Add X-Request-Start header middleware

internal/handler.go

@@ -31,6 +31,7 @@ func NewHandler(options HandlerOptions) http.Handler {
 		handler = http.MaxBytesHandler(handler, int64(options.maxRequestBody))
 	}
 
+	handler = NewRequestStartMiddleware(handler)
 	handler = NewLoggingMiddleware(slog.Default(), handler)
 
 	return handler

internal/handler_test.go

@@ -263,6 +263,23 @@ func TestHandlerXForwardedHeadersDropsExistingHeadersWhenForwardingNotEnabled(t
 	h.ServeHTTP(w, r)
 }
 
+func TestHandlerAddsXRequestStartHeader(t *testing.T) {
+	upstream := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		header := r.Header.Get("X-Request-Start")
+		assert.NotEmpty(t, header, "X-Request-Start header should be present")
+		assert.Regexp(t, `^t=\d+$`, header, "X-Request-Start header should be in format t=msec")
+	}))
+	defer upstream.Close()
+
+	h := NewHandler(handlerOptions(upstream.URL))
+
+	w := httptest.NewRecorder()
+	r := httptest.NewRequest("GET", "/", nil)
+	h.ServeHTTP(w, r)
+
+	assert.Equal(t, http.StatusOK, w.Code)
+}
+
 // Helpers
 
 func handlerOptions(targetUrl string) HandlerOptions {

internal/request_start_middleware.go

@@ -0,0 +1,17 @@
+package internal
+
+import (
+	"fmt"
+	"net/http"
+	"time"
+)
+
+func NewRequestStartMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.Header.Get("X-Request-Start") == "" {
+			timestamp := time.Now().UnixMilli()
+			r.Header.Set("X-Request-Start", fmt.Sprintf("t=%d", timestamp))
+		}
+		next.ServeHTTP(w, r)
+	})
+}

internal/request_start_middleware_test.go

@@ -0,0 +1,52 @@
+package internal
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strconv"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRequestStartMiddleware(t *testing.T) {
+	var capturedHeader string
+	nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		capturedHeader = r.Header.Get("X-Request-Start")
+	})
+
+	middleware := NewRequestStartMiddleware(nextHandler)
+
+	before := time.Now().UnixMilli()
+	req := httptest.NewRequest("GET", "/", nil)
+	w := httptest.NewRecorder()
+	middleware.ServeHTTP(w, req)
+	after := time.Now().UnixMilli()
+
+	assert.NotEmpty(t, capturedHeader)
+	assert.Regexp(t, `^t=\d+$`, capturedHeader)
+
+	timestampStr := capturedHeader[2:]
+	timestamp, err := strconv.ParseInt(timestampStr, 10, 64)
+	assert.NoError(t, err)
+	assert.GreaterOrEqual(t, timestamp, before)
+	assert.LessOrEqual(t, timestamp, after)
+}
+
+func TestRequestStartMiddlewareDoesNotOverwriteExistingHeader(t *testing.T) {
+	existingHeader := "t=1234567890"
+	var capturedHeader string
+	nextHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		capturedHeader = r.Header.Get("X-Request-Start")
+	})
+
+	middleware := NewRequestStartMiddleware(nextHandler)
+
+	req := httptest.NewRequest("GET", "/", nil)
+	req.Header.Set("X-Request-Start", existingHeader)
+	w := httptest.NewRecorder()
+	middleware.ServeHTTP(w, req)
+
+	assert.Equal(t, existingHeader, capturedHeader)
+}