package consul

import (
	"context"
	"errors"
	"fmt"
	"math"
	"strings"
	"time"

	"github.com/armon/go-metrics"
	"github.com/armon/go-metrics/prometheus"
	"github.com/hashicorp/go-hclog"

	"github.com/hashicorp/consul/agent/connect"
	"github.com/hashicorp/consul/logging"
)

var metricsKeyMeshRootCAExpiry = []string{"mesh", "active-root-ca", "expiry"}
var metricsKeyMeshActiveSigningCAExpiry = []string{"mesh", "active-signing-ca", "expiry"}

var LeaderCertExpirationGauges = []prometheus.GaugeDefinition{
	{
		Name: metricsKeyMeshRootCAExpiry,
		Help: "Seconds until the service mesh root certificate expires. Updated every hour",
	},
	{
		Name: metricsKeyMeshActiveSigningCAExpiry,
		Help: "Seconds until the service mesh signing certificate expires. Updated every hour",
	},
}

func rootCAExpiryMonitor(s *Server) CertExpirationMonitor {
	return CertExpirationMonitor{
		Key:    metricsKeyMeshRootCAExpiry,
		Logger: s.logger.Named(logging.Connect),
		Query: func() (time.Duration, error) {
			return getRootCAExpiry(s)
		},
	}
}

func getRootCAExpiry(s *Server) (time.Duration, error) {
	state := s.fsm.State()
	_, root, err := state.CARootActive(nil)
	switch {
	case err != nil:
		return 0, fmt.Errorf("failed to retrieve root CA: %w", err)
	case root == nil:
		return 0, fmt.Errorf("no active root CA")
	}

	return time.Until(root.NotAfter), nil
}

func signingCAExpiryMonitor(s *Server) CertExpirationMonitor {
	return CertExpirationMonitor{
		Key:    metricsKeyMeshActiveSigningCAExpiry,
		Logger: s.logger.Named(logging.Connect),
		Query: func() (time.Duration, error) {
			if s.caManager.isIntermediateUsedToSignLeaf() {
				return getActiveIntermediateExpiry(s)
			}
			return getRootCAExpiry(s)
		},
	}
}

func getActiveIntermediateExpiry(s *Server) (time.Duration, error) {
	state := s.fsm.State()
	_, root, err := state.CARootActive(nil)
	switch {
	case err != nil:
		return 0, fmt.Errorf("failed to retrieve root CA: %w", err)
	case root == nil:
		return 0, fmt.Errorf("no active root CA")
	}

	// the CA used in a secondary DC is the active intermediate,
	// which is the last in the IntermediateCerts stack
	if len(root.IntermediateCerts) == 0 {
		return 0, errors.New("no intermediate available")
	}
	cert, err := connect.ParseCert(root.IntermediateCerts[len(root.IntermediateCerts)-1])
	if err != nil {
		return 0, err
	}
	return time.Until(cert.NotAfter), nil
}

type CertExpirationMonitor struct {
	Key []string
	// Labels to be emitted along with the metric. It is very important that these
	// labels be included in the pre-declaration as well. Otherwise, if
	// telemetry.prometheus_retention_time is less than certExpirationMonitorInterval
	// then the metrics will expire before they are emitted again.
	Labels []metrics.Label
	Logger hclog.Logger
	// Query is called at each interval. It should return the duration until the
	// certificate expires, or an error if the query failed.
	Query func() (time.Duration, error)
}

const certExpirationMonitorInterval = time.Hour

func (m CertExpirationMonitor) Monitor(ctx context.Context) error {
	ticker := time.NewTicker(certExpirationMonitorInterval)
	defer ticker.Stop()

	logger := m.Logger.With("metric", strings.Join(m.Key, "."))

	emitMetric := func() {
		d, err := m.Query()
		if err != nil {
			logger.Warn("failed to emit certificate expiry metric", "error", err)
			return
		}

		if d < 24*time.Hour {
			logger.Warn("certificate will expire soon",
				"time_to_expiry", d, "expiration", time.Now().Add(d))
		}

		expiry := d / time.Second
		metrics.SetGaugeWithLabels(m.Key, float32(expiry), m.Labels)
	}

	// emit the metric immediately so that if a cert was just updated the
	// new metric will be updated to the new expiration time.
	emitMetric()

	for {
		select {
		case <-ctx.Done():
			// "Zero-out" the metric on exit so that when prometheus scrapes this
			// metric from a non-leader, it does not get a stale value.
			metrics.SetGaugeWithLabels(m.Key, float32(math.NaN()), m.Labels)
			return nil
		case <-ticker.C:
			emitMetric()
		}
	}
}

// initLeaderMetrics sets all metrics that are emitted only on leaders to a NaN
// value so that they don't incorrectly report 0 when a server starts as a
// follower.
func initLeaderMetrics() {
	for _, g := range LeaderCertExpirationGauges {
		metrics.SetGaugeWithLabels(g.Name, float32(math.NaN()), g.ConstLabels)
	}
}