terraform cloudinit merge example

main.go

package main

import (
	"fmt"

	"github.com/imdario/mergo"
	"gopkg.in/yaml.v2"
)

type UserData struct {
	SSHKeys []string `yaml:"ssh_authorized_keys,omitempty"`
}

// Convert a UserData instance to a map with string as key and interface as value
func convertUserDataToMap(data UserData) (map[string]interface{}, error) {
	userDataMap := make(map[string]interface{})

	// This is required to get the right names expected by cloud-init
	// For example: SSHKeys -> ssh_authorized_keys
	tmp, err := yaml.Marshal(&data)
	if err != nil {
		return userDataMap, err
	}

	err = yaml.Unmarshal([]byte(tmp), &userDataMap)
	return userDataMap, err
}

func convertUserDataRawToMap(raw string) (map[string]interface{}, error) {
	m := make(map[string]interface{})
	err := yaml.Unmarshal([]byte(raw), &m)
	return m, err
}

func mergeUserDataIntoUserDataRaw(userData UserData, userDataRaw string) {
	userDataMap, err := convertUserDataToMap(userData)
	if err != nil {
		fmt.Printf("error1: %v\n", err)
	}
	fmt.Printf("UserData: %+v\n", userDataMap)

	userDataRawMap, err := convertUserDataRawToMap(userDataRaw)
	if err != nil {
		fmt.Printf("error2: %v\n", err)
	}
	fmt.Printf("UserDataRaw: %+v\n", userDataRawMap)

	if err = mergo.Merge(&userDataRawMap, userDataMap); err != nil {
		fmt.Printf("error3: %v\n", err)
	}
	fmt.Printf("\nMerge result: %+v\n", userDataRawMap)
}

func main() {
	userDataWithSSHKeys := UserData{
		SSHKeys: []string{"key1"},
	}
	userDataEmpty := UserData{}

	var userDataRaw = `
new-key: new-value-set-by-extra
ssh_authorized_keys:
  - key2-from-extra-data
`
	var userDataRawNoSSH = `
new-key: new-value-set-by-extra
`

	mergeUserDataIntoUserDataRaw(userDataWithSSHKeys, userDataRaw)
	fmt.Println("---")
	mergeUserDataIntoUserDataRaw(userDataWithSSHKeys, userDataRawNoSSH)
	fmt.Println("---")
	mergeUserDataIntoUserDataRaw(userDataEmpty, userDataRaw)
	fmt.Println("---")
	mergeUserDataIntoUserDataRaw(userDataEmpty, userDataRawNoSSH)
}

This will produce the following output:

UserData: map[ssh_authorized_keys:[key1]]
UserDataRaw: map[ssh_authorized_keys:[key2-from-extra-data] new-key:new-value-set-by-extra]

Merge result: map[new-key:new-value-set-by-extra ssh_authorized_keys:[key2-from-extra-data]]
---
UserData: map[ssh_authorized_keys:[key1]]
UserDataRaw: map[new-key:new-value-set-by-extra]

Merge result: map[new-key:new-value-set-by-extra ssh_authorized_keys:[key1]]
---
UserData: map[]
UserDataRaw: map[new-key:new-value-set-by-extra ssh_authorized_keys:[key2-from-extra-data]]

Merge result: map[new-key:new-value-set-by-extra ssh_authorized_keys:[key2-from-extra-data]]
---
UserData: map[]
UserDataRaw: map[new-key:new-value-set-by-extra]

Merge result: map[new-key:new-value-set-by-extra]