fix: handle nested config braces

cmd/internal/migrations/v3/common.go

@@ -291,6 +291,8 @@ func extractCall(src string, start int) (int, string) {
 // the opening delimiter at the start position. Start must point to the first
 // character after the opening delimiter. It handles nested delimiters and
 // quoted strings.
+//
+//nolint:unparam // keep open for potential future delimiters
 func extractBlock(src string, start int, open, closeDelim byte) int {
 	depth := 1
 	inStr := false

cmd/internal/migrations/v3/config_listener_fields.go

@@ -19,8 +19,9 @@ func MigrateConfigListenerFields(cmd *cobra.Command, cwd string, _, _ *semver.Ve
 
 	reField := regexp.MustCompile(`\s*(Prefork|Network|DisableStartupMessage|EnablePrintRoutes):\s*([^,}]+),?\s*(//[^\n]*)?`)
 	changed1, err := internal.ChangeFileContent(cwd, func(content string) string {
-		reConfig := regexp.MustCompile(`fiber\.Config\{[^}]*\}`)
-		return reConfig.ReplaceAllStringFunc(content, func(cfg string) string {
+		reConfigStart := regexp.MustCompile(`fiber\.Config{`)
+
+		process := func(cfg string) string {
 			inner := cfg[len("fiber.Config{") : len(cfg)-1]
 			inner = reField.ReplaceAllStringFunc(inner, func(f string) string {
 				sub := reField.FindStringSubmatch(f)
@@ -63,7 +64,25 @@ func MigrateConfigListenerFields(cmd *cobra.Command, cwd string, _, _ *semver.Ve
 			}
 			inner = strings.TrimSuffix(inner, ",")
 			return "fiber.Config{" + inner + "}"
-		})
+		}
+
+		var b strings.Builder
+		last := 0
+		for _, loc := range reConfigStart.FindAllStringIndex(content, -1) {
+			b.WriteString(content[last:loc[0]]) //nolint:errcheck // WriteString never returns an error
+			start := loc[1]
+			end := extractBlock(content, start, '{', '}')
+			if end > start {
+				cfg := content[loc[0]:end]
+				b.WriteString(process(cfg)) //nolint:errcheck // WriteString never returns an error
+				last = end
+			} else {
+				b.WriteString(content[loc[0]:loc[1]]) //nolint:errcheck // WriteString never returns an error
+				last = loc[1]
+			}
+		}
+		b.WriteString(content[last:]) //nolint:errcheck // WriteString never returns an error
+		return b.String()
 	})
 	if err != nil {
 		return fmt.Errorf("failed to migrate listener related config fields: %w", err)

cmd/internal/migrations/v3/config_listener_fields_test.go

@@ -283,3 +283,50 @@ func main() {
 	assert.Contains(t, content, `app.Listen(":3000", fiber.ListenConfig{EnablePrefork: prod})`)
 	assert.Contains(t, buf.String(), "Migrating listener related config fields")
 }
+
+func Test_MigrateConfigListenerFields_WithErrorHandler(t *testing.T) {
+	t.Parallel()
+
+	dir, err := os.MkdirTemp("", "mconf_errhandler")
+	require.NoError(t, err)
+	defer func() { require.NoError(t, os.RemoveAll(dir)) }()
+
+	file1 := filepath.Join(dir, "app.go")
+	require.NoError(t, os.WriteFile(file1, []byte(`package main
+import (
+    "github.com/gofiber/fiber/v2"
+    "net/http"
+)
+func newApp() *fiber.App {
+    var timesHandlingError int
+    return fiber.New(fiber.Config{
+        ErrorHandler: func(ctx *fiber.Ctx, err error) error {
+            timesHandlingError++
+            if err != nil {
+                return fiber.NewError(http.StatusInternalServerError, err.Error())
+            }
+            return nil
+        },
+        Prefork: true,
+    })
+}`), 0o600))
+
+	file2 := filepath.Join(dir, "main.go")
+	require.NoError(t, os.WriteFile(file2, []byte(`package main
+func main() {
+    app := newApp()
+    app.Listen(":3000")
+}`), 0o600))
+
+	var buf bytes.Buffer
+	cmd := newCmd(&buf)
+	require.NoError(t, v3.MigrateConfigListenerFields(cmd, dir, nil, nil))
+
+	content1 := readFile(t, file1)
+	assert.Contains(t, content1, "ErrorHandler: func(ctx *fiber.Ctx, err error) error {")
+	assert.NotContains(t, content1, "Prefork:")
+
+	content2 := readFile(t, file2)
+	assert.Contains(t, content2, `app.Listen(":3000", fiber.ListenConfig{EnablePrefork: true})`)
+	assert.Contains(t, buf.String(), "Migrating listener related config fields")
+}