Port "Use general token signing secret"

Port of https://github.com/go-gitea/gitea/pull/29205

Use a clearly defined "signing secret" for token signing.

(cherry picked from commit 8be198cdef0a486f417663b1fd6878458d7e5d92)

modules/setting/oauth2_test.go

@@ -0,0 +1,34 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/generate"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGetGeneralSigningSecret(t *testing.T) {
+	// when there is no general signing secret, it should be generated, and keep the same value
+	assert.Nil(t, generalSigningSecret.Load())
+	s1 := GetGeneralTokenSigningSecret()
+	assert.NotNil(t, s1)
+	s2 := GetGeneralTokenSigningSecret()
+	assert.Equal(t, s1, s2)
+
+	// the config value should always override any pre-generated value
+	cfg, _ := NewConfigProviderFromData(`
+[oauth2]
+JWT_SECRET = BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
+`)
+	defer test.MockVariableValue(&InstallLock, true)()
+	loadOAuth2From(cfg)
+	actual := GetGeneralTokenSigningSecret()
+	expected, _ := generate.DecodeJwtSecret("BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB")
+	assert.Len(t, actual, 32)
+	assert.EqualValues(t, expected, actual)
+}