cli-plugins: add concept of experimental plugin, only enabled in experimental mode

To test, add $(pwd)/build/plugins-linux-amd64 to "cliPluginsExtraDirs" config and run:
make plugins
make binary
HELLO_EXPERIMENTAL=1 docker helloworld

To show it enabled:
HELLO_EXPERIMENTAL=1 DOCKER_CLI_EXPERIMENTAL=enabled docker helloworld

Signed-off-by: Tibor Vass <tibor@docker.com>

Author: Tibor Vass

cli-plugins/examples/helloworld/main.go

@@ -101,5 +101,6 @@ func main() {
 			SchemaVersion: "0.1.0",
 			Vendor:        "Docker Inc.",
 			Version:       "testing",
+			Experimental:  os.Getenv("HELLO_EXPERIMENTAL") != "",
 		})
 }

cli-plugins/manager/manager.go

@@ -117,7 +117,7 @@ func ListPlugins(dockerCli command.Cli, rootcmd *cobra.Command) ([]Plugin, error
 			continue
 		}
 		c := &candidate{paths[0]}
-		p, err := newPlugin(c, rootcmd)
+		p, err := newPlugin(c, rootcmd, dockerCli.ClientInfo().HasExperimental)
 		if err != nil {
 			return nil, err
 		}
@@ -159,13 +159,12 @@ func PluginRunCommand(dockerCli command.Cli, name string, rootcmd *cobra.Command
 		}
 
 		c := &candidate{path: path}
-		plugin, err := newPlugin(c, rootcmd)
+		plugin, err := newPlugin(c, rootcmd, dockerCli.ClientInfo().HasExperimental)
 		if err != nil {
 			return nil, err
 		}
 		if plugin.Err != nil {
-			// TODO: why are we not returning plugin.Err?
-			return nil, errPluginNotFound(name)
+			return nil, plugin.Err
 		}
 		cmd := exec.Command(plugin.Path, args...)
 		// Using dockerCli.{In,Out,Err}() here results in a hang until something is input.

cli-plugins/manager/metadata.go

@@ -22,4 +22,6 @@ type Metadata struct {
 	ShortDescription string `json:",omitempty"`
 	// URL is a pointer to the plugin's homepage.
 	URL string `json:",omitempty"`
+	// Experimental specifies whether the plugin is experimental
+	Experimental bool `json:",omitempty"`
 }

cli-plugins/manager/plugin.go

@@ -33,7 +33,7 @@ type Plugin struct {
 // is set, and is always a `pluginError`, but the `Plugin` is still
 // returned with no error. An error is only returned due to a
 // non-recoverable error.
-func newPlugin(c Candidate, rootcmd *cobra.Command) (Plugin, error) {
+func newPlugin(c Candidate, rootcmd *cobra.Command, allowExperimental bool) (Plugin, error) {
 	path := c.Path()
 	if path == "" {
 		return Plugin{}, errors.New("plugin candidate path cannot be empty")
@@ -94,6 +94,10 @@ func newPlugin(c Candidate, rootcmd *cobra.Command) (Plugin, error) {
 		p.Err = wrapAsPluginError(err, "invalid metadata")
 		return p, nil
 	}
+	if p.Experimental && !allowExperimental {
+		p.Err = NewPluginError("plugin candidate %q: requires experimental enabled", p.Name)
+		return p, nil
+	}
 
 	if p.Metadata.SchemaVersion != "0.1.0" {
 		p.Err = NewPluginError("plugin SchemaVersion %q is not valid, must be 0.1.0", p.Metadata.SchemaVersion)