transaction: Define TransactionError struct to provide more detailed error

If the transaction fails during start, there's no way to get the error
detail in a programmatic way, so let's inherit from error to allow more
per-type checks.

Author: 3v1n0

transaction.go

@@ -238,7 +238,10 @@ func StartFunc(service, user string, handler func(Style, string) (string, error)
 // transaction provides an interface to the remainder of the API.
 func StartConfDir(service, user string, handler ConversationHandler, confDir string) (*Transaction, error) {
 	if !CheckPamHasStartConfdir() {
-		return nil, errors.New("StartConfDir() was used, but the pam version on the system is not recent enough")
+		return nil, &TransactionError{
+			errors.New("StartConfDir() was used, but the pam version on the system is not recent enough"),
+			SystemErr,
+		}
 	}
 
 	return start(service, user, handler, confDir)
@@ -248,7 +251,10 @@ func start(service, user string, handler ConversationHandler, confDir string) (*
 	switch handler.(type) {
 	case BinaryConversationHandler:
 		if !CheckPamHasBinaryProtocol() {
-			return nil, errors.New("BinaryConversationHandler() was used, but it is not supported by this platform")
+			return nil, &TransactionError{
+				errors.New("BinaryConversationHandler() was used, but it is not supported by this platform"),
+				SystemErr,
+			}
 		}
 	}
 	t := &Transaction{
@@ -272,11 +278,34 @@ func start(service, user string, handler ConversationHandler, confDir string) (*
 		t.status = C.pam_start_confdir(s, u, t.conv, c, &t.handle)
 	}
 	if t.status != Success {
-		return nil, t
+		return nil, &TransactionError{t, ReturnType(t.status)}
 	}
 	return t, nil
 }
 
+// transactionError is a private interface that is implemented by both
+// TransactionError and Transaction
+type transactionError interface {
+	error
+	Status() ReturnType
+}
+
+// TransactionError extends error to provide more detailed information
+type TransactionError struct {
+	error
+	status ReturnType
+}
+
+// Status exposes the ReturnType for the error
+func (e *TransactionError) Status() ReturnType {
+	return ReturnType(e.status)
+}
+
+// Error pretty prints the error from the status message
+func (e *TransactionError) Error() string {
+	return errors.Join(e.error, ReturnType(e.status)).Error()
+}
+
 func (t *Transaction) Error() string {
 	return t.Status().Error()
 }

transaction_test.go

@@ -222,6 +222,14 @@ func TestPAM_ConfDir_FailNoServiceOrUnsupported(t *testing.T) {
 	if len(s) == 0 {
 		t.Fatalf("error #expected an error message")
 	}
+	switch et := err.(type) {
+	case transactionError:
+		if et.Status() != Abort {
+			t.Fatalf("error #unexpected status: %v", et.Status())
+		}
+	default:
+		t.Fatalf("error #unexpected type: %v", et)
+	}
 }
 
 func TestPAM_ConfDir_InfoMessage(t *testing.T) {