class Falcon::Middleware::Proxy

Typically used for implementing virtual servers.
A HTTP middleware for proxying requests to a given set of hosts.

def call(request)

@parameter request [Protocol::HTTP::Request]
Proxy the request if the authority matches a specific host. 
def call(request)
	if host = lookup(request)
		@count += 1
		
		request = self.prepare_request(request, host)
		
		client = connect(host.endpoint)
		
		client.call(request)
	else
		super
	end
rescue
	Async.logger.error(self) {$!}
	return Protocol::HTTP::Response[502, {'content-type' => 'text/plain'}, ["#{$!.inspect}: #{$!.backtrace.join("\n")}"]]
end

def close

Close all the connections to the upstream hosts. 
def close
	@clients.each_value(&:close)
	
	super
end

def connect(endpoint)

@parameter endpoint [Async::HTTP::Endpoint]
Establish a connection to the specified upstream endpoint. 
def connect(endpoint)
	@clients[endpoint] ||= Async::HTTP::Client.new(endpoint)
end

def initialize(app, hosts)

@parameter hosts [Array(Service::Proxy)] The host applications to proxy to.
@parameter app [Protocol::HTTP::Middleware] The middleware to use if a request can't be proxied.
Initialize the proxy middleware. 
def initialize(app, hosts)
	super(app)
	
	@server_context = nil
	
	@hosts = hosts
	@clients = {}
	
	@count = 0
end

def lookup(request)

@returns [Service::Proxy]
@parameter request [Protocol::HTTP::Request]
Lookup the appropriate host for the given request. 
def lookup(request)
	# Trailing dot and port is ignored/normalized.
	if authority = request.authority&.sub(/(\.)?(:\d+)?$/, '')
		return @hosts[authority]
	end
end

def prepare_headers(headers)

In particular, we delete all connection and hop headers.
Prepare the headers to be sent to an upstream host. 
def prepare_headers(headers)
	if connection = headers[CONNECTION]
		headers.extract(connection)
	end
	
	headers.extract(HOP_HEADERS)
end

def prepare_request(request, host)

In particular, we set appropriate {VIA}, {FORWARDED}, {X_FORWARDED_FOR} and {X_FORWARDED_PROTO} headers.
Prepare the request to be proxied to the specified host. 
def prepare_request(request, host)
	forwarded = []
	
	Async.logger.debug(self) do |buffer|
		buffer.puts "Request authority: #{request.authority}"
		buffer.puts "Host authority: #{host.authority}"
		buffer.puts "Request: #{request.method} #{request.path} #{request.version}"
		buffer.puts "Request headers: #{request.headers.inspect}"
	end
	
	# The authority of the request must match the authority of the endpoint we are proxying to, otherwise SNI and other things won't work correctly.
	request.authority = host.authority
	
	if address = request.remote_address
		request.headers.add(X_FORWARDED_FOR, address.ip_address)
		forwarded << "for=#{address.ip_address}"
	end
	
	if scheme = request.scheme
		request.headers.add(X_FORWARDED_PROTO, scheme)
		forwarded << "proto=#{scheme}"
	end
	
	unless forwarded.empty?
		request.headers.add(FORWARDED, forwarded.join(';'))
	end
	
	request.headers.add(VIA, "#{request.version} #{self.class}")
	
	self.prepare_headers(request.headers)
	
	return request
end