[bugfix] Allow unsetting filter expiration dates (#3560)

* Regression tests for #3497 (v1 and v2)
* use Nullable type for v2 form.expires_in

---------

Co-authored-by: tobi <tobi.smethurst@protonmail.com>

internal/api/model/nullable.go

@@ -0,0 +1,107 @@
+// 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 model
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+)
+
+// Nullable is a generic type, which implements a field that can be one of three states:
+//
+// - field is not set in the request
+// - field is explicitly set to `null` in the request
+// - field is explicitly set to a valid value in the request
+//
+// Nullable is intended to be used with JSON unmarshalling.
+//
+// Adapted from https://github.com/oapi-codegen/nullable/blob/main/nullable.go
+type Nullable[T any] struct {
+	state nullableState
+	value T
+}
+
+type nullableState uint8
+
+const (
+	nullableStateUnspecified nullableState = 0
+	nullableStateNull        nullableState = 1
+	nullableStateSet         nullableState = 2
+)
+
+// Get retrieves the underlying value, if present,
+// and returns an error if the value was not present.
+func (t Nullable[T]) Get() (T, error) {
+	var empty T
+	if t.IsNull() {
+		return empty, errors.New("value is null")
+	}
+
+	if !t.IsSpecified() {
+		return empty, errors.New("value is not specified")
+	}
+
+	return t.value, nil
+}
+
+// IsNull indicates whether the field
+// was sent, and had a value of `null`
+func (t Nullable[T]) IsNull() bool {
+	return t.state == nullableStateNull
+}
+
+// IsSpecified indicates whether the field
+// was sent either as a value or as `null`.
+func (t Nullable[T]) IsSpecified() bool {
+	return t.state != nullableStateUnspecified
+}
+
+// If field is unspecified,
+// UnmarshalJSON won't be called.
+func (t *Nullable[T]) UnmarshalJSON(data []byte) error {
+	// If field is specified as `null`.
+	if bytes.Equal(data, []byte("null")) {
+		t.setNull()
+		return nil
+	}
+
+	// Otherwise, we have an
+	// actual value, so parse it.
+	var v T
+	if err := json.Unmarshal(data, &v); err != nil {
+		return err
+	}
+
+	t.set(v)
+	return nil
+}
+
+// setNull indicates that the field
+// was sent, and had a value of `null`
+func (t *Nullable[T]) setNull() {
+	*t = Nullable[T]{state: nullableStateNull}
+}
+
+// set the underlying value to given value.
+func (t *Nullable[T]) set(value T) {
+	*t = Nullable[T]{
+		state: nullableStateSet,
+		value: value,
+	}
+}