[WIP] HTTP::Server refactor

[asterite]

This PR changes the way HTTP::Server and HTTP::Handler work. Instead of having each handler return a Response object that provides the body, headers, etc., now each handler receives a context that has both the request and an initial response, to which you can write. This makes it super simple to stream a response. For example:

require "http/server"

server = HTTP::Server.new("0.0.0.0", 8080) do |context|
  context.response.content_type = "text/plain"
  loop do |i|
    context.response.puts i
    context.response.flush
    sleep 1
  end
end
server.listen

This will print 0, 1, 2... endlessly on the response. Try to do that with the current approach and you'll see it's really hard.

Or, imagine you want to write a JSON object directly to the response (without building it in memory first), or a CSV file. Right now it's really hard, you have to create some sort of IO from which you read the contents.

Another good thing about this change is that HTTP::Server::Context has request and response properties, but you can add more properties to it so handlers down the chain have access to these. For example a handler that sets a session property.

The downside of this change is that response headers must be set before writing the body. There's also the thing that now we have HTTP::Client::Response (returned by an HTTP::Client request) and HTTP::Server::Response, to which you can write the body. But the advantages of this change are much more important than this little downside.

/cc @ysbaddaden @jhass @sdogruyol

[sdogruyol]

This looks pretty cool! I'll give this try with Kemal middlewares.

Meanwhile how do i port something like this to new one https://github.com/sdogruyol/kemal/blob/master/src/kemal/middleware/http_basic_auth.cr ? Is passing around Context enough?

[alex-fedorov]

I like the idea! And code looks 👍

[asterite]

@sdogruyol It changes to this: https://gist.github.com/asterite/57f9d71af44573d4c0db. Basically:

1. Change the request parameter to context, and access it with context.request
2. Instead of returning an HTTP::Response object, set the response properties with context.response.

[sdogruyol]

Thanks @asterite . That's great!

This is not related to issue but how can i profile a code with Instrumentation tool on OS X? Because i'm getting performance degradation after Crystal 0.10.0 and i wanna investigate that.

[asterite]

@sdogruyol I use Instruments, selecting the Time Profiler option. https://developer.apple.com/library/mac/documentation/AnalysisTools/Reference/Instruments_User_Reference/TimeProfilerInstrument/TimeProfilerInstrument.html

[sdogruyol]

Thanks 👍

[luislavena]

Thank you @asterite for working on this!!! ❤️ ❤️ ❤️

The modification suggested here will open the door to functionality like HTTP/2 Server Push.

There is a rich discussion about this between Ilya Grigorik, Aaron Patterson and others in relation to Ruby's Rack implementation of HTTP/2 spec:

tenderlove/the_metal#5

I believe is worth taking a look since redesign of HTTP::Server interface and request/response cycle will need to accommodate HTTP/2 support in the future.