[bugfix] Rework MultiError to wrap + unwrap errors properly (#2057)

* rework multierror a bit

* test multierror

internal/gtserror/multi_test.go

@@ -0,0 +1,64 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package gtserror
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/superseriousbusiness/gotosocial/internal/db"
+)
+
+func TestMultiError(t *testing.T) {
+	errs := MultiError{
+		e: []error{
+			db.ErrNoEntries,
+			errors.New("oopsie woopsie we did a fucky wucky etc"),
+		},
+	}
+	errs.Appendf("appended + wrapped error: %w", db.ErrAlreadyExists)
+
+	err := errs.Combine()
+
+	if !errors.Is(err, db.ErrNoEntries) {
+		t.Error("should be db.ErrNoEntries")
+	}
+
+	if !errors.Is(err, db.ErrAlreadyExists) {
+		t.Error("should be db.ErrAlreadyExists")
+	}
+
+	if errors.Is(err, db.ErrBusyTimeout) {
+		t.Error("should not be db.ErrBusyTimeout")
+	}
+
+	errString := err.Error()
+	expected := `sql: no rows in result set
+oopsie woopsie we did a fucky wucky etc
+appended + wrapped error: already exists`
+	if errString != expected {
+		t.Errorf("errString '%s' should be '%s'", errString, expected)
+	}
+}
+
+func TestMultiErrorEmpty(t *testing.T) {
+	err := new(MultiError).Combine()
+	if err != nil {
+		t.Errorf("should be nil")
+	}
+}