gun_pool feedback

[eproxus]

While looking at the new gun_pool implementation I had some questions / feedback:

1. A pool is created with Host + Port but APIs like info/1 take an argument Authority instead. How is one supposed to compute this if you want to e.g. start a pool and then get its info (without having to store the pid somewhere)?
2. Looking at how Authority is calculated internally, it seems that the same authority will be created for both Host + 80 and Host + 443, e.g. start_pool("example.com", 80) and start_pool("example.com", 443) will both generate a value of "example.com". Is this correct? Does Gun start a pool that can handle both TCP and TLS requests, or will it break if called with the type which it wasn't started with?
3. The pool lookup when performing a request looks for the hardcoded <<"host">> header, but an HTTP header can have any case so this fails if the user supplies e.g. <<"Host">>. Probably some case conversion has to be done here?
4. There is a weird mismatch between the types of the Host argument when creating a pool (which has to be an inet string) and the <<"host">> header which can be a binary. As a user it feels odd when they have an implicit connection but they're different types. In my option, a better API would allow binaries for the Host argument and internally make it a list if needed by inet.

[essen]

1. Right I don't think a function is publicly available for this currently. The relevant code to compute it is in https://github.com/ninenines/gun/blob/master/src/gun_http.erl#L667-L679 and should probably be made available publicly.
2. Yes it's correct. I'm not sure how it will behave in that case. That's a good edge case to look for. If it can't work it can just reject the second pool. If both 80 and 443 are necessary separate scopes can be used anyway. There's more possible edge cases related to tunnels and whatnot.
3. Gun expects header names provided by the user to be all lowercase, like Cowboy. Conversion is only done on header names coming from the network. Lowercase is required in HTTP/2+.
4. Authority and TCP host/port are not the same concept though? Authority (provided in the host header) is an HTTP concept not directly related to the real TCP host/port. In fact some users may want to use a different authority than what they connect to, for example connect to localhost but use example.org as authority. Or use multiple authorities with the same server (if the server has multiple authorities configured). Right now we don't have an option to override the pool used but we could accept the pid when it is known.

[eproxus]

I'm not sure I follow. With the current API it would be impossible to send a request to localhost with the host header set to example.org? (because it would select the pool going to example.org or if it doesn't exist, return an error?)

[essen]

If you connect to localhost explicitly and want to set the host header to example.org it's not going to find the pool yes.

[eproxus]

I find it quite unintuitive that pools are created with a TCP/TLS host + port tuple, but then accessed via the host header which is not the same thing at all, as you correctly point out.

[eproxus]

Another issue I founds is that it seems gun_pool sets its own event handler in the conn_opts. If one wants to collect metrics from connections via a custom event handler, that is currently not possible? I.e. the pool implementation will replace any user event handler with its own internal one?

[essen]

It depends on how many requests you do I guess. It is not an expensive call on its own.

[eproxus]

Which error is returned when all streams are busy in the pool? When trying to use it we sometimes get {error, {stream_error, closed}}, does this mean the pool is busy or is it something else?

[essen]

I guess you got a connection that ended up closing before you could do something with it, or while doing something. It happens. You may get other errors in that scenario. What should be done in that case is try again. The pool cannot guarantee that connections stay up.

[eproxus]

That sounds likely, yes. How can we detect if the pool itself is out of free connections/streams?

[essen]

If you mean when doing requests you should hit this path: https://github.com/ninenines/gun/blob/master/src/gun_pool.erl#L367-L368