town/cmd/appendkeyfile/main.go

package main

/*

The purpose of this command is to be run via sudo as an arbitrary user by the "help" user. It is invoked as part of the "i need to add a new public key" flow from "ssh help@tilde.town".

It's based on the createkeyfile helper and heavily copy pasta'd. They should probably share code or be a single command but I wanted to keep things simple for now.

*/

import (
	"fmt"
	"os"
	"os/user"
	"path"
)

const keyfileName = "authorized_keys2"

func quit(msg string, code int) {
	// TODO print to stderr
	fmt.Println(msg)
	os.Exit(code)
}

func main() {
	username := os.Args[1]
	if username == "" {
		quit("expected username as argument", 1)
	}

	u, err := user.Current()
	if err != nil {
		quit(err.Error(), 2)
	}

	if u.Username != username {
		quit("that's my purse; I don't know you", 3)
	}

	sshPath := path.Join("/home", u.Username, ".ssh")
	keyfilePath := path.Join(sshPath, keyfileName)

	f, err := os.OpenFile(keyfilePath, os.O_APPEND|os.O_WRONLY, 0600)
	if err != nil {
		quit(fmt.Sprintf("failed to open %s: %s", keyfilePath, err.Error()), 5)
	}

	defer f.Close()

	stdin := make([]byte, 90000) // arbitrary limit

	n, err := os.Stdin.Read(stdin)
	if err != nil {
		quit(err.Error(), 6)
	} else if n == 0 {
		quit("nothing passed on STDIN", 7)
	}

	stdin = stdin[0:n]

	n, err = f.Write(stdin)
	if err != nil {
		quit(err.Error(), 9)
	} else if n == 0 {
		quit("wrote nothing to keyfile", 10)
	}
}