Allow to fork repository into the same owner (#32819)

This feature is experimental, not fully tested, and may be changed in
the future.

It is only designed for users who really need it: set
`[repository].ALLOW_FORK_INTO_SAME_OWNER=true` in your app.ini

Doc: https://gitea.com/gitea/docs/pulls/122


![image](https://github.com/user-attachments/assets/38d08c23-9cfc-49d8-9321-ff81edf65395)

3 files changed, 35 insertions, 1 deletions

diff --git a/modules/repository/fork.go b/modules/repository/fork.go
index fbf0008716..d530634071 100644
--- a/modules/repository/fork.go
+++ b/modules/repository/fork.go
@@ -9,14 +9,22 @@ import (
 	"code.gitea.io/gitea/models/organization"
 	repo_model "code.gitea.io/gitea/models/repo"
 	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/setting"
 )
 
+func CanUserForkBetweenOwners(id1, id2 int64) bool {
+	if id1 != id2 {
+		return true
+	}
+	return setting.Repository.AllowForkIntoSameOwner
+}
+
 // CanUserForkRepo returns true if specified user can fork repository.
 func CanUserForkRepo(ctx context.Context, user *user_model.User, repo *repo_model.Repository) (bool, error) {
 	if user == nil {
 		return false, nil
 	}
-	if repo.OwnerID != user.ID && !repo_model.HasForkedRepo(ctx, user.ID, repo.ID) {
+	if CanUserForkBetweenOwners(repo.OwnerID, user.ID) && !repo_model.HasForkedRepo(ctx, user.ID, repo.ID) {
 		return true, nil
 	}
 	ownedOrgs, err := organization.GetOrgsCanCreateRepoByUserID(ctx, user.ID)
diff --git a/modules/repository/fork_test.go b/modules/repository/fork_test.go
new file mode 100644
index 0000000000..f8c76d942d
--- /dev/null
+++ b/modules/repository/fork_test.go
@@ -0,0 +1,25 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repository
+
+import (
+	"testing"
+
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/test"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCanUserForkBetweenOwners(t *testing.T) {
+	defer test.MockVariableValue(&setting.Repository.AllowForkIntoSameOwner)
+
+	setting.Repository.AllowForkIntoSameOwner = true
+	assert.True(t, CanUserForkBetweenOwners(1, 1))
+	assert.True(t, CanUserForkBetweenOwners(1, 2))
+
+	setting.Repository.AllowForkIntoSameOwner = false
+	assert.False(t, CanUserForkBetweenOwners(1, 1))
+	assert.True(t, CanUserForkBetweenOwners(1, 2))
+}
diff --git a/modules/setting/repository.go b/modules/setting/repository.go
index 14cf5805c0..c5619d0f04 100644
--- a/modules/setting/repository.go
+++ b/modules/setting/repository.go
@@ -53,6 +53,7 @@ var (
 		AllowDeleteOfUnadoptedRepositories      bool
 		DisableDownloadSourceArchives           bool
 		AllowForkWithoutMaximumLimit            bool
+		AllowForkIntoSameOwner                  bool
 
 		// Repository editor settings
 		Editor struct {