feat(helm): add helm update command

This resolves issue #640.

cmd/helm/update.go

+package main
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"strings"
+	"sync"
+
+	"github.com/spf13/cobra"
+
+	"github.com/kubernetes/helm/pkg/repo"
+)
+
+var verboseUpdate bool
+
+var updateCommand = &cobra.Command{
+	Use:   "update",
+	Short: "Update information on available charts in the chart repositories.",
+	RunE:  update,
+}
+
+func init() {
+	updateCommand.Flags().BoolVar(&verboseUpdate, "verbose", false, "verbose error messages")
+	RootCommand.AddCommand(updateCommand)
+}
+
+func update(cmd *cobra.Command, args []string) error {
+
+	f, err := repo.LoadRepositoriesFile(repositoriesFile())
+	if err != nil {
+		return err
+	}
+
+	updateCharts(f.Repositories, verboseUpdate)
+	return nil
+}
+
+func updateCharts(repos map[string]string, verbose bool) {
+	fmt.Println("Hang tight while we grab the latest from your chart repositories...")
+	var wg sync.WaitGroup
+	for name, url := range repos {
+		wg.Add(1)
+		go func(n, u string) {
+			defer wg.Done()
+			err := downloadCacheFile(n, u)
+			if err != nil {
+				updateErr := "...Unable to get an update from the " + n + " chart repository"
+				if verbose {
+					updateErr = updateErr + ": " + err.Error()
+				}
+				fmt.Println(updateErr)
+			} else {
+				fmt.Println("...Successfully got an update from the " + n + " chart repository")
+			}
+		}(name, url)
+	}
+	wg.Wait()
+	fmt.Println("Update Complete. Happy Helming!")
+}
+
+func downloadCacheFile(name, url string) error {
+	var cacheURL string
+
+	cacheURL = strings.TrimSuffix(url, "/") + "/cache.yaml"
+	resp, err := http.Get(cacheURL)
+	if err != nil {
+		return err
+	}
+
+	var cacheFile *os.File
+	defer resp.Body.Close()
+	cacheFile, err = os.Create(cacheDirectory(name + "-cache.yaml"))
+	if err != nil {
+		return err
+	}
+
+	if _, err := io.Copy(cacheFile, resp.Body); err != nil {
+		return err
+	}
+
+	return nil
+}