Inside kubectl plugin, prompt for input? - go

Question

I'm writing a kubectl plugin to authenticate users, and I would like to prompt the user for a password after the plugin is invoked. From what I understand, it's fairly trivial to get input from STDIN, but I'm struggling seeing messages written to STDOUT. Currently my code looks like this:
In cmd/kubectl-myauth.go:
// This is mostly boilerplate, but it's needed for the MRE
// https://stackoverflow.com/help/minimal-reproducible-example
package myauth
import (...)
func main() {
pflag.CommandLine = pflag.NewFlagSet("kubectl-myauth", pflag.ExitOnError)
root := cmd.NewCmdAuthOp(genericclioptions.IOStreams{In: os.Stdin, Out: os.Stdout, ErrOut: os.Stderr})
if err := root.Execute(); err != nil {
os.Exit(1)
}
}
In pkg/cmd/auth.go:
package cmd
...
type AuthOpOptions struct {
configFlags *genericclioptions.ConfigFlags
resultingContext *api.Context
rawConfig api.Config
args []string
...
genericclioptions.IOStreams
}
func NewAuthOpOptions(streams genericclioptions.IOStreams) *AuthOpOptions {
return &AuthOpOptions{
configFlags: genericclioptions.NewConfigFlags(true),
IOStreams: streams,
}
}
func NewCmdAuthOp(streams genericclioptions.IOStreams) *cobra.Command {
o := NewAuthOpOptions(streams)
cmd := &cobra.Command{
RunE: func(c *cobra.Command, args []string) error {
return o.Run()
},
}
return cmd
}
func (o *AuthOpOptions) Run() error {
pass, err := getPassword(o)
if err != nil {
return err
}
// Do Auth Stuff
// Eventually print an ExecCredential to STDOUT
return nil
}
func getPassword(o *AuthOpOptions) (string, error) {
var reader *bufio.Reader
reader = nil
pass := ""
for pass == "" {
// THIS IS AN IMPORTANT LINE [1]
fmt.Fprintf(o.IOStreams.Out, "Password with which to authenticate:\n")
// THE REST OF THIS IS STILL IMPORTANT, BUT LESS SO [2]
if reader == nil {
// The first time through, initialize the reader
reader = bufio.NewReader(o.IOStreams.In)
}
pass, err := reader.ReadString('\n')
if err != nil {
return nil, err
}
pass = strings.Trim(pass, "\r\n")
if pass == "" {
// ALSO THIS LINE IS IMPORTANT [3]
fmt.Fprintf(o.IOStreams.Out, `Read password was empty string.
Please input a valid password.
`)
}
}
return pass, nil
}
This works the way that I expect when running from outside of the kubectl context - namely, it prints the string, prompts for input, and continues. However, from inside the kubectl context, I believe the print between the first two all-caps comments ([1] and [2]) is being swallowed by kubectl listening on STDOUT. I can get around this by printing to STDERR, but that feels... wrong. Is there a way that I can bypass kubectl's consumption of STDOUT to communicate with the user?
TL;DR: kubectl appears to be swallowing all of STDOUT for kubectl plugins, but I want to prompt the user for input - is there a simple way to do this?

Answer 1

Sorry I have no better answer than "Works for me" :-) Here are the steps:
git clone https://github.com/kubernetes/kubernetes.git
duplicate sample-cli-plugin as test-cli-plugin (this involves fixing import-restrictions.yaml, rules-godeps.yaml and rules.yaml under staging/publishing - maybe not necessary, but it's safer this way)
change kubectl-ns.go to kubectl-test.go:
package main
import (
"os"
"github.com/spf13/pflag"
"k8s.io/cli-runtime/pkg/genericclioptions"
"k8s.io/test-cli-plugin/pkg/cmd"
)
func main() {
flags := pflag.NewFlagSet("kubectl-test", pflag.ExitOnError)
pflag.CommandLine = flags
root := cmd.NewCmdTest(genericclioptions.IOStreams{In: os.Stdin,
Out: os.Stdout,
ErrOut: os.Stderr})
if err := root.Execute(); err != nil {
os.Exit(1)
}
}
change ns.go to test.go:
package cmd
import (
"fmt"
"os"
"github.com/spf13/cobra"
"k8s.io/cli-runtime/pkg/genericclioptions"
)
type TestOptions struct {
configFlags *genericclioptions.ConfigFlags
genericclioptions.IOStreams
}
func NewTestOptions(streams genericclioptions.IOStreams) *TestOptions {
return &TestOptions{
configFlags: genericclioptions.NewConfigFlags(true),
IOStreams: streams,
}
}
func NewCmdTest(streams genericclioptions.IOStreams) *cobra.Command {
o := NewTestOptions(streams)
cmd := &cobra.Command{
Use: "test",
Short: "Test plugin",
SilenceUsage: true,
RunE: func(c *cobra.Command, args []string) error {
o.Run()
return nil
},
}
return cmd
}
func (o *TestOptions) Run() error {
fmt.Fprintf(os.Stderr, "Testing Fprintf Stderr\n")
fmt.Fprintf(os.Stdout, "Testing Fprintf Stdout\n")
fmt.Printf("Testing Printf\n")
fmt.Fprintf(o.IOStreams.Out, "Testing Fprintf o.IOStreams.Out\n")
return nil
}
fix BUILD files accordingly
build the plugin
run make
copy kubectl-test to /usr/local/bin
run the compiled kubectl binary:
~/k8s/_output/bin$ ./kubectl test
Testing Fprintf Stderr
Testing Fprintf Stdout
Testing Printf
Testing Fprintf o.IOStreams.Out