fix(rules): buffer log output before writing to stdout/stderr

internal/route/rules/do.go

@@ -470,17 +470,29 @@ var commands = map[string]struct {
 		build: func(args any) HandlerFunc {
 			level, f, tmpl := args.(*onLogArgs).Unpack()
 			var logger io.Writer
-			if f == stdout || f == stderr {
+			isStdLogger := f == stdout || f == stderr
+			if isStdLogger {
 				logger = logging.NewLoggerWithFixedLevel(level, f)
 			} else {
 				logger = f
 			}
 			return func(w *httputils.ResponseModifier, r *http.Request, upstream http.HandlerFunc) error {
-				_, err := tmpl.ExpandVars(w, r, logger)
-				if err != nil {
+				if isStdLogger {
+					bufPool := w.BufPool()
+					buf := bufPool.GetBuffer()
+					defer bufPool.PutBuffer(buf)
+
+					if _, err := tmpl.ExpandVars(w, r, buf); err != nil {
+						return err
+					}
+					if buf.Len() == 0 {
+						return nil
+					}
+					_, err := logger.Write(buf.Bytes())
 					return err
 				}
-				return nil
+				_, err := tmpl.ExpandVars(w, r, logger)
+				return err
 			}
 		},
 	},

internal/route/rules/do_log_test.go

@@ -1,6 +1,7 @@
 package rules
 
 import (
+	"bytes"
 	"fmt"
 	"maps"
 	"net/http"
@@ -8,6 +9,7 @@ import (
 	"reflect"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -69,6 +71,17 @@ default {
 }
 
 func TestLogCommand_StdoutAndStderr(t *testing.T) {
+	originalStdout := stdout
+	originalStderr := stderr
+	var stdoutBuf bytes.Buffer
+	var stderrBuf bytes.Buffer
+	stdout = noopWriteCloser{&stdoutBuf}
+	stderr = noopWriteCloser{&stderrBuf}
+	defer func() {
+		stdout = originalStdout
+		stderr = originalStderr
+	}()
+
 	upstream := mockUpstream(http.StatusOK, "success")
 
 	var rules Rules
@@ -88,8 +101,12 @@ default {
 	handler.ServeHTTP(w, req)
 
 	assert.Equal(t, http.StatusOK, w.Code)
-	// Note: We can't easily capture stdout/stderr in unit tests,
-	// but we can verify no errors occurred and the handler completed
+	require.Eventually(t, func() bool {
+		return strings.Contains(stdoutBuf.String(), "stdout: GET 200") &&
+			strings.Contains(stderrBuf.String(), "stderr: /test 200")
+	}, time.Second, 10*time.Millisecond)
+	assert.Equal(t, 1, strings.Count(stdoutBuf.String(), "stdout: GET 200"))
+	assert.Equal(t, 1, strings.Count(stderrBuf.String(), "stderr: /test 200"))
 }
 
 func TestLogCommand_DifferentLogLevels(t *testing.T) {