fixes(knative#814) allow plugins to extend the 'source' command group

* introduce a list of command group that can be extended (currently only source)
* check if plugin main group is in that list and execute, otherwise fail
* add e2e test for both plugin that is allowed and not allowed to extend existing group

CHANGELOG.adoc

@@ -26,6 +26,10 @@
 | Check DeleteTimestamp before updating resource
 | https://github.com/knative/client/pull/805[#805]
 
+| 🎁
+| Allow plugins to extend fixed list of command groups, currently `source`
+| https://github.com/knative/client/pull/818[#818]
+
 |===
 
 ## v0.13.2 (2020-04-15)

pkg/kn/core/root.go

@@ -43,6 +43,12 @@ import (
 	"knative.dev/client/pkg/kn/flags"
 )
 
+// AllowedExtensibleCommandGroups the list of command groups that can be
+// extended with plugins, e.g., a plugin named `kn-source-kafka` for Kafka
+// event sources is allowed. This is defined as a fixed [...]string since
+// cannot defined Golang []string constants
+var AllowedExtensibleCommandGroups = [...]string{"source"}
+
 // NewDefaultKnCommand creates the default `kn` command with a default plugin handler
 func NewDefaultKnCommand() *cobra.Command {
 	rootCmd := NewKnCommand()
@@ -81,7 +87,7 @@ func NewDefaultKnCommandWithArgs(rootCmd *cobra.Command,
 		// only look for suitable extension executables if
 		// the specified command does not already exist
 		foundCmd, innerArgs, err := rootCmd.Find(cmdPathPieces)
-		if err != nil {
+		if err != nil || inAllowedExtensibleCommandGroups(foundCmd.Name()) {
 			err := plugin.HandlePluginCommand(pluginHandler, cmdPathPieces)
 			if err != nil {
 				fmt.Fprintf(rootCmd.OutOrStderr(), "Error: unknown command '%s' \nRun 'kn --help' for usage.\n", args[1])
@@ -373,3 +379,12 @@ func showSubcommands(cmd *cobra.Command, args []string, innerArg string) string
 	}
 	return fmt.Sprintf("Error: unknown subcommand '%s' for '%s'. Available subcommands: %s\nRun 'kn --help' for usage.\n", innerArg, getCommands(args, innerArg), strings.Join(strs, ", "))
 }
+
+func inAllowedExtensibleCommandGroups(name string) bool {
+	for _, groupName := range AllowedExtensibleCommandGroups {
+		if name == groupName {
+			return true
+		}
+	}
+	return false
+}

test/e2e/plugins_test.go

@@ -167,6 +167,30 @@ func TestExecutePluginInPath(t *testing.T) {
 	runPlugin(r, knFlags, "hello2e2e", []string{}, []string{"Hello Knative, I'm a Kn plugin"})
 }
 
+func TestExecutePluginExtendingCommandGroup(t *testing.T) {
+	it, err := test.NewKnTest()
+	assert.NilError(t, err)
+
+	r := test.NewKnRunResultCollector(t, it)
+	defer r.DumpIfFailed()
+
+	pc, oldPath := setupPluginTestConfigWithNewPath(t)
+	defer tearDownWithPath(pc, oldPath)
+
+	pc.knPluginPath, err = createPluginFile("kn-source-test", TestPluginCode, pc.knPluginsDir, FileModeExecutable)
+	assert.NilError(t, err)
+
+	t.Log("execute plugin in $PATH that 'shadows' an allowed command group")
+	knFlags := []string{fmt.Sprintf("--plugins-dir=%s", pc.knPluginsDir), "--lookup-plugins=true"}
+	runPlugin(r, knFlags, "source", []string{"test"}, []string{"Hello Knative, I'm a Kn plugin"})
+
+	t.Log("failed to execute plugin in $PATH that 'shadows' a non-allowed command group")
+	knFlags = []string{fmt.Sprintf("--plugins-dir=%s", pc.knPluginsDir), "--lookup-plugins=true"}
+	runFailedPlugin(r, knFlags, "service", []string{"test"}, []string{"Hello Knative, I'm a Kn plugin"})
+}
+
+// Private
+
 func setupPluginTestConfigWithNewPath(t *testing.T) (pluginTestConfig, string) {
 	pc := pluginTestConfig{}
 	assert.NilError(t, pc.setup())
@@ -180,8 +204,6 @@ func tearDownWithPath(pc pluginTestConfig, oldPath string) {
 	pc.teardown()
 }
 
-// Private
-
 func listPlugin(r *test.KnRunResultCollector, knFlags []string, expectedPlugins []string, unexpectedPlugins []string) {
 	knArgs := append(knFlags, "plugin", "list")
 
@@ -193,12 +215,21 @@ func listPlugin(r *test.KnRunResultCollector, knFlags []string, expectedPlugins
 
 func runPlugin(r *test.KnRunResultCollector, knFlags []string, pluginName string, args []string, expectedOutput []string) {
 	knArgs := append([]string{}, knFlags...)
-	knArgs = append(knArgs, pluginName)
 	knArgs = append(knArgs, args...)
+	knArgs = append(knArgs, pluginName)
 
 	out := test.Kn{}.Run(knArgs...)
 	r.AssertNoError(out)
 	for _, output := range expectedOutput {
 		assert.Check(r.T(), util.ContainsAll(out.Stdout, output))
 	}
 }
+
+func runFailedPlugin(r *test.KnRunResultCollector, knFlags []string, pluginName string, args []string, expectedOutput []string) {
+	knArgs := append([]string{}, knFlags...)
+	knArgs = append(knArgs, args...)
+	knArgs = append(knArgs, pluginName)
+
+	out := test.Kn{}.Run(knArgs...)
+	r.AssertError(out)
+}