/*
Copyright 2017 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package config

import (
	"os"
	"sync"
	"time"

	"github.com/sirupsen/logrus"
)

// Agent watches a path and automatically loads the config stored
// therein.
type Agent struct {
	sync.Mutex
	c             *Config
	subscriptions []chan<- ConfigDelta
}

// Start will begin polling the config file at the path. If the first load
// fails, Start with return the error and abort. Future load failures will log
// the failure message but continue attempting to load.
func (ca *Agent) Start(prowConfig, jobConfig string) error {
	c, err := Load(prowConfig, jobConfig)
	if err != nil {
		return err
	}
	ca.Set(c)
	go func() {
		var lastModTime time.Time
		// Rarely, if two changes happen in the same second, mtime will
		// be the same for the second change, and an mtime-based check would
		// fail. Reload periodically just in case.
		skips := 0
		for range time.Tick(1 * time.Second) {
			if skips < 600 {
				// Check if the file changed to see if it needs to be re-read.
				// os.Stat follows symbolic links, which is how ConfigMaps work.
				prowStat, err := os.Stat(prowConfig)
				if err != nil {
					logrus.WithField("prowConfig", prowConfig).WithError(err).Error("Error loading prow config.")
					continue
				}

				recentModTime := prowStat.ModTime()

				// TODO(krzyzacy): allow empty jobConfig till fully migrate config to subdirs
				if jobConfig != "" {
					jobConfigStat, err := os.Stat(jobConfig)
					if err != nil {
						logrus.WithField("jobConfig", jobConfig).WithError(err).Error("Error loading job configs.")
						continue
					}

					if jobConfigStat.ModTime().After(recentModTime) {
						recentModTime = jobConfigStat.ModTime()
					}
				}

				if !recentModTime.After(lastModTime) {
					skips++
					continue // file hasn't been modified
				}
				lastModTime = recentModTime
			}
			if c, err := Load(prowConfig, jobConfig); err != nil {
				logrus.WithField("prowConfig", prowConfig).
					WithField("jobConfig", jobConfig).
					WithError(err).Error("Error loading config.")
			} else {
				skips = 0
				ca.Set(c)
			}
		}
	}()
	return nil
}

type ConfigDelta struct {
	Before, After Config
}

// Subscribe registers the channel for messages on config reload.
// The caller can expect a copy of the previous and current config
// to be sent down the subscribed channel when a new configuration
// is loaded.
func (ca *Agent) Subscribe(subscription chan<- ConfigDelta) {
	ca.Lock()
	defer ca.Unlock()
	ca.subscriptions = append(ca.subscriptions, subscription)
}

// Config returns the latest config. Do not modify the config.
func (ca *Agent) Config() *Config {
	ca.Lock()
	defer ca.Unlock()
	return ca.c
}

// Set sets the config. Useful for testing.
func (ca *Agent) Set(c *Config) {
	ca.Lock()
	defer ca.Unlock()
	var oldConfig Config
	if ca.c != nil {
		oldConfig = *ca.c
	}
	delta := ConfigDelta{oldConfig, *c}
	for _, subscription := range ca.subscriptions {
		// we can't let unbuffered channels for subscriptions lock us up
		// here, so we will send events best-effort into the channels we have
		go func(out chan<- ConfigDelta) { out <- delta }(subscription)
	}
	ca.c = c
}