package consul

import (
	"encoding/json"
	"flag"
	"fmt"
	"net/url"
	"sync"
	"time"

	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
	"github.com/VictoriaMetrics/VictoriaMetrics/lib/promscrape/discoveryutils"
	"github.com/VictoriaMetrics/metrics"
)

// SDCheckInterval is check interval for Consul service discovery.
var SDCheckInterval = flag.Duration("promscrape.consulSDCheckInterval", 30*time.Second, "Interval for checking for changes in Consul. "+
	"This works only if consul_sd_configs is configured in '-promscrape.config' file. "+
	"See https://prometheus.io/docs/prometheus/latest/configuration/configuration/#consul_sd_config for details")

// consulWatcher is a watcher for consul api, updates services map in background with long-polling.
type consulWatcher struct {
	client *discoveryutils.Client

	serviceNamesQueryArgs string
	serviceNodesQueryArgs string
	watchServices         []string
	watchTags             []string

	// servicesLock protects services
	servicesLock sync.Mutex
	services     map[string]*serviceWatcher

	wg     sync.WaitGroup
	stopCh chan struct{}
}

type serviceWatcher struct {
	serviceName  string
	serviceNodes []ServiceNode
	stopCh       chan struct{}
}

// newConsulWatcher creates new watcher and start background service discovery for Consul.
func newConsulWatcher(client *discoveryutils.Client, sdc *SDConfig, datacenter string) *consulWatcher {
	baseQueryArgs := "?dc=" + url.QueryEscape(datacenter)
	if sdc.AllowStale {
		baseQueryArgs += "&stale"
	}
	for k, v := range sdc.NodeMeta {
		baseQueryArgs += "&node-meta=" + url.QueryEscape(k+":"+v)
	}
	serviceNodesQueryArgs := baseQueryArgs
	for _, tag := range sdc.Tags {
		serviceNodesQueryArgs += "&tag=" + url.QueryEscape(tag)
	}
	cw := &consulWatcher{
		client:                client,
		serviceNamesQueryArgs: baseQueryArgs,
		serviceNodesQueryArgs: serviceNodesQueryArgs,
		watchServices:         sdc.Services,
		watchTags:             sdc.Tags,
		services:              make(map[string]*serviceWatcher),
		stopCh:                make(chan struct{}),
	}
	go cw.watchForServicesUpdates()
	return cw
}

func (cw *consulWatcher) mustStop() {
	close(cw.stopCh)
	// Do not wait for the watcher to stop, since it may take
	// up to discoveryutils.BlockingClientReadTimeout to complete.
	// TODO: add ability to cancel blocking requests.
}

// watchForServicesUpdates watches for new services and updates it in cw.
func (cw *consulWatcher) watchForServicesUpdates() {
	checkInterval := getCheckInterval()
	ticker := time.NewTicker(checkInterval / 2)
	defer ticker.Stop()
	index := int64(0)
	clientAddr := cw.client.Addr()
	f := func() {
		serviceNames, newIndex, err := cw.getBlockingServiceNames(index)
		if err != nil {
			logger.Errorf("cannot obtain Consul serviceNames from %q: %s", clientAddr, err)
			return
		}
		if index == newIndex {
			// Nothing changed.
			return
		}

		cw.servicesLock.Lock()
		// Start watchers for new services.
		for _, serviceName := range serviceNames {
			if _, ok := cw.services[serviceName]; ok {
				// The watcher for serviceName already exists.
				continue
			}
			sw := &serviceWatcher{
				serviceName: serviceName,
				stopCh:      make(chan struct{}),
			}
			cw.services[serviceName] = sw
			cw.wg.Add(1)
			serviceWatchersCreated.Inc()
			go func() {
				serviceWatchersCount.Inc()
				sw.watchForServiceNodesUpdates(cw)
				serviceWatchersCount.Dec()
				cw.wg.Done()
			}()
		}
		// Stop watchers for removed services.
		newServiceNamesMap := make(map[string]struct{}, len(serviceNames))
		for _, serviceName := range serviceNames {
			newServiceNamesMap[serviceName] = struct{}{}
		}
		for serviceName, sw := range cw.services {
			if _, ok := newServiceNamesMap[serviceName]; ok {
				continue
			}
			close(sw.stopCh)
			delete(cw.services, serviceName)
			serviceWatchersStopped.Inc()

			// Do not wait for the watcher goroutine to exit, since this may take for up to maxWaitTime
			// if it is blocked in Consul API request.
		}
		cw.servicesLock.Unlock()

		index = newIndex
	}

	logger.Infof("started Consul service watcher for %q", clientAddr)
	f()
	for {
		select {
		case <-ticker.C:
			f()
		case <-cw.stopCh:
			logger.Infof("stopping Consul service watchers for %q", clientAddr)
			startTime := time.Now()
			cw.servicesLock.Lock()
			for _, sw := range cw.services {
				close(sw.stopCh)
			}
			cw.servicesLock.Unlock()
			cw.wg.Wait()
			logger.Infof("stopped Consul service watcher for %q in %.3f seconds", clientAddr, time.Since(startTime).Seconds())
			return
		}
	}
}

var (
	serviceWatchersCreated = metrics.NewCounter("vm_promscrape_discovery_consul_service_watchers_created_total")
	serviceWatchersStopped = metrics.NewCounter("vm_promscrape_discovery_consul_service_watchers_stopped_total")
	serviceWatchersCount   = metrics.NewCounter("vm_promscrape_discovery_consul_service_watchers")
)

// getBlockingServiceNames obtains serviceNames via blocking request to Consul.
//
// It returns an empty serviceNames list if response contains the same index.
func (cw *consulWatcher) getBlockingServiceNames(index int64) ([]string, int64, error) {
	path := "/v1/catalog/services" + cw.serviceNamesQueryArgs
	data, newIndex, err := getBlockingAPIResponse(cw.client, path, index)
	if err != nil {
		return nil, index, err
	}
	if index == newIndex {
		// Nothing changed - return an empty serviceNames list.
		return nil, index, nil
	}
	var m map[string][]string
	if err := json.Unmarshal(data, &m); err != nil {
		return nil, index, fmt.Errorf("cannot parse response from %q: %w; data=%q", path, err, data)
	}
	serviceNames := make([]string, 0, len(m))
	for serviceName, tags := range m {
		if !shouldCollectServiceByName(cw.watchServices, serviceName) {
			continue
		}
		if !shouldCollectServiceByTags(cw.watchTags, tags) {
			continue
		}
		serviceNames = append(serviceNames, serviceName)
	}
	return serviceNames, newIndex, nil
}

// watchForServiceNodesUpdates watches for Consul serviceNode changes for the given serviceName.
func (sw *serviceWatcher) watchForServiceNodesUpdates(cw *consulWatcher) {
	checkInterval := getCheckInterval()
	ticker := time.NewTicker(checkInterval / 2)
	defer ticker.Stop()
	clientAddr := cw.client.Addr()
	index := int64(0)
	path := "/v1/health/service/" + sw.serviceName + cw.serviceNodesQueryArgs
	f := func() {
		data, newIndex, err := getBlockingAPIResponse(cw.client, path, index)
		if err != nil {
			logger.Errorf("cannot obtain Consul serviceNodes for serviceName=%q from %q: %s", sw.serviceName, clientAddr, err)
			return
		}
		if index == newIndex {
			// Nothing changed.
			return
		}
		sns, err := parseServiceNodes(data)
		if err != nil {
			logger.Errorf("cannot parse Consul serviceNodes response for serviceName=%q from %q: %s", sw.serviceName, clientAddr, err)
			return
		}

		cw.servicesLock.Lock()
		sw.serviceNodes = sns
		cw.servicesLock.Unlock()

		index = newIndex
	}

	f()
	for {
		select {
		case <-ticker.C:
			f()
		case <-sw.stopCh:
			return
		}
	}
}

// getServiceNodesSnapshot returns a snapshot of discovered ServiceNodes.
func (cw *consulWatcher) getServiceNodesSnapshot() []ServiceNode {
	var sns []ServiceNode
	cw.servicesLock.Lock()
	for _, sw := range cw.services {
		sns = append(sns, sw.serviceNodes...)
	}
	cw.servicesLock.Unlock()
	return sns
}

func shouldCollectServiceByName(filterServices []string, serviceName string) bool {
	if len(filterServices) == 0 {
		return true
	}
	for _, filterService := range filterServices {
		if filterService == serviceName {
			return true
		}
	}
	return false
}

func shouldCollectServiceByTags(filterTags, tags []string) bool {
	if len(filterTags) == 0 {
		return true
	}
	for _, filterTag := range filterTags {
		hasTag := false
		for _, tag := range tags {
			if tag == filterTag {
				hasTag = true
				break
			}
		}
		if !hasTag {
			return false
		}
	}
	return true
}

func getCheckInterval() time.Duration {
	d := *SDCheckInterval
	if d <= time.Second {
		return time.Second
	}
	return d
}