fix IsPermanent and add test

Signed-off-by: Joe Elliott <number101010@gmail.com>

consumer/consumererror/permanenterror.go

@@ -17,12 +17,17 @@
 // error type/instance.
 package consumererror
 
+import "errors"
+
 // permanent is an error that will be always returned if its source
 // receives the same inputs.
 type permanent struct {
 	err error
 }
 
+// permanentError exists to test errors for "IsPermanent"
+var permanentError = &permanent{}
+
 // Permanent wraps an error to indicate that it is a permanent error, i.e.: an
 // error that will be always returned if its source receives the same inputs.
 func Permanent(err error) error {
@@ -38,8 +43,7 @@ func (p permanent) Error() string {
 // that its sources receives the same input.
 func IsPermanent(err error) bool {
 	if err != nil {
-		_, isPermanent := err.(permanent)
-		return isPermanent
+		return errors.As(err, permanentError)
 	}
 	return false
 }

consumer/consumererror/permanenterror_test.go

@@ -16,6 +16,7 @@ package consumererror
 
 import (
 	"errors"
+	"fmt"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -27,9 +28,12 @@ func TestPermanent(t *testing.T) {
 
 	err = Permanent(err)
 	require.True(t, IsPermanent(err))
+
+	err = fmt.Errorf("%w", err)
+	require.True(t, IsPermanent(err))
 }
 
 func TestIsPermanent_NilError(t *testing.T) {
 	var err error
 	require.False(t, IsPermanent(err))
-}
+}