// Package proxy passes each request to the app and the app's answer back,
// unchanged, within the size and time limits, and writes one request log
// line for each request.
package proxy

import (
	"io"
	"log"
	"log/slog"
	"net/http"
	"time"

	"sneak.berlin/go/smallwebwaf/internal/config"
	"sneak.berlin/go/smallwebwaf/internal/lookup"
	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
	"sneak.berlin/go/smallwebwaf/internal/requestlog"
)

// The request line and headers a client may send, and how long a
// kept-open client connection may wait for its next request, are fixed
// rather than settings. The limit on the request line and headers is
// 32 KiB, but Go's server reads 4 KiB past its MaxHeaderBytes before it
// refuses, so MaxHeaderBytes is set 4 KiB lower. The idle time is longer
// than the 90 seconds after which traefik closes a connection it is not
// using, so traefik never sends a request on a connection smallwebwaf is
// closing.
const (
	requestHeaderMaxBytes = 32<<10 - 4<<10
	clientIdleTimeout     = 120 * time.Second
)

// How smallwebwaf keeps connections to the app open between requests.
const (
	appIdleConns       = 100
	appIdleConnTimeout = 90 * time.Second
)

// HealthPath is smallwebwaf's health endpoint, which the container's
// health check asks.
const HealthPath = "/_smallwebwaf/healthz"

// Params are what New needs.
type Params struct {
	Config *config.Config
	// RequestLog receives one JSON line per request.
	RequestLog io.Writer
	// ProcessLog receives the process's own messages.
	ProcessLog *slog.Logger
	// GeoJSURL is where clients' countries are looked up, normally
	// lookup.URL. GeoJS is asked only while a country list is set.
	GeoJSURL string
}

// New returns the server smallwebwaf runs: each request it reads passes
// through the proxy. Go's server itself refuses headers over 32 KiB, with
// 431, closes a connection idle for 120 seconds, and applies
// SWWAF_CLIENT_REQUEST_TIMEOUT while the headers arrive; the proxy
// applies the timeouts and size limits from then on.
func New(params Params) *http.Server {
	errorLog := slog.NewLogLogger(params.ProcessLog.Handler(), slog.LevelWarn)

	return &http.Server{
		Addr: params.Config.ListenAddr,
		Handler: &handler{
			config:     params.Config,
			requestLog: params.RequestLog,
			processLog: params.ProcessLog,
			errorLog:   errorLog,
			transport:  newTransport(),
			limiter: ratelimit.New(ratelimit.Limits{
				PerMinute: params.Config.RateLimitPerMinute,
				PerHour:   params.Config.RateLimitPerHour,
				PerDay:    params.Config.RateLimitPerDay,
			}),
			geojs: lookup.New(lookup.Params{
				URL:        params.GeoJSURL,
				Now:        time.Now,
				ProcessLog: params.ProcessLog,
			}),
		},
		ReadHeaderTimeout: params.Config.ClientRequestTimeout,
		IdleTimeout:       clientIdleTimeout,
		MaxHeaderBytes:    requestHeaderMaxBytes,
		ErrorLog:          errorLog,
	}
}

// handler is the proxy. It holds what every request shares; what belongs
// to one request is in a request.
type handler struct {
	config     *config.Config
	requestLog io.Writer
	processLog *slog.Logger
	errorLog   *log.Logger
	transport  http.RoundTripper
	limiter    *ratelimit.Limiter
	geojs      *lookup.GeoJS
}

// newTransport returns what carries requests to the app. It never goes
// through a proxy named in the environment, and leaves the app's answers
// compressed or not as the app sent them.
func newTransport() *http.Transport {
	return &http.Transport{
		MaxIdleConns:        appIdleConns,
		MaxIdleConnsPerHost: appIdleConns,
		IdleConnTimeout:     appIdleConnTimeout,
		DisableCompression:  true,
	}
}

// ServeHTTP handles one request: it works out the client, runs the
// checks, passes the request to the app and the answer back within the
// limits, and writes the request's log line.
func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	rq := h.newRequest(w, r)
	defer rq.finish()

	// The health endpoint is answered at once, before any check, so that
	// a health checker is never refused. It does not ask the app.
	if r.Method == http.MethodGet && r.URL.Path == HealthPath {
		rq.line.Action = requestlog.ActionAdmin
		_, _ = io.WriteString(rq.out, "ok\n")

		return
	}

	refused := rq.check(r.Context())
	if refused != nil {
		rq.answer(*refused)

		return
	}

	rq.forward(r.Context())
}