netwatch/backend/internal/middleware/middleware.go

// Package middleware provides HTTP middleware for logging,
// CORS, and other cross-cutting concerns.
package middleware

import (
	"errors"
	"fmt"
	"io"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"runtime/debug"
	"strings"
	"time"

	"sneak.berlin/go/netwatch/internal/config"
	"sneak.berlin/go/netwatch/internal/globals"
	"sneak.berlin/go/netwatch/internal/logger"

	"github.com/go-chi/chi/v5/middleware"
	"github.com/go-chi/cors"
	"github.com/go-chi/httprate"
	"go.uber.org/fx"
)

const corsMaxAgeSec = 300

// jsonErrorBody is the body written for errors raised inside
// middleware, matching the {"status":"error"} shape the handlers
// return so clients see one error contract across the API.
const (
	jsonContentType = "application/json; charset=utf-8"
	jsonErrorBody   = "{\"status\":\"error\"}\n"
)

// Security header values. The backend is a JSON API with no
// HTML surface, so the CSP forbids every resource type and
// framing outright.
const (
	hstsValue              = "max-age=31536000; includeSubDomains"
	cspValue               = "default-src 'none'; frame-ancestors 'none'"
	permissionsPolicyValue = "camera=(), microphone=(), geolocation=()"
)

// Params defines the dependencies for Middleware.
type Params struct {
	fx.In

	Config  *config.Config
	Globals *globals.Globals
	Logger  *logger.Logger
}

// Middleware holds shared state for middleware factories.
type Middleware struct {
	log            *slog.Logger
	params         *Params
	trustedProxies []netip.Prefix
}

// New creates a Middleware instance.
func New(
	_ fx.Lifecycle,
	params Params,
) (*Middleware, error) {
	trusted, err := parseTrustedProxies(params.Config.TrustedProxies)
	if err != nil {
		return nil, err
	}

	s := new(Middleware)
	s.params = &params
	s.log = params.Logger.Get()
	s.trustedProxies = trusted

	return s, nil
}

// parseTrustedProxies converts the TRUSTED_PROXIES entries into
// prefixes, failing fast on any malformed entry.
func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
	prefixes := make([]netip.Prefix, 0, len(cidrs))

	for _, cidr := range cidrs {
		prefix, err := netip.ParsePrefix(cidr)
		if err != nil {
			return nil, fmt.Errorf(
				"TRUSTED_PROXIES %q: %w", cidr, err,
			)
		}

		prefixes = append(prefixes, prefix.Masked())
	}

	return prefixes, nil
}

type loggingResponseWriter struct {
	http.ResponseWriter

	statusCode int
}

func newLoggingResponseWriter(
	w http.ResponseWriter,
) *loggingResponseWriter {
	return &loggingResponseWriter{w, http.StatusOK}
}

func (lrw *loggingResponseWriter) WriteHeader(code int) {
	lrw.statusCode = code
	lrw.ResponseWriter.WriteHeader(code)
}

func ipFromHostPort(hostPort string) string {
	host, _, err := net.SplitHostPort(hostPort)
	if err != nil {
		return hostPort
	}

	return host
}

// clientIP resolves the caller's address. X-Forwarded-For and
// X-Real-IP are honoured only when the direct peer is a
// trusted proxy; otherwise the direct peer is returned so a
// spoofed header cannot forge the logged address.
func clientIP(
	remoteAddr string,
	header http.Header,
	trusted []netip.Prefix,
) string {
	peer := ipFromHostPort(remoteAddr)

	if !addrInAny(peer, trusted) {
		return peer
	}

	if xff := firstForwardedFor(header.Get("X-Forwarded-For")); xff != "" {
		return xff
	}

	if xr := strings.TrimSpace(header.Get("X-Real-IP")); validIP(xr) {
		return xr
	}

	return peer
}

// firstForwardedFor returns the left-most valid address in an
// X-Forwarded-For list (the original client), or "" if none.
func firstForwardedFor(value string) string {
	for part := range strings.SplitSeq(value, ",") {
		candidate := strings.TrimSpace(part)
		if validIP(candidate) {
			return candidate
		}
	}

	return ""
}

func validIP(s string) bool {
	_, err := netip.ParseAddr(s)

	return err == nil
}

// addrInAny reports whether s parses as an address contained
// in any of the trusted prefixes.
func addrInAny(s string, trusted []netip.Prefix) bool {
	addr, err := netip.ParseAddr(s)
	if err != nil {
		return false
	}

	addr = addr.Unmap()

	for _, prefix := range trusted {
		if prefix.Contains(addr) {
			return true
		}
	}

	return false
}

// Logging returns middleware that logs each request with
// timing, status code, and client information.
func (s *Middleware) Logging() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(
			func(w http.ResponseWriter, r *http.Request) {
				start := time.Now().UTC()
				lrw := newLoggingResponseWriter(w)
				ctx := r.Context()

				defer func() {
					latency := time.Since(start)
					s.log.InfoContext(ctx, "request",
						"request_start", start,
						"method", r.Method,
						"url", r.URL.String(),
						"useragent", r.UserAgent(),
						"request_id",
						ctx.Value(
							middleware.RequestIDKey,
						),
						"referer", r.Referer(),
						"proto", r.Proto,
						"remote_ip",
						clientIP(
							r.RemoteAddr,
							r.Header,
							s.trustedProxies,
						),
						"status", lrw.statusCode,
						"latency_ms",
						latency.Milliseconds(),
					)
				}()

				next.ServeHTTP(lrw, r)
			},
		)
	}
}

// SecurityHeaders returns middleware that sets response
// security headers. It runs before CORS so the headers are
// present on preflight responses the CORS handler writes.
func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(
			func(w http.ResponseWriter, r *http.Request) {
				h := w.Header()
				h.Set("Strict-Transport-Security", hstsValue)
				h.Set("Content-Security-Policy", cspValue)
				h.Set("X-Frame-Options", "DENY")
				h.Set("X-Content-Type-Options", "nosniff")
				h.Set("Referrer-Policy", "no-referrer")
				h.Set("Permissions-Policy", permissionsPolicyValue)

				next.ServeHTTP(w, r)
			},
		)
	}
}

// writeJSONError writes the shared JSON error body with the
// given status. Used where middleware must reject a request
// before it reaches a handler.
func writeJSONError(w http.ResponseWriter, status int) {
	w.Header().Set("Content-Type", jsonContentType)
	w.WriteHeader(status)
	_, _ = io.WriteString(w, jsonErrorBody)
}

// MaxBodyBytes returns middleware that caps the request body at
// limit bytes. A declared Content-Length over the limit is
// rejected immediately with 413. Bodies without a declared
// length (or that understate it) are capped as they are read, so
// a handler that reads the body sees a *http.MaxBytesError it can
// map to 413. Mounted again on a route group, it can only lower
// the limit: a cap applied earlier in the chain still holds.
func (s *Middleware) MaxBodyBytes(
	limit int64,
) func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(
			func(w http.ResponseWriter, r *http.Request) {
				if r.ContentLength > limit {
					writeJSONError(
						w,
						http.StatusRequestEntityTooLarge,
					)

					return
				}

				r.Body = http.MaxBytesReader(w, r.Body, limit)

				next.ServeHTTP(w, r)
			},
		)
	}
}

// Recoverer returns middleware that recovers from a panic in a
// downstream handler, logs the panic and stack trace through
// slog, and responds 500 with no body. http.ErrAbortHandler is
// re-panicked so the server can abort the response as intended.
func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
	return func(next http.Handler) http.Handler {
		return http.HandlerFunc(
			func(w http.ResponseWriter, r *http.Request) {
				defer func() {
					rec := recover()
					if rec == nil {
						return
					}

					err, ok := rec.(error)
					if ok && errors.Is(err, http.ErrAbortHandler) {
						panic(rec)
					}

					s.log.ErrorContext(r.Context(),
						"panic recovered",
						"panic", fmt.Sprintf("%v", rec),
						"stack", string(debug.Stack()),
					)

					w.WriteHeader(http.StatusInternalServerError)
				}()

				next.ServeHTTP(w, r)
			},
		)
	}
}

// CORS returns middleware that lets pages served from the given
// origins call the API. With no origins it adds no CORS headers at
// all, so only same-origin pages can use the API. That case must not
// reach cors.Handler, which treats an empty origin list as "allow
// every origin".
func (s *Middleware) CORS(
	origins []string,
) func(http.Handler) http.Handler {
	if len(origins) == 0 {
		return func(next http.Handler) http.Handler { return next }
	}

	return cors.Handler(cors.Options{
		AllowedOrigins:   origins,
		AllowedMethods:   []string{http.MethodGet, http.MethodPost},
		AllowedHeaders:   []string{"Content-Type"},
		AllowCredentials: false,
		MaxAge:           corsMaxAgeSec,
	})
}

// RateLimit returns middleware that allows each client address
// perMinute requests a minute and answers the rest with 429, the
// Retry-After header httprate sets, and the usual error body. The
// address is the one clientIP resolves, so clients behind the reverse
// proxy are limited one by one, not together as the proxy.
func (s *Middleware) RateLimit(
	perMinute int,
) func(http.Handler) http.Handler {
	return httprate.LimitBy(perMinute, time.Minute,
		func(r *http.Request) (string, error) {
			return clientIP(r.RemoteAddr, r.Header, s.trustedProxies), nil
		},
		httprate.WithLimitHandler(
			func(w http.ResponseWriter, _ *http.Request) {
				writeJSONError(w, http.StatusTooManyRequests)
			},
		),
	)
}