package link

import (
	"context"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"github.com/novox/mesh-controller/internal/broker"
	"log"
	"os"
	"time"

	amqp "github.com/rabbitmq/amqp091-go"

	"github.com/novox/mesh-controller/internal/envfile"
)

// AMQPVar is the controller's own connection to the bus the mesh runs on today.
const AMQPVar = "MESH_BROKER_AMQP"

// Enroller is what the controller does with an enrolment request.
//
// An interface so the serving loop can be tested against a real bus without a database, and so the
// two concerns — moving messages, and deciding — stay apart.
type Enroller interface {
	// Enrol spends the token, records the key, and reports the node's name. The error is
	// returned to the node as a refusal; it must be the same for every reason a token can fail.
	Enrol(ctx context.Context, request EnrolRequest) (EnrolReply, error)
}

// Listener is what the controller does with a report. Separate from Enroller so the two can be
// given independently, and so a server that only sends declarations needs neither.
type Listener interface {
	Heard(ctx context.Context, report Report) error
}

// Recorder keeps what builders say.
//
// Separate from Listener because they are different things arriving: a report is a node saying
// what it did with a declaration, and a build result is a machine saying what came of some work.
// One interface carrying both would mean an implementation of one having to say something about
// the other.
type Recorder interface {
	Built(ctx context.Context, result BuildResult) error
}

// Upgrader is what the controller does when the catalogue says a module moved.
//
// An interface for the same reason Enroller is one: deciding what an upgrade means for the
// machines running it is a different concern from noticing that one was announced, and only the
// first needs a database.
type Upgrader interface {
	// Upgraded is told which module moved and between which commits. An error is logged and the
	// message is not handed back: an upgrade the controller could not act on is not one it will
	// act on by being given the same message again, and a poison message on a durable queue would
	// stop every upgrade behind it — except the store unreachable for the moment, which is asked
	// again for a bounded time (novox/hq issue 083).
	Upgraded(ctx context.Context, u Upgraded) error
}

// Server acts on what nodes and modules say.
//
// **It holds no transport.** What arrives comes through Inbound and what it publishes goes through
// Bus, so this file is the controller's *decisions* about messages and nothing about wires. The
// connection below is the bus the mesh runs on today, kept because the command line publishes over
// the same one until the rollout (ADR 0116).
type Server struct {
	inbound Inbound
	bus     Bus
	conn    *amqp.Connection
	channel *amqp.Channel
	js      *broker.JetStream

	enroller Enroller
	listener Listener
	recorder Recorder
	upgrader Upgrader
	replayer Replayer

	log *log.Logger
	// giveUp is how long one message is held for the store; zero means GiveUpAfter.
	giveUp time.Duration
}

// ErrTryAgain marks a listener's failure as "not now": what it was given is worth keeping and
// asking again, as when the store is restarting (novox/hq issue 082).
var ErrTryAgain = errors.New("not now, try again")

// GiveUpAfter bounds how long one message is kept trying. A store that has not come back in this
// long is not restarting, and the message is let go with a line saying it was lost.
const GiveUpAfter = 2 * time.Minute

// Records tells the server where to keep build results.
//
// Set after Connect rather than passed to it, because a controller that only publishes — the
// `build` command, which waits for its own answer — needs a connection and no recorder, and making
// it supply one would have it construct something it never uses.
func (s *Server) Records(r Recorder) { s.recorder = r }

// Follows says what to do about upgrades, and asks for them to be delivered.
func (s *Server) Follows(u Upgrader) error {
	if err := s.inbound.Also(KindModuleMoved); err != nil {
		return err
	}
	s.upgrader = u
	return nil
}

// Answers says what to do about a catalogue's catch-up request, and asks for them to be delivered.
func (s *Server) Answers(r Replayer) error {
	if err := s.inbound.Also(KindCatchUp); err != nil {
		return err
	}
	s.replayer = r
	return nil
}

// Connect opens the controller's own connection to the bus.
//
// On the port MESH_BROKER_AMQP_PORT names when the node's settings moved the broker (novox/hq
// 04-ISSUES/102) — the URL is genesis's, sealed, and its port is the one thing in it the node may
// have moved since.
func Connect(enroller Enroller, listener Listener) (*Server, error) {
	url, err := envfile.Placed(AMQPVar)
	if err != nil {
		return nil, err
	}
	if url == "" {
		return nil, fmt.Errorf(
			"this control plane has no %s, so it cannot reach its broker. Nodes talk to it over "+
				"the broker and nowhere else, so without this it can hold records and answer "+
				"nothing", AMQPVar)
	}

	conn, err := amqp.Dial(url)
	if err != nil {
		// Not quoted back: the URL carries the controller's own bus password.
		return nil, fmt.Errorf("cannot reach the broker named in %s: %w", AMQPVar, err)
	}
	channel, err := conn.Channel()
	if err != nil {
		conn.Close()
		return nil, err
	}

	// Declared here rather than assumed. The controller is the only thing that may create them — a
	// node's account can write to this exchange and read its own queue, and configure nothing
	// else, so a node arriving before the controller has ever run finds nothing and says so,
	// rather than quietly creating a topology nobody designed.
	if err := channel.ExchangeDeclare(Exchange, "direct", true, false, false, false, nil); err != nil {
		conn.Close()
		return nil, fmt.Errorf("cannot declare the %s exchange: %w", Exchange, err)
	}
	// The events exchange too. The controller is not the only publisher on it — modules announce
	// onto it with their own accounts — but it is the only thing permitted to create it, for the
	// same reason it is the only thing permitted to create the direct one.
	if err := channel.ExchangeDeclare(EventsExchange, "topic", true, false, false, false, nil); err != nil {
		conn.Close()
		return nil, fmt.Errorf("cannot declare the %s exchange: %w", EventsExchange, err)
	}
	if _, err := channel.QueueDeclare(ControlQueue, true, false, false, false, nil); err != nil {
		conn.Close()
		return nil, fmt.Errorf("cannot declare the %s queue: %w", ControlQueue, err)
	}
	// Every key a node may publish. Binding one and forgetting another is a message the bus
	// accepts, finds no queue for, and drops — the publisher sees success and the consumer sees
	// nothing. That is exactly what happened to reports: `report` was left unbound while `enrol`
	// worked, so nodes announced what they had applied into a void for an afternoon.
	for _, key := range []string{KeyEnrol, KeyReport, KeyAlive, KeyBuilt} {
		if err := channel.QueueBind(ControlQueue, key, Exchange, false, nil); err != nil {
			conn.Close()
			return nil, fmt.Errorf("cannot bind %s to %s/%s: %w", ControlQueue, Exchange, key, err)
		}
	}

	return &Server{
		inbound:  Current(conn, channel),
		bus:      OverCurrent{Channel: channel},
		conn:     conn,
		channel:  channel,
		enroller: enroller,
		listener: listener,
		log:      newLog(),
	}, nil
}

func newLog() *log.Logger { return log.New(os.Stdout, "", log.LstdFlags) }

// Channel is the controller's channel, for the command line's own publishing.
func (s *Server) Channel() *amqp.Channel { return s.channel }

func (s *Server) Close() {
	if s.inbound != nil {
		s.inbound.Close()
	}
	if s.channel != nil {
		_ = s.channel.Close()
	}
	if s.conn != nil {
		_ = s.conn.Close()
	}
	if s.js != nil {
		s.js.Close()
	}
}

// Serve acts on what arrives until the context ends.
func (s *Server) Serve(ctx context.Context) error {
	s.log.Printf("consuming what nodes say: %s, %s, %s, %s",
		KindEnrolment, KindReport, KindHeartbeat, KindBuilt)
	if s.upgrader != nil {
		s.log.Printf("following %s", KindModuleMoved)
	}
	if s.replayer != nil {
		s.log.Printf("answering %s", KindCatchUp)
	}
	return s.inbound.Receive(ctx, s.act)
}

// act is one message, whichever bus it came over.
func (s *Server) act(ctx context.Context, m Control) {
	switch m.Kind() {
	case KindEnrolment:
		s.enrolling(ctx, m)
	case KindReport:
		s.reported(ctx, m)
	case KindHeartbeat:
		s.heartbeat(m)
	case KindBuilt:
		s.wasBuilt(ctx, m)
	case KindModuleMoved:
		s.moved(ctx, m)
	case KindCatchUp:
		s.catchingUp(ctx, m)
	default:
		// Dropped: a message nothing understands will not be understood on the next attempt
		// either, and asking for it again would spin.
		s.log.Printf("refusing a message the mesh has no name for: %q", m.Kind())
		_ = m.Drop()
	}
}

// decide asks the window what to do about one message, and holds it when that is the answer —
// because holding is the one verdict that means the same thing everywhere.
//
// The other three come back, because "settled without acting" is a build result dropped and an
// upgrade taken, and only the handler knows which its message is. Hold means the message is the
// bus's problem now and **the caller must not settle it**; that is also what a cancelled context
// gets, because shutting down is not an answer about a message (novox/hq issue 083, on review).
//
// declaredIn is the declaration this message is about, empty when it is about none; outstanding is
// what the mesh last sent that node, empty when it has sent none or could not be asked.
func (s *Server) decide(ctx context.Context, m Control, what, declaredIn, outstanding string,
	err error) Verdict {

	if ctx.Err() != nil {
		return Hold
	}
	window := StoreWindow{GiveUpAfter: s.giveUpAfter()}
	switch v := window.Decide(err, declaredIn, outstanding, m.HeldFor()); v {
	case Hold:
		// Said once, on the first hold. Every redelivery saying it again would fill the log with
		// one store restart.
		first := m.HeldFor() == 0
		if held := m.Hold(RedeliverAfter(m.HeldFor())); held != nil {
			s.log.Printf("LOST %s: it could not be held for the store (%v): %v", what, held, err)
			return GiveUp
		}
		if first {
			s.log.Printf("could not keep %s yet; holding it to try again: %v", what, err)
		}
		return Hold
	case Stale:
		s.log.Printf("set aside %s: the mesh has moved past that declaration", what)
		return Stale
	case GiveUp:
		s.log.Printf("LOST %s: the store has not come back in %s: %v", what, s.giveUpAfter(), err)
		return GiveUp
	default:
		return v
	}
}

func (s *Server) giveUpAfter() time.Duration {
	if s.giveUp == 0 {
		return GiveUpAfter
	}
	return s.giveUp
}

// outstanding is the digest of the declaration the mesh last sent a node.
//
// Nothing is guessed when it cannot be answered: a listener that keeps no record of what was sent
// gives a report nothing to be stale against, and a store that cannot be read will hold the report
// anyway, so there is nothing for the check to decide.
func (s *Server) outstanding(ctx context.Context, node string) string {
	if node == "" {
		return ""
	}
	asks, ok := s.listener.(Outstanding)
	if !ok {
		return ""
	}
	digest, err := asks.Outstanding(ctx, node)
	if err != nil {
		return ""
	}
	return digest
}

// heartbeat records that a node was heard from, and nothing else.
//
// Deliberately silent: a node saying it is there every minute would fill the log with the ordinary
// case, and a log where the ordinary case is loud is a log nobody reads. Not held for the store
// either — the next heartbeat is a minute away, and a heartbeat kept for two minutes to be written
// late says nothing the one after it will not say better.
func (s *Server) heartbeat(m Control) {
	var alive Alive
	if err := json.Unmarshal(m.Body(), &alive); err != nil || alive.Node == "" {
		_ = m.Drop()
		return
	}
	if s.listener != nil {
		if err := s.listener.Heard(context.Background(), Report{Node: alive.Node}); err != nil {
			s.log.Printf("could not record that %s is here: %v", alive.Node, err)
		}
	}
	_ = m.Took()
}

// reported records what a node says it did.
//
// A node states; nothing here writes anything the node claimed about itself beyond that it was
// heard from. What it applied is its own account of its own machine, and the mesh keeps the last
// one as a copy for recovery rather than as a source (novox/hq 09-the-node-lifecycle).
func (s *Server) reported(ctx context.Context, m Control) {
	var report Report
	if err := json.Unmarshal(m.Body(), &report); err != nil {
		s.log.Printf("a report could not be read: %v", err)
		_ = m.Drop()
		return
	}
	m.About("report " + report.Node)
	what := fmt.Sprintf("%s's report of declaration %s", report.Node, report.Declared)

	if s.listener != nil {
		// **Asked before the store, not after** (design 25 §3). A report about a declaration the
		// mesh has moved past would otherwise wait out a restarting store to be written and then
		// overwrite what the node is doing now — and on the bus being built, where the holding is
		// the server's, it comes back after the newer was applied whatever the controller does.
		outstanding := s.outstanding(ctx, report.Node)
		declaredIn := staleAgainst(report)
		if Superseded(declaredIn, outstanding) {
			s.log.Printf("set aside %s: the mesh has moved past that declaration", what)
			_ = m.Took()
			return
		}

		err := s.listener.Heard(context.Background(), report)
		switch s.decide(ctx, m, what, declaredIn, outstanding, err) {
		case Hold:
			// Held, not settled, while the store cannot take it: the node reports an apply once,
			// and a report lost here is a node the mesh never hears from again — the store
			// restarting under the adoption that node just applied lost exactly that (issue 082).
			return
		case Stale, GiveUp:
			_ = m.Took()
			return
		}
		if err != nil {
			// Said rather than swallowed. A report the mesh heard and failed to write down is a
			// node whose recovery copy is silently older than it looks.
			s.log.Printf("could not record %s's report: %v", report.Node, err)
		}
	}

	switch {
	case report.Refused != "":
		s.log.Printf("%s refused a declaration: %s", report.Node, report.Refused)
	case len(report.Failed) > 0:
		s.log.Printf("%s applied %d and failed: %v", report.Node, len(report.Applied), report.Failed)
	default:
		s.log.Printf("%s applied %d resource(s)", report.Node, len(report.Applied))
	}
	_ = m.Took()
}

// staleAgainst is the declaration a report may be judged stale against, and it is empty for a
// report that is not only an account of an apply.
//
// **A report carries two different things, and only one of them is about a declaration.** What the
// node applied is; what the machine *is* — the tunnel it took over, the ports its own bundle holds,
// what an adopted node found and is keeping, a node moving its overlay key — is not. Those reach
// the mesh on a report because a report is the message a node sends, and nowhere else: a rekey
// dropped as stale is a node whose overlay key never moves, and no retry is coming, because the
// node said it once.
//
// So staleness is asked only of a report that is purely an apply's account. The rest is acted on
// whenever it arrives, which is the behaviour the mesh has had all along.
func staleAgainst(report Report) string {
	if report.Rekey != nil || report.Tunnel != nil || len(report.Held) > 0 ||
		report.Firewall != "" || len(report.Reachable) > 0 || len(report.Carried) > 0 {
		return ""
	}
	return report.Declared
}

func (s *Server) enrolling(ctx context.Context, m Control) {
	reply := EnrolReply{Refusal: "that token cannot be used"}

	var request EnrolRequest
	if err := json.Unmarshal(m.Body(), &request); err != nil {
		s.log.Printf("an enrolment request could not be read: %v", err)
	} else {
		request.Redelivered = m.Redelivered()
		accepted, err := s.enroller.Enrol(ctx, request)
		switch {
		case errors.Is(err, ErrTryAgain):
			// Not a refusal: nothing was spent, and the same request asked again will be
			// answered. Replied at once rather than held, so the node — which is waiting on this
			// answer — decides when to ask, and the queue behind it moves (issue 083).
			reply = EnrolReply{TryAgain: true, Refusal: "the mesh cannot answer right now; ask again"}
			s.log.Printf("asked %q to enrol again shortly: %v", request.Node, err)
		case err != nil && request.Redelivered:
			// Said as what it most likely is: the bus handed this request over again after the
			// controller stopped mid-answer, and an enrolment already spent is not finished a
			// second time. The node may need a new token.
			s.log.Printf("refusing a redelivered enrolment for %q — it may have finished before "+
				"the control plane stopped, and if the node did not get its answer it needs a new "+
				"token: %v", request.Node, err)
		case err != nil:
			// Logged in full here, where an operator can see it; sent back as one refusal, so
			// that somebody guessing learns nothing from which reason came back.
			s.log.Printf("refusing enrolment for %q: %v", request.Node, err)
		default:
			reply = accepted
			s.log.Printf("enrolled %s", accepted.Node)
		}
	}

	if body, err := json.Marshal(reply); err != nil {
		s.log.Printf("cannot encode a reply: %v", err)
	} else {
		answer, cancel := context.WithTimeout(ctx, 10*time.Second)
		if err := m.Answer(answer, body); err != nil {
			s.log.Printf("cannot answer an enrolment: %v", err)
		}
		cancel()
	}

	// Settled after the answer is sent, so a controller that dies mid-answer leaves the request on
	// the bus rather than having consumed it silently. Asked again by the same presenter, an
	// enrolment finishes: the token is held for its key and spent last (issue 083).
	_ = m.Took()
}

// wasBuilt keeps what a builder said, whichever way it went.
//
// This is for results nobody was waiting for. A build asked for with `build` is answered directly
// to the asker; one triggered any other way is published here, and without this it would be
// reported into the void — which is the same as not reporting it.
func (s *Server) wasBuilt(ctx context.Context, m Control) {
	var result BuildResult
	if err := json.Unmarshal(m.Body(), &result); err != nil {
		s.log.Printf("a build result could not be read: %v", err)
		_ = m.Drop()
		return
	}
	if s.recorder == nil {
		// Nothing to keep it in. Dropped rather than swallowed, so the bus's own counters show
		// something arriving that nothing handles.
		s.log.Printf("a build result arrived and this control plane keeps none")
		_ = m.Drop()
		return
	}
	// Each build result its own subject: none supersedes another, and recording one twice is
	// harmless — the build is kept by its id.
	m.About("build " + digest(m.Body()))
	err := s.recorder.Built(ctx, result)
	switch s.decide(ctx, m, fmt.Sprintf("a build result from %s", result.On), "", "", err) {
	case Hold:
		// Held while the store cannot take it: a build result lost here is never announced (083).
		return
	case Stale, GiveUp:
		_ = m.Drop()
		return
	}
	if err != nil {
		s.log.Printf("cannot keep a build result from %s: %v", result.On, err)
		_ = m.Drop()
		return
	}
	switch {
	case result.Failed != "":
		s.log.Printf("%s could not build %s", result.On, result.Repository)
	default:
		s.log.Printf("%s built %s from %s", result.On, result.Repository, result.Commit)
	}
	_ = m.Took()
}

// catchingUp answers a catalogue that has just started and may have missed builds.
//
// Settled after the work. A replay that fails for a reason other than the store is not one that
// succeeds by being handed the same request again, so that is settled and said; but a store that
// could not be read right now is held and asked again, bounded, rather than the request lost until
// the catalogue next restarts (issue 083). One request stands for all: a newer one supersedes one
// still held.
func (s *Server) catchingUp(ctx context.Context, m Control) {
	m.About("catch-up")
	if s.replayer == nil {
		s.log.Printf("a catalogue asked to catch up and this control plane has nothing to replay")
		_ = m.Took()
		return
	}
	announcements, err := s.replayer.Announceable(ctx)
	switch s.decide(ctx, m, "a catalogue's request to catch up", "", "", err) {
	case Hold:
		return
	case Stale, GiveUp:
		_ = m.Took()
		return
	}
	if err != nil {
		s.log.Printf("a catalogue asked to catch up and the mesh could not read its builds: %v", err)
		_ = m.Took()
		return
	}
	sent := 0
	for _, a := range announcements {
		a.Replay = true
		if err := EmitEvent(ctx, s.bus, KeyModuleBuilt, "control-plane", "", a); err != nil {
			// Said and abandoned rather than retried: the catalogue asks again every time it
			// starts, and half a graph delivered twice is no better than half delivered once.
			s.log.Printf("replaying %s at %s failed, and the rest is abandoned: %v",
				a.Module, short(a.Commit), err)
			_ = m.Took()
			return
		}
		sent++
	}
	s.log.Printf("a catalogue asked to catch up; re-announced %d build(s)", sent)
	_ = m.Took()
}

// moved hands one announcement to whatever is following them.
//
// **Settled whatever happens, but one thing.** A failure here is usually the controller being
// unable to act on an upgrade — a machine that cannot be resolved, a bus that will not take a
// declaration — and none of those get better by being handed the same message again. The one
// exception is the upgrader saying the store could not be read for the moment (ErrTryAgain): that
// is held and asked again, bounded (novox/hq issue 083). Only the upgrader's word counts here, not
// an error that merely looks like an outage — a push that timed out on the second machine is not
// asked again, or the first would be pushed every few seconds for two minutes. A newer move of the
// same module supersedes one still held.
func (s *Server) moved(ctx context.Context, m Control) {
	var u Upgraded
	if err := json.Unmarshal(m.Body(), &u); err != nil {
		s.log.Printf("an upgrade announcement could not be read: %v", err)
		_ = m.Took()
		return
	}
	m.About("upgrade " + u.Module)
	if u.Module == "" {
		s.log.Printf("an upgrade announcement named no module; ignored")
		_ = m.Took()
		return
	}
	err := s.upgrader.Upgraded(ctx, u)
	// Only "not now" is worth holding. Anything else is an answer, and the window would read a
	// timed-out push as an outage and ask for it again.
	holdable := err
	if !errors.Is(err, ErrTryAgain) {
		holdable = nil
	}
	what := fmt.Sprintf("%s's move to %s", u.Module, short(u.Commit))
	switch s.decide(ctx, m, what, "", "", holdable) {
	case Hold:
		return
	case Stale, GiveUp:
		_ = m.Took()
		return
	}
	if err != nil {
		s.log.Printf("%s moved to %s and the mesh could not act on it: %v",
			u.Module, short(u.Commit), err)
	}
	_ = m.Took()
}

// digest names a message by its content.
func digest(body []byte) string {
	sum := sha256.Sum256(body)
	return hex.EncodeToString(sum[:])
}

// short is a commit as people read it.
func short(commit string) string {
	if len(commit) > 8 {
		return commit[:8]
	}
	return commit
}