feat(util/http_client): Adds generic HTTP client

Supports retries with exponential backoff as well as connection
timeouts and the ability to skip SSL/TLS verification.

This commit also refactors datasource and initialize packages
in order to use the new HTTP client.

datasource/datasource.go

@@ -1,104 +1,6 @@
 package datasource
 
-import (
-	"errors"
-	"fmt"
-	"io/ioutil"
-	"log"
-	"math"
-	"net"
-	"net/http"
-	neturl "net/url"
-	"strings"
-	"time"
-)
-
-const (
-	HTTP_2xx = 2
-	HTTP_4xx = 4
-
-	maxTimeout = time.Second * 5
-	maxRetries = 15
-)
-
 type Datasource interface {
 	Fetch() ([]byte, error)
 	Type() string
 }
-
-// HTTP client timeout
-// This one is low since exponential backoff will kick off too.
-var timeout = time.Duration(2) * time.Second
-
-func dialTimeout(network, addr string) (net.Conn, error) {
-	deadline := time.Now().Add(timeout)
-	c, err := net.DialTimeout(network, addr, timeout)
-	if err != nil {
-		return nil, err
-	}
-	c.SetDeadline(deadline)
-	return c, nil
-}
-
-// Fetches user-data url with support for exponential backoff and maximum retries
-func fetchURL(rawurl string) ([]byte, error) {
-	if rawurl == "" {
-		return nil, errors.New("user-data URL is empty. Skipping.")
-	}
-
-	url, err := neturl.Parse(rawurl)
-	if err != nil {
-		return nil, err
-	}
-
-	// Unfortunately, url.Parse is too generic to throw errors if a URL does not
-	// have a valid HTTP scheme. So, we have to do this extra validation
-	if !strings.HasPrefix(url.Scheme, "http") {
-		return nil, fmt.Errorf("user-data URL %s does not have a valid HTTP scheme. Skipping.", rawurl)
-	}
-
-	userdataURL := url.String()
-
-	// We need to create our own client in order to add timeout support.
-	// TODO(c4milo) Replace it once Go 1.3 is officially used by CoreOS
-	// More info: https://code.google.com/p/go/source/detail?r=ada6f2d5f99f
-	transport := &http.Transport{
-		Dial: dialTimeout,
-	}
-
-	client := &http.Client{
-		Transport: transport,
-	}
-
-	for retry := 1; retry <= maxRetries; retry++ {
-		log.Printf("Fetching user-data from %s. Attempt #%d", userdataURL, retry)
-
-		resp, err := client.Get(userdataURL)
-
-		if err == nil {
-			defer resp.Body.Close()
-			status := resp.StatusCode / 100
-
-			if status == HTTP_2xx {
-				return ioutil.ReadAll(resp.Body)
-			}
-
-			if status == HTTP_4xx {
-				return nil, fmt.Errorf("user-data not found. HTTP status code: %d", resp.StatusCode)
-			}
-
-			log.Printf("user-data not found. HTTP status code: %d", resp.StatusCode)
-		} else {
-			log.Printf("unable to fetch user-data: %s", err.Error())
-		}
-
-		duration := time.Millisecond * time.Duration((math.Pow(float64(2), float64(retry)) * 100))
-		if duration > maxTimeout {
-			duration = maxTimeout
-		}
-
-		time.Sleep(duration)
-	}
-
-	return nil, fmt.Errorf("unable to fetch user-data. Maximum retries reached: %d", maxRetries)
-}