Add release notes to commit message

commit_msg.go

package main

import (
	"bytes"
	"fmt"
	"io/ioutil"
	"os"
	"regexp"
)

func hookCommitMsg(args []string) {
	if len(args) != 1 {
		fmt.Println("usage: commit-msg message.txt\n")
		return
	}

	file := args[0]
	oldData, err := ioutil.ReadFile(file)
	if err != nil {
		fmt.Println("%v", err)
		return
	}

	data := append([]byte{}, oldData...)
	data = stripComments(data)

	// Empty message not allowed.
	if len(bytes.TrimSpace(data)) == 0 {
		return
	}

	// Insert a blank line between first line and subsequent lines if not present.
	eol := bytes.IndexByte(data, '\n')
	if eol != -1 && len(data) > eol+1 && data[eol+1] != '\n' {
		data = append(data, 0)
		copy(data[eol+1:], data[eol:])
		data[eol+1] = '\n'
	}

	// Complain if two release notes are present.
	// This can happen during an interactive rebase;
	// it is easy to forget to remove one of them.
	numRelNotes := bytes.Count(data, []byte("\n```release-note"))
	if numRelNotes > 1 {
		fmt.Println("multiple release-note lines")
		return
	}

	// Add release-note to commit message if not present.
	if numRelNotes == 0 {
		data = append(data, []byte("```release-note\nNONE\n```")...)
	}

	if !bytes.Equal(data, oldData) {
		if err := ioutil.WriteFile(file, data, 0666); err != nil {
			fmt.Println("%v", err)
			return
		}
	}
}

func stripComments(in []byte) []byte {
	return regexp.MustCompile(`(?m)^#.*\n`).ReplaceAll(in, nil)
}

func main() {
	hookCommitMsg(os.Args[1:])
}