Release note:

The plugin mechanism functionality now closely follows the git plugin design

Replace the existing plugin mechanism with the design proposed in kubernetes/community#2437.

The full implementation of the plugin mechanism itself is entirely contained within the first commit.

Walkthrough

Under the new design, there is no plugin installation or loading required to use plugins.
A plugin is simply any executable file on a user's PATH whose name begins with kubectl-.

• Plugins receive the inherited environment from the kubectl binary. All environment variables
  accessible by kubectl become accessible by the plugin.
• Plugins decide which command path they wish to implement based on their name. For example, a plugin wanting to provide a new command foo, would simply be named kubectl-foo.

Creating a plugin

Below is an example plugin, that we will use for this walkthrough. Plugins may be written in any language, and handle arguments and flags in any way, optionally (as a convention) providing a way to retrieve their version via a version subcommand.

#!/bin/bash

# optional argument handling
if [[ "$1" == "version" ]]
then
    echo "1.0.0"
    exit 0
fi

# optional argument handling
if [[ "$1" == "config" ]]
then
    echo $KUBECONFIG
    exit 0
fi

echo "I am a plugin named kubectl-foo"

Using a plugin

To use a plugin, simply make it executable:

sudo chmod +x ./kubectl-foo

and place it anywhere in your PATH:

sudo mv ./kubectl-foo /usr/local/bin

You may now invoke your plugin as a kubectl command:

$ kubectl foo
I am a plugin named kubectl-foo

All args and flags are passed as-is to the executable:

$ kubectl foo version
1.0.0

All environment variables are also passed as-is to the executable:

$ export KUBECONFIG=~/.kube/config
$ kubectl foo config
/home/<user>/.kube/config

$ KUBECONFIG=/etc/kube/config kubectl foo config
/etc/kube/config

Additionally, the first argument that is passed to a plugin will always be the full path to the location where it was invoked ($0 would equal /usr/local/bin/kubectl-foo in our example above).

Plugin discoverability

Seeing as how the kubectl plugin command is left as a no-op with this PR (perhaps it could serve as an entrypoint towards additional plugin functionality in the future), a small subcommand has been included that lists all available plugin executables on a user's PATH, along with any warnings it finds.

Example usage of this new subcommand is included below:

$ kubectl plugin list
The following kubectl-compatible plugins are available:

test/fixtures/pkg/kubectl/plugins/kubectl-foo
plugins/kubectl-foo
  - warning: plugins/kubectl-foo is overshadowed by a similarly named plugin: test/fixtures/pkg/kubectl/plugins/kubectl-foo
plugins/kubectl-invalid
  - warning: plugins/kubectl-invalid identified as a kubectl plugin, but it is not executable
plugins/kubectl-bar

error: 2 plugin warnings were found

cc @kubernetes/kubectl-maintainers @kubernetes/sig-cli-pr-reviews @soltysh @seans3 @mengqiy