// Copyright 2016 The Prometheus 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 marathon

import (
	"encoding/json"
	"fmt"
	"io/ioutil"
	"math/rand"
	"net"
	"net/http"
	"strconv"
	"strings"
	"time"

	"golang.org/x/net/context"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/common/log"
	"github.com/prometheus/common/model"
	"github.com/prometheus/prometheus/config"
	"github.com/prometheus/prometheus/util/httputil"
	"github.com/prometheus/prometheus/util/strutil"
)

const (
	// metaLabelPrefix is the meta prefix used for all meta labels in this discovery.
	metaLabelPrefix = model.MetaLabelPrefix + "marathon_"
	// appLabelPrefix is the prefix for the application labels.
	appLabelPrefix = metaLabelPrefix + "app_label_"

	// appLabel is used for the name of the app in Marathon.
	appLabel model.LabelName = metaLabelPrefix + "app"
	// imageLabel is the label that is used for the docker image running the service.
	imageLabel model.LabelName = metaLabelPrefix + "image"
	// portIndexLabel is the integer port index when multiple ports are defined;
	// e.g. PORT1 would have a value of '1'
	portIndexLabel model.LabelName = metaLabelPrefix + "port_index"
	// taskLabel contains the mesos task name of the app instance.
	taskLabel model.LabelName = metaLabelPrefix + "task"

	// portMappingLabelPrefix is the prefix for the application portMappings labels.
	portMappingLabelPrefix = metaLabelPrefix + "port_mapping_label_"
	// portDefinitionLabelPrefix is the prefix for the application portDefinitions labels.
	portDefinitionLabelPrefix = metaLabelPrefix + "port_definition_label_"

	// Constants for instrumentation.
	namespace = "prometheus"
)

var (
	refreshFailuresCount = prometheus.NewCounter(
		prometheus.CounterOpts{
			Namespace: namespace,
			Name:      "sd_marathon_refresh_failures_total",
			Help:      "The number of Marathon-SD refresh failures.",
		})
	refreshDuration = prometheus.NewSummary(
		prometheus.SummaryOpts{
			Namespace: namespace,
			Name:      "sd_marathon_refresh_duration_seconds",
			Help:      "The duration of a Marathon-SD refresh in seconds.",
		})
)

func init() {
	prometheus.MustRegister(refreshFailuresCount)
	prometheus.MustRegister(refreshDuration)
}

const appListPath string = "/v2/apps/?embed=apps.tasks"

// Discovery provides service discovery based on a Marathon instance.
type Discovery struct {
	client          *http.Client
	servers         []string
	refreshInterval time.Duration
	lastRefresh     map[string]*config.TargetGroup
	appsClient      AppListClient
	token           string
	logger          log.Logger
}

// NewDiscovery returns a new Marathon Discovery.
func NewDiscovery(conf *config.MarathonSDConfig, logger log.Logger) (*Discovery, error) {
	tls, err := httputil.NewTLSConfig(conf.TLSConfig)
	if err != nil {
		return nil, err
	}

	token := string(conf.BearerToken)
	if conf.BearerTokenFile != "" {
		bf, err := ioutil.ReadFile(conf.BearerTokenFile)
		if err != nil {
			return nil, err
		}
		token = strings.TrimSpace(string(bf))
	}

	client := &http.Client{
		Timeout: time.Duration(conf.Timeout),
		Transport: &http.Transport{
			TLSClientConfig: tls,
		},
	}

	return &Discovery{
		client:          client,
		servers:         conf.Servers,
		refreshInterval: time.Duration(conf.RefreshInterval),
		appsClient:      fetchApps,
		token:           token,
		logger:          logger,
	}, nil
}

// Run implements the TargetProvider interface.
func (d *Discovery) Run(ctx context.Context, ch chan<- []*config.TargetGroup) {
	for {
		select {
		case <-ctx.Done():
			return
		case <-time.After(d.refreshInterval):
			err := d.updateServices(ctx, ch)
			if err != nil {
				d.logger.Errorf("Error while updating services: %s", err)
			}
		}
	}
}

func (d *Discovery) updateServices(ctx context.Context, ch chan<- []*config.TargetGroup) (err error) {
	t0 := time.Now()
	defer func() {
		refreshDuration.Observe(time.Since(t0).Seconds())
		if err != nil {
			refreshFailuresCount.Inc()
		}
	}()

	targetMap, err := d.fetchTargetGroups()
	if err != nil {
		return err
	}

	all := make([]*config.TargetGroup, 0, len(targetMap))
	for _, tg := range targetMap {
		all = append(all, tg)
	}

	select {
	case <-ctx.Done():
		return ctx.Err()
	case ch <- all:
	}

	// Remove services which did disappear.
	for source := range d.lastRefresh {
		_, ok := targetMap[source]
		if !ok {
			select {
			case <-ctx.Done():
				return ctx.Err()
			case ch <- []*config.TargetGroup{{Source: source}}:
				d.logger.Debugf("Removing group for %s", source)
			}
		}
	}

	d.lastRefresh = targetMap
	return nil
}

func (d *Discovery) fetchTargetGroups() (map[string]*config.TargetGroup, error) {
	url := RandomAppsURL(d.servers)
	apps, err := d.appsClient(d.client, url, d.token)
	if err != nil {
		return nil, err
	}

	groups := AppsToTargetGroups(apps)
	return groups, nil
}

// Task describes one instance of a service running on Marathon.
type Task struct {
	ID    string   `json:"id"`
	Host  string   `json:"host"`
	Ports []uint32 `json:"ports"`
}

// PortMappings describes in which port the process are binding inside the docker container.
type PortMappings struct {
	Labels map[string]string `json:"labels"`
}

// DockerContainer describes a container which uses the docker runtime.
type DockerContainer struct {
	Image        string         `json:"image"`
	PortMappings []PortMappings `json:"portMappings"`
}

// Container describes the runtime an app in running in.
type Container struct {
	Docker DockerContainer `json:"docker"`
}

// PortDefinitions describes which load balancer port you should access to access the service.
type PortDefinitions struct {
	Labels map[string]string `json:"labels"`
}

// App describes a service running on Marathon.
type App struct {
	ID              string            `json:"id"`
	Tasks           []Task            `json:"tasks"`
	RunningTasks    int               `json:"tasksRunning"`
	Labels          map[string]string `json:"labels"`
	Container       Container         `json:"container"`
	PortDefinitions []PortDefinitions `json:"portDefinitions"`
}

// AppList is a list of Marathon apps.
type AppList struct {
	Apps []App `json:"apps"`
}

// AppListClient defines a function that can be used to get an application list from marathon.
type AppListClient func(client *http.Client, url, token string) (*AppList, error)

// fetchApps requests a list of applications from a marathon server.
func fetchApps(client *http.Client, url, token string) (*AppList, error) {
	request, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return nil, err
	}

	// According to  https://dcos.io/docs/1.8/administration/id-and-access-mgt/managing-authentication
	// DC/OS wants with "token=" a different Authorization header than implemented in httputil/client.go
	// so we set this implicitly here
	if token != "" {
		request.Header.Set("Authorization", "token="+token)
	}

	resp, err := client.Do(request)
	if err != nil {
		return nil, err
	}

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}

	return parseAppJSON(body)
}

func parseAppJSON(body []byte) (*AppList, error) {
	apps := &AppList{}
	err := json.Unmarshal(body, apps)
	if err != nil {
		return nil, err
	}
	return apps, nil
}

// RandomAppsURL randomly selects a server from an array and creates
// an URL pointing to the app list.
func RandomAppsURL(servers []string) string {
	// TODO: If possible update server list from Marathon at some point.
	server := servers[rand.Intn(len(servers))]
	return fmt.Sprintf("%s%s", server, appListPath)
}

// AppsToTargetGroups takes an array of Marathon apps and converts them into target groups.
func AppsToTargetGroups(apps *AppList) map[string]*config.TargetGroup {
	tgroups := map[string]*config.TargetGroup{}
	for _, a := range apps.Apps {
		group := createTargetGroup(&a)
		tgroups[group.Source] = group
	}
	return tgroups
}

func createTargetGroup(app *App) *config.TargetGroup {
	var (
		targets = targetsForApp(app)
		appName = model.LabelValue(app.ID)
		image   = model.LabelValue(app.Container.Docker.Image)
	)
	tg := &config.TargetGroup{
		Targets: targets,
		Labels: model.LabelSet{
			appLabel:   appName,
			imageLabel: image,
		},
		Source: app.ID,
	}

	for ln, lv := range app.Labels {
		ln = appLabelPrefix + strutil.SanitizeLabelName(ln)
		tg.Labels[model.LabelName(ln)] = model.LabelValue(lv)
	}

	return tg
}

func targetsForApp(app *App) []model.LabelSet {
	targets := make([]model.LabelSet, 0, len(app.Tasks))
	for _, t := range app.Tasks {
		if len(t.Ports) == 0 {
			continue
		}
		for i := 0; i < len(t.Ports); i++ {
			targetAddress := targetForTask(&t, i)
			target := model.LabelSet{
				model.AddressLabel: model.LabelValue(targetAddress),
				taskLabel:          model.LabelValue(t.ID),
				portIndexLabel:     model.LabelValue(strconv.Itoa(i)),
			}
			if i < len(app.PortDefinitions) {
				for ln, lv := range app.PortDefinitions[i].Labels {
					ln = portDefinitionLabelPrefix + strutil.SanitizeLabelName(ln)
					target[model.LabelName(ln)] = model.LabelValue(lv)
				}
			}
			if i < len(app.Container.Docker.PortMappings) {
				for ln, lv := range app.Container.Docker.PortMappings[i].Labels {
					ln = portMappingLabelPrefix + strutil.SanitizeLabelName(ln)
					target[model.LabelName(ln)] = model.LabelValue(lv)
				}
			}
			targets = append(targets, target)
		}
	}
	return targets
}

func targetForTask(task *Task, index int) string {
	return net.JoinHostPort(task.Host, fmt.Sprintf("%d", task.Ports[index]))
}