add new appendkeyfile helper

cmd/appendkeyfile/main.go

@@ -0,0 +1,67 @@
+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.
+
+*/
+
+import (
+	"fmt"
+	"os"
+	"os/user"
+	"path"
+)
+
+const keyfileName = "authorized_keys2"
+
+func quit(msg string, code int) {
+	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)
+	}
+}