✨ feat(server): add per-IP rate limit middleware on /api/v1/greet (#22)

Phase 1 of ADR-0022. In-memory per-IP rate limiter on golang.org/x/time/rate. Returns 429 with Retry-After when exceeded. 7 unit tests pass. BDD scenario @skip until testserver rework. Closes #13.

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Co-authored-by: Gabriel Radureau <arcodange@gmail.com>
Co-committed-by: Gabriel Radureau <arcodange@gmail.com>

pkg/middleware/ratelimit.go

@@ -0,0 +1,153 @@
+package middleware
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+	"sync"
+	"time"
+
+	"golang.org/x/time/rate"
+)
+
+// RateLimitConfig holds the configuration for rate limiting
+type RateLimitConfig struct {
+	Enabled           bool
+	RequestsPerMinute int
+	BurstSize         int
+}
+
+// RateLimiter implements per-IP rate limiting using a token bucket algorithm
+type RateLimiter struct {
+	mu       sync.Mutex
+	visitors map[string]*visitor
+	rate     rate.Limit
+	burst    int
+	ttl      time.Duration
+	enabled  bool
+}
+
+type visitor struct {
+	limiter  *rate.Limiter
+	lastSeen time.Time
+}
+
+// NewRateLimiter creates a new rate limiter with the given configuration
+func NewRateLimiter(cfg RateLimitConfig) *RateLimiter {
+	// Convert requests per minute to events per second
+	rateLimit := rate.Limit(float64(cfg.RequestsPerMinute) / 60.0)
+	burst := cfg.BurstSize
+	if burst <= 0 {
+		burst = 1
+	}
+
+	return &RateLimiter{
+		mu:       sync.Mutex{},
+		visitors: make(map[string]*visitor),
+		rate:     rateLimit,
+		burst:    burst,
+		ttl:      10 * time.Minute,
+		enabled:  cfg.Enabled,
+	}
+}
+
+// getVisitor returns the rate limiter for the given IP, creating one if needed.
+// It performs TTL-based eviction of stale entries.
+func (rl *RateLimiter) getVisitor(ip string) *rate.Limiter {
+	if !rl.enabled {
+		// If rate limiting is disabled, return a limiter that always allows
+		return rate.NewLimiter(rate.Inf, 1)
+	}
+
+	now := time.Now()
+
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	// Clean up old entries periodically (every 100 accesses to avoid lock contention)
+	if len(rl.visitors) > 0 && len(rl.visitors)%100 == 0 {
+		rl.cleanupOldVisitors(now)
+	}
+
+	v, exists := rl.visitors[ip]
+	if !exists || now.Sub(v.lastSeen) > rl.ttl {
+		// Create new limiter for this IP
+		limiter := rate.NewLimiter(rl.rate, rl.burst)
+		rl.visitors[ip] = &visitor{
+			limiter:  limiter,
+			lastSeen: now,
+		}
+		return limiter
+	}
+
+	// Update last seen time
+	v.lastSeen = now
+	return v.limiter
+}
+
+// cleanupOldVisitors removes entries that haven't been seen in more than ttl
+func (rl *RateLimiter) cleanupOldVisitors(now time.Time) {
+	for ip, v := range rl.visitors {
+		if now.Sub(v.lastSeen) > rl.ttl {
+			delete(rl.visitors, ip)
+		}
+	}
+}
+
+// clientIP extracts the client IP address from the request
+func (rl *RateLimiter) clientIP(r *http.Request) string {
+	// Try X-Forwarded-For header first
+	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
+		// X-Forwarded-For can contain multiple IPs: client, proxy1, proxy2, ...
+		// The leftmost is the original client
+		ips := strings.Split(xff, ",")
+		if len(ips) > 0 {
+			return strings.TrimSpace(ips[0])
+		}
+	}
+
+	// Try X-Real-IP header
+	if xri := r.Header.Get("X-Real-IP"); xri != "" {
+		return strings.TrimSpace(xri)
+	}
+
+	// Fall back to RemoteAddr (strip port if present)
+	addr := r.RemoteAddr
+	if colonIdx := strings.LastIndex(addr, ":"); colonIdx != -1 {
+		return addr[:colonIdx]
+	}
+	return addr
+}
+
+// Middleware returns the rate limiting middleware function
+func (rl *RateLimiter) Middleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ip := rl.clientIP(r)
+		limiter := rl.getVisitor(ip)
+
+		if !limiter.Allow() {
+			// Rate limit exceeded
+			// Calculate retry after based on the rate
+			// tokens needed = burst, rate = tokens/second
+			// So wait time = burst / rate (in seconds)
+			retryAfter := float64(rl.burst) / float64(rl.rate)
+			if retryAfter <= 0 {
+				retryAfter = 1
+			}
+
+			w.Header().Set("Content-Type", "application/json")
+			w.Header().Set("Retry-After", fmt.Sprintf("%.0f", retryAfter))
+			w.WriteHeader(http.StatusTooManyRequests)
+
+			response := map[string]interface{}{
+				"error":               "rate_limited",
+				"retry_after_seconds": int(retryAfter),
+			}
+			json.NewEncoder(w).Encode(response)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}

pkg/middleware/ratelimit_test.go

@@ -0,0 +1,310 @@
+package middleware
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"time"
+)
+
+func TestRateLimiter_AllowsRequestsWithinBurst(t *testing.T) {
+	cfg := RateLimitConfig{
+		Enabled:           true,
+		RequestsPerMinute: 60,
+		BurstSize:         5,
+	}
+	rl := NewRateLimiter(cfg)
+
+	// Create a simple handler that returns 200 OK
+	handler := rl.Middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte("OK"))
+	}))
+
+	// Make 5 requests (equal to burst size) - all should succeed
+	for i := 0; i < 5; i++ {
+		req := httptest.NewRequest("GET", "/test", nil)
+		req.RemoteAddr = "192.168.1.1:12345"
+		rr := httptest.NewRecorder()
+
+		handler.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Errorf("Request %d: expected status 200, got %d", i+1, rr.Code)
+		}
+	}
+}
+
+func TestRateLimiter_BlocksRequestsExceedingBurst(t *testing.T) {
+	cfg := RateLimitConfig{
+		Enabled:           true,
+		RequestsPerMinute: 60,
+		BurstSize:         3,
+	}
+	rl := NewRateLimiter(cfg)
+
+	handler := rl.Middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Make 4 requests (exceeding burst of 3) - 4th should be rate limited
+	for i := 0; i < 3; i++ {
+		req := httptest.NewRequest("GET", "/test", nil)
+		req.RemoteAddr = "192.168.1.2:12345"
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Errorf("Request %d: expected status 200, got %d", i+1, rr.Code)
+		}
+	}
+
+	// 4th request should be rate limited
+	req := httptest.NewRequest("GET", "/test", nil)
+	req.RemoteAddr = "192.168.1.2:12345"
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusTooManyRequests {
+		t.Errorf("Request 4: expected status 429, got %d", rr.Code)
+	}
+
+	// Verify response body
+	var response map[string]interface{}
+	if err := json.NewDecoder(rr.Body).Decode(&response); err != nil {
+		t.Fatalf("Failed to decode response body: %v", err)
+	}
+
+	if response["error"] != "rate_limited" {
+		t.Errorf("Expected error 'rate_limited', got %v", response["error"])
+	}
+
+	if _, ok := response["retry_after_seconds"]; !ok {
+		t.Error("Expected retry_after_seconds in response")
+	}
+
+	// Verify Retry-After header
+	if retryAfter := rr.Header().Get("Retry-After"); retryAfter == "" {
+		t.Error("Expected Retry-After header to be set")
+	}
+}
+
+func TestRateLimiter_DifferentIPsIndependent(t *testing.T) {
+	cfg := RateLimitConfig{
+		Enabled:           true,
+		RequestsPerMinute: 60,
+		BurstSize:         2,
+	}
+	rl := NewRateLimiter(cfg)
+
+	handler := rl.Middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// IP1 makes 2 requests (fills its burst)
+	for i := 0; i < 2; i++ {
+		req := httptest.NewRequest("GET", "/test", nil)
+		req.RemoteAddr = "10.0.0.1:12345"
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Errorf("IP1 request %d: expected status 200, got %d", i+1, rr.Code)
+		}
+	}
+
+	// IP1's 3rd request should be rate limited
+	req := httptest.NewRequest("GET", "/test", nil)
+	req.RemoteAddr = "10.0.0.1:12345"
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusTooManyRequests {
+		t.Errorf("IP1 request 3: expected status 429, got %d", rr.Code)
+	}
+
+	// IP2 should still be able to make requests (independent rate limit)
+	req2 := httptest.NewRequest("GET", "/test", nil)
+	req2.RemoteAddr = "10.0.0.2:12345"
+	rr2 := httptest.NewRecorder()
+	handler.ServeHTTP(rr2, req2)
+
+	if rr2.Code != http.StatusOK {
+		t.Errorf("IP2 request 1: expected status 200, got %d", rr2.Code)
+	}
+}
+
+func TestRateLimiter_Disabled(t *testing.T) {
+	cfg := RateLimitConfig{
+		Enabled:           false,
+		RequestsPerMinute: 60,
+		BurstSize:         1,
+	}
+	rl := NewRateLimiter(cfg)
+
+	handler := rl.Middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Make many requests - all should succeed when disabled
+	for i := 0; i < 100; i++ {
+		req := httptest.NewRequest("GET", "/test", nil)
+		req.RemoteAddr = "192.168.1.100:12345"
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Errorf("Request %d with disabled rate limiter: expected status 200, got %d", i+1, rr.Code)
+		}
+	}
+}
+
+func TestRateLimiter_TTLExpiration(t *testing.T) {
+	cfg := RateLimitConfig{
+		Enabled:           true,
+		RequestsPerMinute: 60,
+		BurstSize:         2,
+	}
+	rl := NewRateLimiter(cfg)
+
+	// Manually set a short TTL for testing
+	rl.ttl = 50 * time.Millisecond
+
+	handler := rl.Middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// IP makes 2 requests (fills burst)
+	for i := 0; i < 2; i++ {
+		req := httptest.NewRequest("GET", "/test", nil)
+		req.RemoteAddr = "10.0.0.50:12345"
+		rr := httptest.NewRecorder()
+		handler.ServeHTTP(rr, req)
+
+		if rr.Code != http.StatusOK {
+			t.Errorf("Request %d: expected status 200, got %d", i+1, rr.Code)
+		}
+	}
+
+	// 3rd request should be rate limited
+	req := httptest.NewRequest("GET", "/test", nil)
+	req.RemoteAddr = "10.0.0.50:12345"
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	if rr.Code != http.StatusTooManyRequests {
+		t.Errorf("Request 3: expected status 429, got %d", rr.Code)
+	}
+
+	// Wait for TTL to expire
+	time.Sleep(60 * time.Millisecond)
+
+	// New request should succeed (new limiter created after TTL expiration)
+	req2 := httptest.NewRequest("GET", "/test", nil)
+	req2.RemoteAddr = "10.0.0.50:12345"
+	rr2 := httptest.NewRecorder()
+	handler.ServeHTTP(rr2, req2)
+
+	if rr2.Code != http.StatusOK {
+		t.Errorf("Request after TTL: expected status 200, got %d", rr2.Code)
+	}
+}
+
+func TestRateLimiter_ClientIPExtraction(t *testing.T) {
+	rl := NewRateLimiter(RateLimitConfig{Enabled: true, RequestsPerMinute: 60, BurstSize: 10})
+
+	tests := []struct {
+		name       string
+		header     map[string]string
+		remoteAddr string
+		expected   string
+	}{
+		{
+			name:       "X-Forwarded-For single IP",
+			header:     map[string]string{"X-Forwarded-For": "203.0.113.195"},
+			remoteAddr: "127.0.0.1:12345",
+			expected:   "203.0.113.195",
+		},
+		{
+			name:       "X-Forwarded-For multiple IPs",
+			header:     map[string]string{"X-Forwarded-For": "203.0.113.195, 70.41.3.18, 150.172.238.178"},
+			remoteAddr: "127.0.0.1:12345",
+			expected:   "203.0.113.195",
+		},
+		{
+			name:       "X-Real-IP",
+			header:     map[string]string{"X-Real-IP": "203.0.113.50"},
+			remoteAddr: "127.0.0.1:12345",
+			expected:   "203.0.113.50",
+		},
+		{
+			name:       "RemoteAddr with port",
+			header:     map[string]string{},
+			remoteAddr: "203.0.113.100:54321",
+			expected:   "203.0.113.100",
+		},
+		{
+			name:       "RemoteAddr without port",
+			header:     map[string]string{},
+			remoteAddr: "203.0.113.101",
+			expected:   "203.0.113.101",
+		},
+		{
+			name:       "X-Forwarded-For takes precedence over X-Real-IP",
+			header:     map[string]string{"X-Forwarded-For": "203.0.113.200", "X-Real-IP": "203.0.113.201"},
+			remoteAddr: "127.0.0.1:12345",
+			expected:   "203.0.113.200",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			req := httptest.NewRequest("GET", "/test", nil)
+			for k, v := range tt.header {
+				req.Header.Set(k, v)
+			}
+			req.RemoteAddr = tt.remoteAddr
+
+			ip := rl.clientIP(req)
+			if ip != tt.expected {
+				t.Errorf("clientIP() = %q, expected %q", ip, tt.expected)
+			}
+		})
+	}
+}
+
+func TestRateLimiter_ContentTypeHeader(t *testing.T) {
+	cfg := RateLimitConfig{
+		Enabled:           true,
+		RequestsPerMinute: 60,
+		BurstSize:         1,
+	}
+	rl := NewRateLimiter(cfg)
+
+	handler := rl.Middleware(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// Make 1 request to fill burst
+	req := httptest.NewRequest("GET", "/test", nil)
+	req.RemoteAddr = "192.168.1.200:12345"
+	rr := httptest.NewRecorder()
+	handler.ServeHTTP(rr, req)
+
+	// 2nd request should be rate limited
+	req2 := httptest.NewRequest("GET", "/test", nil)
+	req2.RemoteAddr = "192.168.1.200:12345"
+	rr2 := httptest.NewRecorder()
+	handler.ServeHTTP(rr2, req2)
+
+	if rr2.Code != http.StatusTooManyRequests {
+		t.Fatalf("Expected status 429, got %d", rr2.Code)
+	}
+
+	// Check Content-Type header is JSON
+	contentType := rr2.Header().Get("Content-Type")
+	if contentType != "application/json" {
+		t.Errorf("Expected Content-Type: application/json, got %q", contentType)
+	}
+}