Implements Kn plugins re-using some code from kubectl plugins.

This version contains the following:

1. wraps the main root Kn command to support plugin
2. plugins are any executable in kn's config new pluginDir
   variable which defaults to $PATH
3. plugins must have name kn-*
4. 'kn plugin list' sub-command to list found kn plugins
5. skips any kn plugins found with name that match core
   commands, e.g., kn-service would be ignored
6. can execute any valid kn plugins found, e.g.,
   `kn valid` where the plugin file `kn-valid` is in path
   specified in 2.
7. unit tests (using gotest.tools)

And is missing:

1. integration tests
2. plugin install command
3. plugin repository command
4. plugin / Knative server version negotiation
5. anything else we agree on in plugin req doc

I plan to create issues for the things missing so we don't
end up with an even bigger PR. It's already big as is but is a
good MVP as per plugins requirement doc.

cmd/kn/main.go

@@ -19,12 +19,22 @@ import (
 	"os"
 
 	"github.com/knative/client/pkg/kn/core"
+	"github.com/spf13/viper"
 )
 
+func init() {
+	core.InitializeConfig()
+}
+
 func main() {
-	err := core.NewKnCommand().Execute()
+	defer cleanup()
+	err := core.NewDefaultKnCommand().Execute()
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
 }
+
+func cleanup() {
+	viper.WriteConfig()
+}

docs/cmd/kn.md

@@ -16,11 +16,13 @@ Eventing: Manage event subscriptions and channels. Connect up event sources.
       --config string       config file (default is $HOME/.kn/config.yaml)
   -h, --help                help for kn
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO
 
 * [kn completion](kn_completion.md)	 - Output shell completion code (default Bash)
+* [kn plugin](kn_plugin.md)	 - Plugin command group
 * [kn revision](kn_revision.md)	 - Revision command group
 * [kn route](kn_route.md)	 - Route command group
 * [kn service](kn_service.md)	 - Service command group

docs/cmd/kn_completion.md

@@ -22,6 +22,7 @@ kn completion [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_plugin.md

@@ -0,0 +1,34 @@
+## kn plugin
+
+Plugin command group
+
+### Synopsis
+
+Provides utilities for interacting with kn plugins.
+
+Plugins provide extended functionality that is not part of the major command-line distribution.
+Please refer to the documentation and examples for more information about how write your own plugins.
+
+```
+kn plugin [flags]
+```
+
+### Options
+
+```
+  -h, --help   help for plugin
+```
+
+### Options inherited from parent commands
+
+```
+      --config string       config file (default is $HOME/.kn/config.yaml)
+      --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
+```
+
+### SEE ALSO
+
+* [kn](kn.md)	 - Knative client
+* [kn plugin list](kn_plugin_list.md)	 - List all visible plugin executables on a user's PATH
+

docs/cmd/kn_plugin_list.md

@@ -0,0 +1,36 @@
+## kn plugin list
+
+List all visible plugin executables on a user's PATH
+
+### Synopsis
+
+List all visible plugin executables on a user's PATH.
+
+		Available plugin files are those that are:
+		- executable
+		- anywhere on the user's PATH
+		- begin with "kn-
+
+```
+kn plugin list [flags]
+```
+
+### Options
+
+```
+  -h, --help        help for list
+      --name-only   If true, display only the binary name of each plugin, rather than its full path
+```
+
+### Options inherited from parent commands
+
+```
+      --config string       config file (default is $HOME/.kn/config.yaml)
+      --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
+```
+
+### SEE ALSO
+
+* [kn plugin](kn_plugin.md)	 - Plugin command group
+

docs/cmd/kn_revision.md

@@ -21,6 +21,7 @@ kn revision [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_revision_delete.md

@@ -30,6 +30,7 @@ kn revision delete NAME [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_revision_describe.md

@@ -25,6 +25,7 @@ kn revision describe NAME [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_revision_list.md

@@ -38,6 +38,7 @@ kn revision list [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_route.md

@@ -21,6 +21,7 @@ kn route [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_route_list.md

@@ -40,6 +40,7 @@ kn route list NAME [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_service.md

@@ -21,6 +21,7 @@ kn service [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_service_create.md

@@ -62,6 +62,7 @@ kn service create NAME --image IMAGE [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_service_delete.md

@@ -33,6 +33,7 @@ kn service delete NAME [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_service_describe.md

@@ -25,6 +25,7 @@ kn service describe NAME [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_service_list.md

@@ -26,6 +26,7 @@ kn service list [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_service_update.md

@@ -47,6 +47,7 @@ kn service update NAME [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

docs/cmd/kn_version.md

@@ -21,6 +21,7 @@ kn version [flags]
 ```
       --config string       config file (default is $HOME/.kn/config.yaml)
       --kubeconfig string   kubectl config file (default is $HOME/.kube/config)
+      --plugin-dir string   kn plugin directory (default is value in kn config or $PATH)
 ```
 
 ### SEE ALSO

pkg/kn/commands/plugin/path_verifier.go

@@ -0,0 +1,102 @@
+// Copyright © 2018 The Knative Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package plugin
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+
+	"github.com/spf13/cobra"
+)
+
+// PathVerifier receives a path and determines if it is valid or not
+type PathVerifier interface {
+	// Verify determines if a given path is valid
+	Verify(path string) []error
+}
+
+// CommandOverrideVerifier verifies that existing kn commands are not overriden
+type CommandOverrideVerifier struct {
+	Root        *cobra.Command
+	SeenPlugins map[string]string
+}
+
+// Verify implements PathVerifier and determines if a given path
+// is valid depending on whether or not it overwrites an existing
+// kn command path, or a previously seen plugin.
+func (v *CommandOverrideVerifier) Verify(path string) []error {
+	if v.Root == nil {
+		return []error{fmt.Errorf("unable to verify path with nil root")}
+	}
+
+	// extract the plugin binary name
+	segs := strings.Split(path, "/")
+	binName := segs[len(segs)-1]
+
+	cmdPath := strings.Split(binName, "-")
+	if len(cmdPath) > 1 {
+		// the first argument is always "kn" for a plugin binary
+		cmdPath = cmdPath[1:]
+	}
+
+	errors := []error{}
+	isExec, err := isExecutable(path)
+	if err == nil && !isExec {
+		errors = append(errors, fmt.Errorf("warning: %s identified as a kn plugin, but it is not executable", path))
+	} else if err != nil {
+		errors = append(errors, fmt.Errorf("error: unable to identify %s as an executable file: %v", path, err))
+	}
+
+	if existingPath, ok := v.SeenPlugins[binName]; ok {
+		errors = append(errors, fmt.Errorf("warning: %s is overshadowed by a similarly named plugin: %s", path, existingPath))
+	} else {
+		v.SeenPlugins[binName] = path
+	}
+
+	cmd, _, err := v.Root.Find(cmdPath)
+	if err == nil {
+		errors = append(errors, fmt.Errorf("warning: %s overwrites existing command: %q", binName, cmd.CommandPath()))
+	}
+
+	return errors
+}
+
+// Private functions
+
+func isExecutable(fullPath string) (bool, error) {
+	info, err := os.Stat(fullPath)
+	if err != nil {
+		return false, err
+	}
+
+	if runtime.GOOS == "windows" {
+		fileExt := strings.ToLower(filepath.Ext(fullPath))
+
+		switch fileExt {
+		case ".bat", ".cmd", ".com", ".exe", ".ps1":
+			return true, nil
+		}
+		return false, nil
+	}
+
+	if m := info.Mode(); !m.IsDir() && m&0111 != 0 {
+		return true, nil
+	}
+
+	return false, nil
+}