watch.go

// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

package flyscrape

import (
	"errors"
	"fmt"
	"os"
	"time"

	"github.com/fsnotify/fsnotify"
)

var StopWatch = errors.New("stop watch")

func Watch(path string, fn func(string) error) error {
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		return fmt.Errorf("creating file watcher: %w", err)
	}
	defer watcher.Close()

	if err := watcher.Add(path); err != nil {
		return fmt.Errorf("watching file %q: %w", path, err)
	}

	update := func() error {
		data, err := os.ReadFile(path)
		if err != nil {
			return err
		}
		return fn(string(data))
	}

	if err := update(); errors.Is(err, StopWatch) {
		return nil
	}

	for {
		select {
		case e, ok := <-watcher.Events:
			if !ok {
				return nil
			}
			if e.Has(fsnotify.Rename) {
				time.Sleep(10 * time.Millisecond)
				watcher.Remove(path)
				watcher.Add(path)
			}
			if e.Has(fsnotify.Write) || e.Has(fsnotify.Rename) {
				if err := update(); errors.Is(err, StopWatch) {
					return nil
				}
			}
		case err, ok := <-watcher.Errors:
			if !ok {
				return nil
			}
			if err != nil {
				return fmt.Errorf("watcher: %w", err)
			}
		}
	}
}