Skip reinstalling existing plugins

This change introduces logic to skip installing plugins that already
exist, unless --reinstall is explicitly passed to `pulumi plugin install`.

cmd/plugin_install.go

@@ -18,6 +18,7 @@ import (
 func newPluginInstallCmd() *cobra.Command {
 	var cloudURL string
 	var file string
+	var reinstall bool
 	var cmd = &cobra.Command{
 		Use:   "install [KIND NAME VERSION]",
 		Args:  cmdutil.MaximumNArgs(3),
@@ -63,19 +64,26 @@ func newPluginInstallCmd() *cobra.Command {
 
 			// Target the cloud URL for downloads.
 			var releases cloud.Backend
-			if len(installs) > 0 && file != "" {
+			if len(installs) > 0 && file == "" {
 				releases = cloud.New(cmdutil.Diag(), cloud.ValueOrDefaultURL(cloudURL))
 			}
 
 			// Now for each kind, name, version pair, download it from the release website, and install it.
 			for _, install := range installs {
+				// If the plugin already exists, don't download it unless --reinstall was passed.
+				if !reinstall && workspace.HasPlugin(install) {
+					continue
+				}
+
+				// If we got here, actually try to do the download.
 				var source string
 				var tarball io.ReadCloser
 				var err error
 				if file == "" {
 					source = releases.CloudURL()
 					if tarball, err = releases.DownloadPlugin(install, true); err != nil {
-						return errors.Wrapf(err, "downloading %s from %s", install.String(), source)
+						return errors.Wrapf(err,
+							"downloading %s plugin %s from %s", install.Kind, install.String(), source)
 					}
 				} else {
 					source = file
@@ -96,6 +104,8 @@ func newPluginInstallCmd() *cobra.Command {
 		"cloud-url", "c", "", "A cloud URL to download releases from")
 	cmd.PersistentFlags().StringVarP(&file,
 		"file", "f", "", "Install a plugin from a tarball file, instead of downloading it")
+	cmd.PersistentFlags().BoolVar(&reinstall,
+		"reinstall", false, "Reinstall a plugin even if it already exists")
 
 	return cmd
 }

pkg/workspace/plugins.go

@@ -12,6 +12,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strings"
 	"time"
 
@@ -35,11 +36,24 @@ type PluginInfo struct {
 	LastUsedTime time.Time       // the last time the plugin was used.
 }
 
+// File gets the expected filename for this plugin.
+func (info PluginInfo) File() string {
+	return info.FilePrefix() + info.FileSuffix()
+}
+
 // FilePrefix gets the expected default file prefix for the plugin.
 func (info PluginInfo) FilePrefix() string {
 	return filePrefix(info.Kind, info.Name, info.Version)
 }
 
+// FileSuffix returns the suffix for the plugin (if any).
+func (info PluginInfo) FileSuffix() string {
+	if runtime.GOOS == "windows" {
+		return ".exe"
+	}
+	return ""
+}
+
 // filePrefix gets the expected default file prefix for the plugin.
 func filePrefix(kind PluginKind, name string, version *semver.Version) string {
 	prefix := fmt.Sprintf("pulumi-%s-%s", kind, name)
@@ -49,6 +63,15 @@ func filePrefix(kind PluginKind, name string, version *semver.Version) string {
 	return prefix
 }
 
+// DefaultPath returns the path where this plugin is normally installed to.
+func (info PluginInfo) DefaultPath() (string, error) {
+	dir, err := GetPluginDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, info.File()), nil
+}
+
 // Delete removes the plugin from the cache.  It also deletes any supporting files in the cache, which includes
 // any files that contain the same prefix as the plugin itself.
 func (info PluginInfo) Delete() error {
@@ -133,6 +156,18 @@ func IsPluginKind(k string) bool {
 	}
 }
 
+// HasPlugin returns true if the given plugin exists.
+func HasPlugin(plug PluginInfo) bool {
+	path, err := plug.DefaultPath()
+	if err == nil {
+		_, err := os.Stat(path)
+		if err == nil {
+			return true
+		}
+	}
+	return false
+}
+
 // GetPluginDir returns the directory in which plugins on the current machine are managed.
 func GetPluginDir() (string, error) {
 	home, err := homedir.Dir()