add ErrFlagsAlreadyParsed; fix tests

box_test.go

@@ -3,7 +3,6 @@ package box_test
 import (
 	"context"
 	"errors"
-	"flag"
 	"log/slog"
 	"net/http"
 	"testing"
@@ -53,15 +52,20 @@ func TestWithConfigFromPath(t *testing.T) {
 }
 
 func TestWithFlags(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			// we expect to panic due to go test already parsing flags
+			if !errors.Is(r.(error), box.ErrFlagsAlreadyParsed) {
+				t.Errorf("expected %s, got %s", box.ErrFlagsAlreadyParsed, r)
+			}
+		}
+	}()
+
 	b := box.New(box.WithFlags())
 	if b == nil {
 		t.Error("box.New() returned nil")
 		return
 	}
-
-	if !flag.Parsed() {
-		t.Errorf("flag.Parsed() should be true")
-	}
 }
 
 func TestWithGlobalLogger(t *testing.T) {

options.go

@@ -1,6 +1,7 @@
 package box
 
 import (
+	"errors"
 	"flag"
 	"net/http"
 	"os"
@@ -11,6 +12,9 @@ import (
 	"gopkg.in/yaml.v3"
 )
 
+// ErrFlagsAlreadyParsed indicates that the flag.Parse function has already been called.
+var ErrFlagsAlreadyParsed = errors.New("flag.Parse() has already been called")
+
 // Option is a modifier function which can alter the provided functionality of a Box.
 type Option func(*Box)
 
@@ -59,7 +63,7 @@ func WithFlags() Option {
 		flag.StringVar(&box.Config.TLSKeyFile, "tls-key-file", box.Config.TLSKeyFile, "Webserver TLS key file")
 
 		if flag.Parsed() {
-			panic("flag.Parse() has already been called")
+			panic(ErrFlagsAlreadyParsed)
 		}
 
 		flag.Parse()