Show updated plugin manifests and available upgrades (#457)

* Show information about updated plugin during update command

* Refactor code following suggestions from review

* Improve code following suggestions from review

- Do not raise an error if no index was found before an update
- Do not print updated plugins output if no index was found before an update
- Split output lines on chuncks of 5 plugins
- Fix complains from linter

cmd/krew/cmd/update.go

@@ -15,15 +15,21 @@
 package cmd
 
 import (
+	"bytes"
 	"fmt"
+	"io"
 	"os"
+	"strings"
 
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
 	"k8s.io/klog"
 
 	"sigs.k8s.io/krew/internal/gitutil"
+	"sigs.k8s.io/krew/internal/index/indexscanner"
+	"sigs.k8s.io/krew/internal/installation"
 	"sigs.k8s.io/krew/pkg/constants"
+	"sigs.k8s.io/krew/pkg/index"
 )
 
 // updateCmd represents the update command
@@ -41,12 +47,94 @@ Remarks:
 	RunE: ensureIndexUpdated,
 }
 
+func showFormattedPluginsInfo(out io.Writer, header string, plugins []string) {
+	var b bytes.Buffer
+	b.WriteString(fmt.Sprintf("  %s: ", header))
+
+	chunkSize := 5
+	var s []string
+	for i := 0; i < len(plugins); i += chunkSize {
+		end := i + chunkSize
+		if end > len(plugins) {
+			end = len(plugins)
+		}
+		s = append(s, strings.Join(plugins[i:end], ", "))
+	}
+
+	b.WriteString(strings.Join(s, "\n\t"))
+	fmt.Fprintln(out, b.String())
+}
+
+func showUpdatedPlugins(out io.Writer, preUpdate, posUpdate []index.Plugin, installedPlugins map[string]string) {
+	var newPlugins []index.Plugin
+	var updatedPlugins []index.Plugin
+
+	oldIndex := make(map[string]index.Plugin)
+	for _, p := range preUpdate {
+		oldIndex[p.Name] = p
+	}
+
+	for _, p := range posUpdate {
+		old, ok := oldIndex[p.Name]
+		if !ok {
+			newPlugins = append(newPlugins, p)
+			continue
+		}
+
+		if _, ok := installedPlugins[p.Name]; !ok {
+			continue
+		}
+
+		if old.Spec.Version != p.Spec.Version {
+			updatedPlugins = append(updatedPlugins, p)
+		}
+	}
+
+	if len(newPlugins) > 0 {
+		var s []string
+		for _, p := range newPlugins {
+			s = append(s, fmt.Sprintf("%s %s", p.Name, p.Spec.Version))
+		}
+
+		showFormattedPluginsInfo(out, "New plugins available", s)
+	}
+
+	if len(updatedPlugins) > 0 {
+		var s []string
+		for _, p := range updatedPlugins {
+			old := oldIndex[p.Name]
+			s = append(s, fmt.Sprintf("%s %s -> %s", p.Name, old.Spec.Version, p.Spec.Version))
+		}
+
+		showFormattedPluginsInfo(out, "Upgrades available", s)
+	}
+}
+
 func ensureIndexUpdated(_ *cobra.Command, _ []string) error {
+	preUpdateIndex, _ := indexscanner.LoadPluginListFromFS(paths.IndexPluginsPath())
+
 	klog.V(1).Infof("Updating the local copy of plugin index (%s)", paths.IndexPath())
 	if err := gitutil.EnsureUpdated(constants.IndexURI, paths.IndexPath()); err != nil {
 		return errors.Wrap(err, "failed to update the local index")
 	}
 	fmt.Fprintln(os.Stderr, "Updated the local copy of plugin index.")
+
+	if len(preUpdateIndex) == 0 {
+		return nil
+	}
+
+	posUpdateIndex, err := indexscanner.LoadPluginListFromFS(paths.IndexPluginsPath())
+	if err != nil {
+		return errors.Wrap(err, "failed to load plugin index after update")
+	}
+
+	installedPlugins, err := installation.ListInstalledPlugins(paths.InstallReceiptsPath())
+	if err != nil {
+		return errors.Wrap(err, "failed to load installed plugins list after update")
+	}
+
+	showUpdatedPlugins(os.Stderr, preUpdateIndex, posUpdateIndex, installedPlugins)
+
 	return nil
 }