feat: add keyvalue.Remember function

pkg/modules/keyvalue/keyvalue.go

@@ -86,3 +86,27 @@ func ListKeys(prefix string) ([]string, error) {
 func DelPrefix(prefix string) error {
 	return store.DelPrefix(prefix)
 }
+
+// Remember returns the value for a key if it exists.
+// If the key is not present, it executes fn to calculate the value,
+// stores it and then returns it.
+func Remember(key string, fn func() (any, error)) (any, error) {
+	val, exists, err := Get(key)
+	if err != nil {
+		return nil, err
+	}
+	if exists {
+		return val, nil
+	}
+
+	val, err = fn()
+	if err != nil {
+		return nil, err
+	}
+
+	if err := Put(key, val); err != nil {
+		return nil, err
+	}
+
+	return val, nil
+}

pkg/modules/keyvalue/keyvalue_test.go

@@ -0,0 +1,83 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// 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 <https://www.gnu.org/licenses/>.
+
+package keyvalue
+
+import (
+	"errors"
+	"testing"
+
+	"code.vikunja.io/api/pkg/modules/keyvalue/memory"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRememberReturnsExisting(t *testing.T) {
+	store = memory.NewStorage()
+	err := Put("foo", "bar")
+	require.NoError(t, err)
+
+	called := false
+	val, err := Remember("foo", func() (interface{}, error) {
+		called = true
+		return "baz", nil
+	})
+
+	require.NoError(t, err)
+	assert.Equal(t, "bar", val)
+	assert.False(t, called)
+}
+
+func TestRememberComputesAndStores(t *testing.T) {
+	store = memory.NewStorage()
+
+	called := 0
+	val, err := Remember("foo", func() (interface{}, error) {
+		called++
+		return "bar", nil
+	})
+
+	require.NoError(t, err)
+	assert.Equal(t, "bar", val)
+	assert.Equal(t, 1, called)
+
+	v, exists, err := Get("foo")
+	require.NoError(t, err)
+	assert.True(t, exists)
+	assert.Equal(t, "bar", v)
+
+	val, err = Remember("foo", func() (interface{}, error) {
+		called++
+		return "baz", nil
+	})
+
+	require.NoError(t, err)
+	assert.Equal(t, "bar", val)
+	assert.Equal(t, 1, called)
+}
+
+func TestRememberErrorDoesNotStore(t *testing.T) {
+	store = memory.NewStorage()
+
+	_, err := Remember("foo", func() (interface{}, error) {
+		return nil, errors.New("fail")
+	})
+
+	require.Error(t, err)
+	_, exists, err2 := Get("foo")
+	require.NoError(t, err2)
+	assert.False(t, exists)
+}