pad/internal/server/middleware_ratelimit.go

package server

import (
	"net/http"
	"strconv"
	"strings"
	"sync"
	"time"

	"golang.org/x/time/rate"
)

// rateLimitConfig holds the rate and burst for a limiter.
type rateLimitConfig struct {
	Rate  rate.Limit // events per second
	Burst int        // max burst
}

// ipRateLimiter tracks per-key rate limiters with automatic cleanup.
type ipRateLimiter struct {
	mu       sync.Mutex
	limiters map[string]*rateLimiterEntry
	config   rateLimitConfig
}

type rateLimiterEntry struct {
	limiter  *rate.Limiter
	lastSeen time.Time
}

func newIPRateLimiter(cfg rateLimitConfig) *ipRateLimiter {
	rl := &ipRateLimiter{
		limiters: make(map[string]*rateLimiterEntry),
		config:   cfg,
	}
	// Background cleanup of stale entries every 5 minutes
	go rl.cleanup()
	return rl
}

func (rl *ipRateLimiter) getLimiter(key string) *rate.Limiter {
	rl.mu.Lock()
	defer rl.mu.Unlock()

	entry, exists := rl.limiters[key]
	if !exists {
		limiter := rate.NewLimiter(rl.config.Rate, rl.config.Burst)
		rl.limiters[key] = &rateLimiterEntry{
			limiter:  limiter,
			lastSeen: time.Now(),
		}
		return limiter
	}
	entry.lastSeen = time.Now()
	return entry.limiter
}

func (rl *ipRateLimiter) cleanup() {
	for {
		time.Sleep(5 * time.Minute)
		rl.mu.Lock()
		for key, entry := range rl.limiters {
			if time.Since(entry.lastSeen) > 30*time.Minute {
				delete(rl.limiters, key)
			}
		}
		rl.mu.Unlock()
	}
}

// RateLimiters holds all the rate limiters used by the server.
type RateLimiters struct {
	// Auth endpoints: strict limits per IP
	Auth *ipRateLimiter
	// Password reset: per-IP
	PasswordReset *ipRateLimiter
	// Registration: per-IP
	Register *ipRateLimiter
	// API: per-user (authenticated)
	API *ipRateLimiter
	// Search: per-user or per-IP
	Search *ipRateLimiter
}

// NewRateLimiters creates rate limiters with sensible defaults.
func NewRateLimiters() *RateLimiters {
	return &RateLimiters{
		// Login: 5 attempts per minute per IP (= 5/60 per second, burst 5)
		Auth: newIPRateLimiter(rateLimitConfig{
			Rate:  rate.Limit(5.0 / 60.0),
			Burst: 5,
		}),
		// Password reset: 3 per hour per IP (= 3/3600 per second, burst 3)
		PasswordReset: newIPRateLimiter(rateLimitConfig{
			Rate:  rate.Limit(3.0 / 3600.0),
			Burst: 3,
		}),
		// Registration: 5 per hour per IP (= 5/3600 per second, burst 5)
		Register: newIPRateLimiter(rateLimitConfig{
			Rate:  rate.Limit(5.0 / 3600.0),
			Burst: 5,
		}),
		// API: 100 requests per minute per user/IP (= 100/60 per second, burst 20)
		API: newIPRateLimiter(rateLimitConfig{
			Rate:  rate.Limit(100.0 / 60.0),
			Burst: 20,
		}),
		// Search: 30 requests per minute per user/IP (= 30/60 per second, burst 10)
		Search: newIPRateLimiter(rateLimitConfig{
			Rate:  rate.Limit(30.0 / 60.0),
			Burst: 10,
		}),
	}
}

// RateLimit is the general-purpose rate limiting middleware.
// It applies different limits based on the endpoint being hit.
func (s *Server) RateLimit(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if s.rateLimiters == nil {
			next.ServeHTTP(w, r)
			return
		}

		path := r.URL.Path

		// Only rate-limit API endpoints
		if !strings.HasPrefix(path, "/api/") {
			next.ServeHTTP(w, r)
			return
		}

		ip := clientIP(r)

		// Auth-specific rate limits
		if strings.HasPrefix(path, "/api/v1/auth/") {
			var limiter *ipRateLimiter
			switch {
			case path == "/api/v1/auth/login" || path == "/api/v1/auth/bootstrap":
				limiter = s.rateLimiters.Auth
			case path == "/api/v1/auth/forgot-password" || path == "/api/v1/auth/reset-password":
				limiter = s.rateLimiters.PasswordReset
			case path == "/api/v1/auth/register":
				limiter = s.rateLimiters.Register
			default:
				// Other auth endpoints (session check, logout) — use general API limit
				limiter = s.rateLimiters.API
			}

			if limiter != nil && !limiter.getLimiter(ip).Allow() {
				writeTooManyRequests(w)
				return
			}
			next.ServeHTTP(w, r)
			return
		}

		// Search endpoint
		if path == "/api/v1/search" {
			key := rateLimitKey(r, ip)
			if !s.rateLimiters.Search.getLimiter(key).Allow() {
				writeTooManyRequests(w)
				return
			}
			next.ServeHTTP(w, r)
			return
		}

		// General API rate limit
		key := rateLimitKey(r, ip)
		if !s.rateLimiters.API.getLimiter(key).Allow() {
			writeTooManyRequests(w)
			return
		}

		next.ServeHTTP(w, r)
	})
}

// rateLimitKey returns a key for rate limiting: user ID if authenticated, IP otherwise.
func rateLimitKey(r *http.Request, ip string) string {
	if user := currentUser(r); user != nil {
		return "user:" + user.ID
	}
	return "ip:" + ip
}

// clientIP extracts the client IP from RemoteAddr. This is safe because
// chimiddleware.RealIP runs earlier in the chain and overwrites RemoteAddr
// with the trusted value from X-Real-IP / X-Forwarded-For. We deliberately
// do NOT read proxy headers here to prevent clients from spoofing their IP
// to bypass rate limits.
func clientIP(r *http.Request) string {
	host := r.RemoteAddr
	if idx := strings.LastIndex(host, ":"); idx != -1 {
		return host[:idx]
	}
	return host
}

// writeTooManyRequests sends a 429 response with a Retry-After header.
func writeTooManyRequests(w http.ResponseWriter) {
	w.Header().Set("Retry-After", strconv.Itoa(60)) // suggest retry after 60s
	writeError(w, http.StatusTooManyRequests, "rate_limited", "Too many requests. Please try again later.")
}