// Package certs obtains and renews the web UI's certificate from an ACME CA
// (Let's Encrypt), with Jarl publishing the DNS-01 challenge
// (docs/certificates.md). Before every order a preflight check proves the
// challenge can be published and seen, so a broken setup never spends the
// CA's rate limits.
package certs

import (
	"context"
	"crypto"
	"crypto/ecdsa"
	"crypto/elliptic"
	"crypto/rand"
	"crypto/tls"
	"crypto/x509"
	"encoding/json"
	"encoding/pem"
	"errors"
	"fmt"
	"log/slog"
	"net/http"
	"os"
	"regexp"
	"strings"
	"sync"
	"time"

	"github.com/go-acme/lego/v4/certificate"
	"github.com/go-acme/lego/v4/challenge/dns01"
	"github.com/go-acme/lego/v4/lego"
	legolog "github.com/go-acme/lego/v4/log"
	"github.com/go-acme/lego/v4/registration"

	"steward/internal/api"
	"steward/internal/config"
	"steward/internal/jarl"
)

// Store keeps the account and certificate (the database's meta table).
type Store interface {
	GetMeta(ctx context.Context, key string) (string, error)
	SetMeta(ctx context.Context, key, value string) error
}

const (
	metaAccount = "acme.account"
	metaCert    = "acme.certificate"
)

// Config configures the manager.
type Config struct {
	ACME    config.ACME
	Version string
}

// Status is api.CertStatus.
type Status = api.CertStatus

// Manager keeps the web UI's certificate.
type Manager struct {
	cfg   Config
	jarl  *jarl.Client
	store Store
	log   *slog.Logger
	now   func() time.Time

	mu          sync.Mutex
	cert        *tls.Certificate
	leaf        *x509.Certificate
	issuedBy    string
	working     bool
	lastErr     string
	lastAttempt time.Time
	nextAttempt time.Time
	lastCheck   *Check
	failures    int
	force       bool // renew now, even if not due (Kick)

	lego sync.Mutex // one ACME order at a time
	kick chan struct{}
}

// New makes a manager and loads the stored certificate.
func New(ctx context.Context, cfg Config, j *jarl.Client, st Store, log *slog.Logger) *Manager {
	if log == nil {
		log = slog.Default()
	}
	// lego logs each step of an order; send it to the journal with the rest.
	legolog.Logger = slog.NewLogLogger(log.With("component", "acme").Handler(), slog.LevelInfo)
	m := &Manager{cfg: cfg, jarl: j, store: st, log: log, now: time.Now, kick: make(chan struct{}, 1)}
	m.load(ctx)
	return m
}

type stored struct {
	Domain    string    `json:"domain"`
	Directory string    `json:"directory"`
	CertPEM   string    `json:"cert_pem"`
	KeyPEM    string    `json:"key_pem"`
	Obtained  time.Time `json:"obtained"`
}

func (m *Manager) load(ctx context.Context) {
	raw, err := m.store.GetMeta(ctx, metaCert)
	if err != nil || raw == "" {
		return
	}
	var s stored
	if json.Unmarshal([]byte(raw), &s) != nil {
		return
	}
	if err := m.use(s); err != nil {
		m.log.Warn("the stored certificate can't be used; getting a new one", "err", err)
	}
}

// use makes a stored certificate the one served.
func (m *Manager) use(s stored) error {
	c, err := tls.X509KeyPair([]byte(s.CertPEM), []byte(s.KeyPEM))
	if err != nil {
		return err
	}
	leaf, err := x509.ParseCertificate(c.Certificate[0])
	if err != nil {
		return err
	}
	c.Leaf = leaf
	m.mu.Lock()
	defer m.mu.Unlock()
	// A certificate for another name isn't served.
	if !strings.EqualFold(s.Domain, m.cfg.ACME.Domain) {
		m.cert, m.leaf = nil, nil
		return fmt.Errorf("it is for %s, not %s", s.Domain, m.cfg.ACME.Domain)
	}
	m.cert, m.leaf, m.issuedBy = &c, leaf, s.Directory
	return nil
}

// Certificate is the certificate to serve (nil when there is none yet).
func (m *Manager) Certificate() *tls.Certificate {
	m.mu.Lock()
	defer m.mu.Unlock()
	if m.cert == nil || m.now().After(m.leaf.NotAfter) {
		return nil
	}
	return m.cert
}

// Status describes the certificate.
func (m *Manager) Status() Status {
	m.mu.Lock()
	defer m.mu.Unlock()
	st := Status{Domain: m.cfg.ACME.Domain, Directory: m.cfg.ACME.DirectoryURL(), Staging: m.cfg.ACME.Staging(), State: "none",
		IssuedBy: m.issuedBy, LastError: m.lastErr, Check: m.lastCheck}
	if m.leaf != nil {
		nb, na := m.leaf.NotBefore, m.leaf.NotAfter
		ra := m.renewAt()
		st.NotBefore, st.NotAfter, st.RenewAt, st.Issuer, st.State = &nb, &na, &ra, m.leaf.Issuer.CommonName, "ok"
	}
	if m.lastErr != "" {
		st.State = "error"
	}
	if m.working {
		st.State = "working"
	}
	if !m.lastAttempt.IsZero() {
		t := m.lastAttempt
		st.LastAttempt = &t
	}
	if !m.nextAttempt.IsZero() {
		t := m.nextAttempt
		st.NextAttempt = &t
	}
	return st
}

// renewAt is when the certificate is renewed: a third of its lifetime
// before it ends (30 days for Let's Encrypt's 90), m.mu held.
func (m *Manager) renewAt() time.Time {
	life := m.leaf.NotAfter.Sub(m.leaf.NotBefore)
	return m.leaf.NotAfter.Add(-life / 3).UTC().Truncate(time.Second)
}

// due reports whether a new certificate is needed: none yet, another
// directory configured, or renewal time.
func (m *Manager) due() bool {
	m.mu.Lock()
	defer m.mu.Unlock()
	return m.force || m.leaf == nil || m.issuedBy != m.cfg.ACME.DirectoryURL() || !m.now().Before(m.renewAt())
}

// Run keeps the certificate: it gets one when needed, checks twice a day
// whether it is due, and retries failures with a growing wait.
func (m *Manager) Run(ctx context.Context) {
	timer := time.NewTimer(5 * time.Second)
	defer timer.Stop()
	for {
		select {
		case <-ctx.Done():
			return
		case <-m.kick:
		case <-timer.C:
		}
		wait := 12 * time.Hour
		if m.due() {
			if err := m.Renew(ctx); err != nil {
				m.mu.Lock()
				m.failures++
				// 1h, 3h, 6h, then 12h.
				waits := []time.Duration{time.Hour, 3 * time.Hour, 6 * time.Hour, 12 * time.Hour}
				wait = waits[min(m.failures, len(waits))-1]
				m.mu.Unlock()
				m.log.Warn("getting the web UI's certificate failed; trying again later", "domain", m.cfg.ACME.Domain, "err", err, "retry_in", wait)
			}
		}
		m.mu.Lock()
		m.nextAttempt = m.now().Add(wait).UTC()
		m.mu.Unlock()
		if !timer.Stop() {
			select {
			case <-timer.C:
			default:
			}
		}
		timer.Reset(wait)
	}
}

// Kick asks Run to renew now, even if the certificate isn't due.
func (m *Manager) Kick() {
	m.mu.Lock()
	m.force, m.failures = true, 0
	m.mu.Unlock()
	select {
	case m.kick <- struct{}{}:
	default:
	}
}

// ErrBusy is returned while another order runs.
var ErrBusy = errors.New("a certificate order is already running")

// Renew gets a new certificate now: the preflight check, then the order.
func (m *Manager) Renew(ctx context.Context) error {
	if !m.lego.TryLock() {
		return ErrBusy
	}
	defer m.lego.Unlock()
	m.mu.Lock()
	m.working, m.lastAttempt = true, m.now().UTC()
	m.mu.Unlock()
	err := m.renew(ctx)
	m.mu.Lock()
	m.working, m.force = false, false
	if err != nil {
		m.lastErr = err.Error()
	} else {
		m.lastErr, m.failures = "", 0
	}
	m.mu.Unlock()
	return err
}

func (m *Manager) renew(ctx context.Context) error {
	c := m.Preflight(ctx)
	if !c.OK {
		for _, s := range c.Steps {
			if s.Status == "fail" {
				return fmt.Errorf("the check before asking the CA failed at %s: %s", strings.ToLower(s.Name), s.Detail)
			}
		}
	}
	m.log.Info("the check passed; asking the CA for a certificate", "domain", m.cfg.ACME.Domain, "directory", m.cfg.ACME.DirectoryURL())
	res, err := m.obtain(ctx)
	if err != nil {
		return err
	}
	s := stored{Domain: m.cfg.ACME.Domain, Directory: m.cfg.ACME.DirectoryURL(), CertPEM: string(res.Certificate), KeyPEM: string(res.PrivateKey), Obtained: m.now().UTC()}
	if err := m.use(s); err != nil {
		return fmt.Errorf("the CA's certificate can't be used: %w", err)
	}
	b, _ := json.Marshal(s)
	if err := m.store.SetMeta(ctx, metaCert, string(b)); err != nil {
		return err
	}
	m.mu.Lock()
	na := m.leaf.NotAfter
	m.mu.Unlock()
	m.log.Info("got a new certificate for the web UI", "domain", s.Domain, "until", na.Format(time.RFC3339), "staging", m.cfg.ACME.Staging())
	return nil
}

// ---- ACME

type account struct {
	Directory string `json:"directory"`
	Email     string `json:"email,omitempty"`
	URI       string `json:"uri,omitempty"`
	KeyPEM    string `json:"key_pem"`
}

type legoUser struct {
	email string
	reg   *registration.Resource
	key   crypto.PrivateKey
}

func (u *legoUser) GetEmail() string                        { return u.email }
func (u *legoUser) GetRegistration() *registration.Resource { return u.reg }
func (u *legoUser) GetPrivateKey() crypto.PrivateKey        { return u.key }

func (m *Manager) httpClient() (*http.Client, error) {
	tr := http.DefaultTransport.(*http.Transport).Clone()
	tr.TLSClientConfig = &tls.Config{MinVersion: tls.VersionTLS12}
	if f := m.cfg.ACME.CABundle; f != "" {
		pemData, err := os.ReadFile(f)
		if err != nil {
			return nil, fmt.Errorf("tls.acme.ca_bundle: %w", err)
		}
		pool, err := x509.SystemCertPool()
		if err != nil || pool == nil {
			pool = x509.NewCertPool()
		}
		if !pool.AppendCertsFromPEM(pemData) {
			return nil, fmt.Errorf("tls.acme.ca_bundle: no certificates in %s", f)
		}
		tr.TLSClientConfig.RootCAs = pool
	}
	return &http.Client{Timeout: 2 * time.Minute, Transport: tr}, nil
}

// client returns a lego client for the account, registering it if needed.
func (m *Manager) client(ctx context.Context) (*lego.Client, error) {
	dir := m.cfg.ACME.DirectoryURL()
	hc, err := m.httpClient()
	if err != nil {
		return nil, err
	}
	user := &legoUser{email: m.cfg.ACME.Email}
	var a account
	if raw, _ := m.store.GetMeta(ctx, metaAccount); raw != "" && json.Unmarshal([]byte(raw), &a) == nil && a.Directory == dir && a.KeyPEM != "" {
		b, _ := pem.Decode([]byte(a.KeyPEM))
		if b == nil {
			return nil, errors.New("the stored ACME account key isn't PEM")
		}
		if user.key, err = x509.ParsePKCS8PrivateKey(b.Bytes); err != nil {
			return nil, fmt.Errorf("the stored ACME account key: %w", err)
		}
		if a.URI != "" {
			user.reg = &registration.Resource{URI: a.URI}
		}
	} else {
		if user.key, err = ecdsa.GenerateKey(elliptic.P256(), rand.Reader); err != nil {
			return nil, err
		}
	}
	cfg := lego.NewConfig(user)
	cfg.CADirURL, cfg.HTTPClient, cfg.UserAgent = dir, hc, "steward/"+m.cfg.Version
	cfg.Certificate.Timeout = 90 * time.Second
	c, err := lego.NewClient(cfg)
	if err != nil {
		return nil, fmt.Errorf("the ACME server %s: %w", dir, err)
	}
	if user.reg != nil {
		return c, nil
	}
	if !m.cfg.ACME.AgreeTOS {
		tos := c.GetToSURL()
		if tos == "" {
			tos = "on the CA's website"
		}
		return nil, fmt.Errorf("Steward registers an account with %s only after you accept its terms of service (%s): set tls.acme.agree_tos: true in steward.yaml and restart stewardd", dir, tos)
	}
	reg, err := c.Registration.Register(registration.RegisterOptions{TermsOfServiceAgreed: true})
	if err != nil {
		return nil, fmt.Errorf("registering the ACME account: %w", err)
	}
	user.reg = reg
	der, err := x509.MarshalPKCS8PrivateKey(user.key)
	if err != nil {
		return nil, err
	}
	b, _ := json.Marshal(account{Directory: dir, Email: user.email, URI: reg.URI,
		KeyPEM: string(pem.EncodeToMemory(&pem.Block{Type: "PRIVATE KEY", Bytes: der}))})
	if err := m.store.SetMeta(ctx, metaAccount, string(b)); err != nil {
		return nil, err
	}
	m.log.Info("ACME account registered", "directory", dir, "account", reg.URI)
	return c, nil
}

// provider publishes the challenge through Jarl.
type provider struct {
	m   *Manager
	ctx context.Context
}

func (p *provider) set(domain, keyAuth string, set bool) error {
	info := dns01.GetChallengeInfo(domain, keyAuth)
	ctx := p.ctx
	if !set {
		ctx = context.WithoutCancel(ctx)
	}
	all, err := p.m.jarl.Zones(ctx)
	if err != nil {
		return err
	}
	zones := jarl.ZonesFor(info.EffectiveFQDN, all, "")
	if len(zones) == 0 {
		return fmt.Errorf("no primary zone in Jarl holds %s", strings.TrimSuffix(info.EffectiveFQDN, "."))
	}
	return p.m.jarl.SetTXT(ctx, info.EffectiveFQDN, info.Value, zones, set, false, "Steward's certificate")
}

func (p *provider) Present(domain, token, keyAuth string) error { return p.set(domain, keyAuth, true) }
func (p *provider) CleanUp(domain, token, keyAuth string) error { return p.set(domain, keyAuth, false) }

// Timeout is how long lego waits for the record to reach the name servers.
func (p *provider) Timeout() (timeout, interval time.Duration) {
	return 5 * time.Minute, 5 * time.Second
}

func (m *Manager) obtain(ctx context.Context) (*certificate.Resource, error) {
	c, err := m.client(ctx)
	if err != nil {
		return nil, err
	}
	opts := []dns01.ChallengeOption{dns01.AddDNSTimeout(10 * time.Second)}
	if len(m.cfg.ACME.Resolvers) > 0 {
		opts = append(opts, dns01.AddRecursiveNameservers(m.cfg.ACME.Resolvers))
	}
	if !m.cfg.ACME.CheckPropagation {
		opts = append(opts, dns01.DisableAuthoritativeNssPropagationRequirement())
	}
	if err := c.Challenge.SetDNS01Provider(&provider{m: m, ctx: ctx}, opts...); err != nil {
		return nil, err
	}
	res, err := c.Certificate.Obtain(certificate.ObtainRequest{Domains: []string{m.cfg.ACME.Domain}, Bundle: true})
	if err != nil {
		return nil, cleanLegoError(err)
	}
	return res, nil
}

// cleanLegoError shortens lego's errors (as Jarl does): "[name] acme:
// error: 403 :: urn:ietf:params:acme:error:unauthorized :: MSG" becomes
// "name: the CA refused (unauthorized): MSG".
func cleanLegoError(err error) error {
	msg := strings.TrimSpace(strings.Replace(err.Error(), "error: one or more domains had a problem:", "", 1))
	var out []string
	for _, line := range strings.Split(msg, "\n") {
		line = strings.TrimSpace(line)
		if line == "" {
			continue
		}
		domain := ""
		for strings.HasPrefix(line, "[") {
			end := strings.Index(line, "] ")
			if end < 0 {
				break
			}
			if domain == "" {
				domain = line[1:end]
			}
			line = line[end+2:]
		}
		for _, p := range []string{"acme: error presenting token: ", "acme: error: ", "acme: "} {
			line = strings.TrimPrefix(line, p)
		}
		if mm := problemRe.FindStringSubmatch(line); mm != nil {
			line = "the CA refused (" + mm[2] + "): " + mm[3]
		}
		if domain != "" {
			line = domain + ": " + line
		}
		out = append(out, line)
	}
	if len(out) == 0 {
		return err
	}
	return errors.New(strings.Join(out, "; "))
}

var problemRe = regexp.MustCompile(`^(\d+) :: urn:ietf:params:acme:error:(\w+) :: (.*)$`)