/*
DESCRIPTION

	watcher.go provides a tool for watching a file for modifications and
	performing an action when the file is modified.

AUTHORS

	Saxon A. Nelson-Milton <saxon@ausocean.org>

LICENSE

	Copyright (C) 2022 the Australian Ocean Lab (AusOcean)

	It is free software: you can redistribute it and/or modify them
	under the terms of the GNU General Public License as published by the
	Free Software Foundation, either version 3 of the License, or (at your
	option) any later version.

	It is distributed in the hope that it will be useful, but WITHOUT
	ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	for more details.

	You should have received a copy of the GNU General Public License
	along with revid in gpl.txt. If not, see http://www.gnu.org/licenses.
*/

package main

import (
	"fmt"
	"path"

	"bitbucket.org/ausocean/utils/logging"
	"github.com/fsnotify/fsnotify"
)

// watchFile watches a file for modifications and calls onWrite when the file
// is modified. Technically, the directory is watched instead of the file.
// This is because watching the file itself will cause problems if changes
// are done atomically.
// See fsnotify documentation:
// https://godocs.io/github.com/fsnotify/fsnotify#hdr-Watching_files
func watchFile(file string, onWrite func(), l logging.Logger) error {
	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		return fmt.Errorf("could not create watcher: %w", err)
	}

	go func() {
		for {
			select {
			case event, ok := <-watcher.Events:
				if !ok {
					l.Warning("watcher events chan closed, terminating")
					return
				}
				if event.Op&fsnotify.Write == fsnotify.Write && event.Name == file {
					l.Info("file modification event", "file", file)
					onWrite()
				}
			case err, ok := <-watcher.Errors:
				if !ok {
					l.Warning("watcher error chan closed, terminating")
					return
				}
				l.Error("file watcher error", "error", err)
			}
		}
	}()

	// Watch the directory over the file.
	err = watcher.Add(path.Dir(file))
	if err != nil {
		return fmt.Errorf("could not add file %s to watcher: %w", file, err)
	}
	return nil
}