fix: do not save .runner unless it is modified (#1006)

Resolves forgejo/runner#548

<!--start release-notes-assistant-->
<!--URL:https://code.forgejo.org/forgejo/runner-->
- bug fixes
  - [PR](https://code.forgejo.org/forgejo/runner/pulls/1006): <!--number 1006 --><!--line 0 --><!--description Zml4OiBkbyBub3Qgc2F2ZSAucnVubmVyIHVubGVzcyBpdCBpcyBtb2RpZmllZA==-->fix: do not save .runner unless it is modified<!--description-->
<!--end release-notes-assistant-->

Reviewed-on: https://code.forgejo.org/forgejo/runner/pulls/1006
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

go.mod

@@ -19,6 +19,7 @@ require (
 	github.com/go-git/go-billy/v5 v5.6.2
 	github.com/go-git/go-git/v5 v5.16.2
 	github.com/gobwas/glob v0.2.3
+	github.com/google/go-cmp v0.7.0
 	github.com/google/uuid v1.6.0
 	github.com/joho/godotenv v1.5.1
 	github.com/julienschmidt/httprouter v1.3.0
@@ -62,7 +63,6 @@ require (
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/golang/groupcache v0.0.0-20241129210726-2c02b8208cf8 // indirect
-	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/google/shlex v0.0.0-20191202100458-e7afc7fbc510 // indirect
 	github.com/inconshreveable/mousetrap v1.1.0 // indirect
 	github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 // indirect

internal/pkg/config/registration.go

@@ -1,3 +1,4 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
 // Copyright 2023 The Gitea Authors. All rights reserved.
 // SPDX-License-Identifier: MIT
 
@@ -6,6 +7,8 @@ package config
 import (
 	"encoding/json"
 	"os"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 const registrationWarning = "This file is automatically generated by act-runner. Do not edit it manually unless you know what you are doing. Removing this file will cause act runner to re-register as a new runner."
@@ -34,12 +37,28 @@ func LoadRegistration(file string) (*Registration, error) {
 		return nil, err
 	}
 
-	reg.Warning = ""
-
 	return &reg, nil
 }
 
+func isEqualRegistration(file string, reg *Registration) (bool, error) {
+	existing, err := LoadRegistration(file)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil
+		}
+		return false, err
+	}
+	return cmp.Equal(*reg, *existing), nil
+}
+
 func SaveRegistration(file string, reg *Registration) error {
+	equal, err := isEqualRegistration(file, reg)
+	if err != nil {
+		return err
+	}
+	if equal {
+		return nil
+	}
 	f, err := os.Create(file)
 	if err != nil {
 		return err

internal/pkg/config/registration_test.go

@@ -0,0 +1,63 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestConfig_Registration(t *testing.T) {
+	reg := Registration{
+		Warning: registrationWarning,
+		ID:      1234,
+		UUID:    "UUID",
+		Name:    "NAME",
+		Token:   "TOKEN",
+		Address: "ADDRESS",
+		Labels:  []string{"LABEL1", "LABEL2"},
+	}
+
+	file := filepath.Join(t.TempDir(), ".runner")
+
+	// when the file does not exist, it is never equal
+	equal, err := isEqualRegistration(file, &reg)
+	require.NoError(t, err)
+	assert.False(t, equal)
+
+	require.NoError(t, SaveRegistration(file, &reg))
+
+	regReloaded, err := LoadRegistration(file)
+	require.NoError(t, err)
+	assert.Equal(t, reg, *regReloaded)
+
+	equal, err = isEqualRegistration(file, &reg)
+	require.NoError(t, err)
+	assert.True(t, equal)
+
+	// if the registration is not modified, it is not saved
+	time.Sleep(2 * time.Second) // file system precision on modification time is one second
+	before, err := os.Stat(file)
+	require.NoError(t, err)
+	require.NoError(t, SaveRegistration(file, &reg))
+	after, err := os.Stat(file)
+	require.NoError(t, err)
+	assert.Equal(t, before.ModTime(), after.ModTime())
+
+	reg.Labels = []string{"LABEL3"}
+	equal, err = isEqualRegistration(file, &reg)
+	require.NoError(t, err)
+	assert.False(t, equal)
+
+	// if the registration is modified, it is saved
+	require.NoError(t, SaveRegistration(file, &reg))
+	after, err = os.Stat(file)
+	require.NoError(t, err)
+	assert.NotEqual(t, before.ModTime(), after.ModTime())
+}