// Package server provides the HTTP server.
package server

import (
	"context"
	"errors"
	"fmt"
	"log/slog"
	"net/http"
	"time"

	"github.com/getsentry/sentry-go"
	"github.com/go-chi/chi/v5"
	"go.uber.org/fx"

	"sneak.berlin/go/dnswatcher/internal/config"
	"sneak.berlin/go/dnswatcher/internal/globals"
	"sneak.berlin/go/dnswatcher/internal/handlers"
	"sneak.berlin/go/dnswatcher/internal/logger"
	"sneak.berlin/go/dnswatcher/internal/middleware"
)

// Params contains dependencies for Server.
type Params struct {
	fx.In

	Logger     *logger.Logger
	Globals    *globals.Globals
	Config     *config.Config
	Middleware *middleware.Middleware
	Handlers   *handlers.Handlers
}

// shutdownTimeout is how long to wait for graceful shutdown.
const shutdownTimeout = 30 * time.Second

// sentryFlushTimeout is how long shutdown waits for Sentry to send the
// error reports it still holds.
const sentryFlushTimeout = 2 * time.Second

// Socket-level timeouts for the HTTP server.
//
// These bound time spent on the connection itself and are a distinct
// control from the per-request handler budget enforced by
// chimw.Timeout(requestTimeout) in routes.go: that one cancels the
// request context after requestTimeout but never touches the socket,
// so without the values below a peer can hold a connection open
// forever (slowloris, unreaped keep-alives).
//
// The one hard constraint between the two controls is
// writeTimeout > requestTimeout. net/http arms the write deadline
// once the request headers have been read, so on a plaintext
// connection it covers handler execution AND the response flush. If
// writeTimeout were <= requestTimeout the server would sever the
// connection before a handler that legitimately consumed its full
// budget could emit anything, making the 60s budget unreachable in
// practice. The margin between them is the response-flush allowance.
//
// The only clients of this service are browsers loading the dashboard
// and a Prometheus scraper; the values are sized for those.
const (
	// readHeaderTimeout is the max duration for reading request
	// headers.
	readHeaderTimeout = 10 * time.Second

	// readTimeout bounds reading the entire request, headers plus
	// body. Every route here is a GET with no body, so this only
	// ever needs to cover headers; the extra 5s over
	// readHeaderTimeout is slack, not a real allowance, and keeps a
	// body dribbled one byte at a time from holding the read side
	// open indefinitely.
	readTimeout = 15 * time.Second

	// writeTimeout must exceed the requestTimeout handler budget
	// (60s) per the note above. The 15s difference is the allowance
	// for flushing a completed response to a slow client.
	writeTimeout = 75 * time.Second

	// idleTimeout reaps keep-alive connections between requests. It
	// is deliberately longer than the common Prometheus scrape
	// intervals (15s/30s/60s) so the scraper reuses its connection
	// rather than reconnecting every cycle, while a browser tab
	// left open on the dashboard stops occupying a connection
	// within two minutes of going quiet.
	idleTimeout = 120 * time.Second
)

// Server is the HTTP server.
type Server struct {
	startupTime   time.Time
	port          int
	sentryEnabled bool
	log           *slog.Logger
	router        *chi.Mux
	httpServer    *http.Server
	params        Params
	mw            *middleware.Middleware
	handlers      *handlers.Handlers
}

// New creates a new Server instance.
func New(
	lifecycle fx.Lifecycle,
	params Params,
) (*Server, error) {
	srv := &Server{
		port:     params.Config.Port,
		log:      params.Logger.Get(),
		params:   params,
		mw:       params.Middleware,
		handlers: params.Handlers,
	}

	lifecycle.Append(fx.Hook{
		OnStart: func(_ context.Context) error {
			srv.startupTime = time.Now()

			err := srv.enableSentry()
			if err != nil {
				return err
			}

			go srv.Run()

			return nil
		},
		OnStop: func(ctx context.Context) error {
			return srv.Shutdown(ctx)
		},
	})

	return srv, nil
}

// newHTTPServer builds the listening http.Server with every
// socket-level timeout set. All four are set deliberately: a zero
// value in net/http means "no limit", not "some default".
func newHTTPServer(
	listenAddr string,
	handler http.Handler,
) *http.Server {
	return &http.Server{
		Addr:              listenAddr,
		Handler:           handler,
		ReadTimeout:       readTimeout,
		ReadHeaderTimeout: readHeaderTimeout,
		WriteTimeout:      writeTimeout,
		IdleTimeout:       idleTimeout,
	}
}

// Run starts the HTTP server.
func (s *Server) Run() {
	s.SetupRoutes()

	listenAddr := fmt.Sprintf(":%d", s.port)
	s.httpServer = newHTTPServer(listenAddr, s)

	s.log.Info("http server starting", "addr", listenAddr)

	err := s.httpServer.ListenAndServe()
	if err != nil && !errors.Is(err, http.ErrServerClosed) {
		s.log.Error("http server error", "error", err)
	}
}

// Shutdown gracefully shuts down the server, then sends the error
// reports Sentry still holds.
func (s *Server) Shutdown(ctx context.Context) error {
	defer s.flushSentry()

	if s.httpServer == nil {
		return nil
	}

	s.log.Info("shutting down http server")

	shutdownCtx, cancel := context.WithTimeout(
		ctx, shutdownTimeout,
	)
	defer cancel()

	err := s.httpServer.Shutdown(shutdownCtx)
	if err != nil {
		s.log.Error("http server shutdown error", "error", err)

		return fmt.Errorf("shutting down http server: %w", err)
	}

	s.log.Info("http server stopped")

	return nil
}

// ServeHTTP implements http.Handler.
func (s *Server) ServeHTTP(
	writer http.ResponseWriter,
	request *http.Request,
) {
	s.router.ServeHTTP(writer, request)
}

// enableSentry turns on Sentry error reporting when
// DNSWATCHER_SENTRY_DSN is set, and does nothing when it is not. A DSN
// that Sentry cannot parse is an error, so that startup stops instead
// of running without the error reporting the operator asked for.
func (s *Server) enableSentry() error {
	if s.params.Config.SentryDSN == "" {
		return nil
	}

	err := sentry.Init(sentry.ClientOptions{
		Dsn:     s.params.Config.SentryDSN,
		Release: s.params.Globals.Appname + "-" + s.params.Globals.Version,
		// Use the transport that queues each report as it is made. With
		// the default one, Flush can return before sending a report made
		// just before it, such as one from the last request at shutdown.
		DisableTelemetryBuffer: true,
		// Send panic reports only, not Sentry's counts of what it dropped,
		// such as the transaction it starts for every request.
		DisableClientReports: true,
	})
	if err != nil {
		return fmt.Errorf("invalid DNSWATCHER_SENTRY_DSN: %w", err)
	}

	s.log.Info("sentry error reporting activated")
	s.sentryEnabled = true

	return nil
}

// flushSentry sends the error reports Sentry still holds, waiting at
// most sentryFlushTimeout.
func (s *Server) flushSentry() {
	if !s.sentryEnabled {
		return
	}

	if !sentry.Flush(sentryFlushTimeout) {
		s.log.Warn("sentry flush timed out; some error reports were not sent")
	}
}