't []string?

[papanikge]

It returns:

./server.go:169:36: cannot use topic_names (type []string) as type []interface {} in argument to writer.WriteObjects

I think it has to do with the internal implementation of interfaces, the only thing I found is this article and the relevant part is:

By running this, you can see that we encounter the following error: cannot use names (type []string) as type []interface {} in function argument. If we want to actually make that work, we would have to convert the []string to an []interface{}:

package main

import (
    "fmt"
)

func PrintAll(vals []interface{}) {
    for _, val := range vals {
        fmt.Println(val)
    }
}

func main() {
    names := []string{"stanley", "david", "oscar"}
    vals := make([]interface{}, len(names))
    for i, v := range names {
        vals[i] = v
    }
    PrintAll(vals)
}
Run it here:http://play.golang.org/p/Dhg1YS6BJS

That’s pretty ugly, but c'est la vie. Not everything is perfect. (in reality, this doesn’t come up very often, because []interface{} turns out to be less useful than you would initially expect)

At first I had the topic_names` var as a slice of strings as you say, and has something to "unwrap" the strings and convert them to interfaces but we discussed it a bit with @dtheodor and we converged on just using an interface slice.

Feel free to clarify it more for me, and/or suggest anything else, cause I'm a beginner in go :)

[agis]

Ah I see. Fair enough. A more detailed answer on this can be found here.

[agis]

How about a simpler and more consistent with redis conventions:

List all topics

We should also use the same phrasing in the README.

[papanikge]

agreed, will change.

[agis]

You can simplify this by returning when/if the topic is found. Otherwise, fail. So no need for found.

[papanikge]

@agis comments addressed.. :)

[agis]

As a sidenote, we can just do a WriteObjects() when secmask/go-redisproto#2 is merged.

[agis]

@papanikge Can you please rebase on top of master?

[papanikge]

@agis rebased and squashed. sorry for the delay..

[dtheodor]

The test failure seems legit

[papanikge]

oh yes, that' mine I forgot to change the test to account for the new prefix, sorry about that. On it.