package gcplog

import (
	"context"
	"io"
	"sync"
	"time"

	"cloud.google.com/go/pubsub"
	"github.com/go-kit/log"
	"github.com/go-kit/log/level"
	"github.com/grafana/dskit/backoff"
	"github.com/prometheus/common/model"
	"github.com/prometheus/prometheus/model/relabel"
	"google.golang.org/api/option"

	"github.com/grafana/loki/v3/clients/pkg/promtail/api"
	"github.com/grafana/loki/v3/clients/pkg/promtail/scrapeconfig"
	"github.com/grafana/loki/v3/clients/pkg/promtail/targets/target"
)

var defaultBackoff = backoff.Config{
	MinBackoff: 1 * time.Second,
	MaxBackoff: 10 * time.Second,
	MaxRetries: 0, // Retry forever
}

// pubsubSubscription allows us to mock pubsub for testing
type pubsubSubscription interface {
	Receive(ctx context.Context, f func(context.Context, *pubsub.Message)) error
}

// pullTarget represents the target specific to GCP project, with a pull subscription type.
// It collects logs from GCP and push it to Loki.
// nolint:revive
type pullTarget struct {
	metrics       *Metrics
	logger        log.Logger
	handler       api.EntryHandler
	config        *scrapeconfig.GcplogTargetConfig
	relabelConfig []*relabel.Config
	jobName       string

	// lifecycle management
	ctx     context.Context
	cancel  context.CancelFunc
	wg      sync.WaitGroup
	backoff *backoff.Backoff

	// pubsub
	ps   io.Closer
	sub  pubsubSubscription
	msgs chan *pubsub.Message
}

// newPullTarget returns the new instance of pullTarget for
// the given `project-id`. It scraps logs from the GCP project
// and push it Loki via given `api.EntryHandler.`
// It starts the `run` loop to consume log entries that can be
// stopped via `target.Stop()`
// nolint:revive,govet
func newPullTarget(
	metrics *Metrics,
	logger log.Logger,
	handler api.EntryHandler,
	relabel []*relabel.Config,
	jobName string,
	config *scrapeconfig.GcplogTargetConfig,
	clientOptions ...option.ClientOption,
) (*pullTarget, error) {
	ctx, cancel := context.WithCancel(context.Background())
	ps, err := pubsub.NewClient(ctx, config.ProjectID, clientOptions...)
	if err != nil {
		cancel()
		return nil, err
	}

	target := &pullTarget{
		metrics:       metrics,
		logger:        logger,
		handler:       handler,
		relabelConfig: relabel,
		config:        config,
		jobName:       jobName,
		ctx:           ctx,
		cancel:        cancel,
		ps:            ps,
		sub:           ps.SubscriptionInProject(config.Subscription, config.ProjectID),
		backoff:       backoff.New(ctx, defaultBackoff),
		msgs:          make(chan *pubsub.Message),
	}

	go func() {
		_ = target.run()
	}()

	return target, nil
}

func (t *pullTarget) run() error {
	t.wg.Add(1)
	defer t.wg.Done()

	go t.consumeSubscription()

	for {
		select {
		case <-t.ctx.Done():
			return t.ctx.Err()
		case m := <-t.msgs:
			entry, err := parseGCPLogsEntry(m.Data, t.config.Labels, nil, t.config.UseIncomingTimestamp, t.config.UseFullLine, t.relabelConfig)
			if err != nil {
				level.Error(t.logger).Log("event", "error formating log entry", "cause", err)
				m.Ack()
				break
			}
			t.handler.Chan() <- entry
			m.Ack() // Ack only after log is sent.
			t.metrics.gcplogEntries.WithLabelValues(t.config.ProjectID).Inc()
		}
	}
}

func (t *pullTarget) consumeSubscription() {
	// NOTE(kavi): `cancel` the context as exiting from this goroutine should stop main `run` loop
	// It makesense as no more messages will be received.
	defer t.cancel()

	for t.backoff.Ongoing() {
		err := t.sub.Receive(t.ctx, func(ctx context.Context, m *pubsub.Message) {
			t.msgs <- m
			t.backoff.Reset()
		})
		if err != nil {
			level.Error(t.logger).Log("msg", "failed to receive pubsub messages", "error", err)
			t.metrics.gcplogErrors.WithLabelValues(t.config.ProjectID).Inc()
			t.metrics.gcplogTargetLastSuccessScrape.WithLabelValues(t.config.ProjectID, t.config.Subscription).SetToCurrentTime()
			t.backoff.Wait()
		}
	}
}

func (t *pullTarget) Type() target.TargetType {
	return target.GcplogTargetType
}

func (t *pullTarget) Ready() bool {
	// Return true just like all other targets.
	// Rationale is gcplog scraping shouldn't stop because of some transient timeout errors.
	// This transient failure can cause promtail readyness probe to fail which may prevent pod from starting.
	// We have metrics now to track if scraping failed (`gcplog_target_last_success_scrape`).
	return true
}

func (t *pullTarget) DiscoveredLabels() model.LabelSet {
	return nil
}

func (t *pullTarget) Labels() model.LabelSet {
	return t.config.Labels
}

func (t *pullTarget) Details() interface{} {
	return nil
}

func (t *pullTarget) Stop() error {
	t.cancel()
	t.wg.Wait()
	t.handler.Stop()
	t.ps.Close()
	return nil
}