Factor out httpRendezvous separate from BrokerChannel.

Makes BrokerChannel abstract over a rendezvousMethod. BrokerChannel
itself is responsible for keepLocalAddresses and the NAT type state, as
well as encoding and decoding client poll messages. rendezvousMethod is
only responsible for delivery of encoded messages.

client/lib/rendezvous_http.go

@@ -0,0 +1,77 @@
+package lib
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"io/ioutil"
+	"log"
+	"net/http"
+	"net/url"
+)
+
+// httpRendezvous is a rendezvousMethod that communicates with the .../client
+// route of the broker over HTTP or HTTPS, with optional domain fronting.
+type httpRendezvous struct {
+	brokerURL *url.URL
+	front     string            // Optional front domain to replace url.Host in requests.
+	transport http.RoundTripper // Used to make all requests.
+}
+
+// newHTTPRendezvous creates a new httpRendezvous that contacts the broker at
+// the given URL, with an optional front domain. transport is the
+// http.RoundTripper used to make all requests.
+func newHTTPRendezvous(broker, front string, transport http.RoundTripper) (*httpRendezvous, error) {
+	brokerURL, err := url.Parse(broker)
+	if err != nil {
+		return nil, err
+	}
+	return &httpRendezvous{
+		brokerURL: brokerURL,
+		front:     front,
+		transport: transport,
+	}, nil
+}
+
+func (r *httpRendezvous) Exchange(encPollReq []byte) ([]byte, error) {
+	log.Println("Negotiating via HTTP rendezvous...")
+	log.Println("Target URL: ", r.brokerURL.Host)
+	log.Println("Front URL:  ", r.front)
+
+	// Suffix the path with the broker's client registration handler.
+	reqURL := r.brokerURL.ResolveReference(&url.URL{Path: "client"})
+	req, err := http.NewRequest("POST", reqURL.String(), bytes.NewReader(encPollReq))
+	if err != nil {
+		return nil, err
+	}
+
+	if r.front != "" {
+		// Do domain fronting. Replace the domain in the URL's with the
+		// front, and store the original domain the HTTP Host header.
+		req.Host = req.URL.Host
+		req.URL.Host = r.front
+	}
+
+	resp, err := r.transport.RoundTrip(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	log.Printf("HTTP rendezvous response: %s", resp.Status)
+	if resp.StatusCode != http.StatusOK {
+		return nil, errors.New(BrokerErrorUnexpected)
+	}
+
+	return limitedRead(resp.Body, readLimit)
+}
+
+func limitedRead(r io.Reader, limit int64) ([]byte, error) {
+	p, err := ioutil.ReadAll(&io.LimitedReader{R: r, N: limit + 1})
+	if err != nil {
+		return p, err
+	} else if int64(len(p)) == limit+1 {
+		return p[0:limit], io.ErrUnexpectedEOF
+	}
+	return p, err
+}