command: allow callers to activate a plugin cache

Here we add a new caller-settable field to command.Meta that activates
a read-through cache directory for plugin installation.

command/init.go

@@ -71,10 +71,11 @@ func (c *InitCommand) Run(args []string) int {
 		c.getPlugins = false
 	}
 
-	// set getProvider if we don't have a test version already
+	// set providerInstaller if we don't have a test version already
 	if c.providerInstaller == nil {
 		c.providerInstaller = &discovery.ProviderInstaller{
-			Dir: c.pluginDir(),
+			Dir:   c.pluginDir(),
+			Cache: c.pluginCache(),
 			PluginProtocolVersion: plugin.Handshake.ProtocolVersion,
 			SkipVerify:            !flagVerifyPlugins,
 			Ui:                    c.Ui,

command/meta.go

@@ -55,6 +55,10 @@ type Meta struct {
 	// the specific commands being run.
 	RunningInAutomation bool
 
+	// PluginCacheDir, if non-empty, enables caching of downloaded plugins
+	// into the given directory.
+	PluginCacheDir string
+
 	//----------------------------------------------------------
 	// Protected: commands can set these
 	//----------------------------------------------------------

command/plugins.go

@@ -168,6 +168,17 @@ func (m *Meta) pluginDirs(includeAutoInstalled bool) []string {
 	return dirs
 }
 
+func (m *Meta) pluginCache() discovery.PluginCache {
+	dir := m.PluginCacheDir
+	if dir == "" {
+		return nil // cache disabled
+	}
+
+	dir = filepath.Join(dir, pluginMachineName)
+
+	return discovery.NewLocalPluginCache(dir)
+}
+
 // providerPluginSet returns the set of valid providers that were discovered in
 // the defined search paths.
 func (m *Meta) providerPluginSet() discovery.PluginMetaSet {