Improve error message for TransferService

* Contrbutes to https://gitlab.com/gitlab-org/gitlab/-/issues/27843
* Inspired by
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68371 (Thanks
@1ace)

**Problem**

Previous error message is not clear and can be triggered by multiple
reasons.

**Solution**

Add descriptive error messages for each error case

Changelog: changed

app/services/projects/transfer_service.rb

@@ -20,8 +20,16 @@ module Projects
         raise TransferError, s_('TransferProject|Please select a new namespace for your project.')
       end
 
-      unless allowed_transfer?(current_user, project)
-        raise TransferError, s_('TransferProject|Transfer failed, please contact an admin.')
+      if @new_namespace.id == project.namespace_id
+        raise TransferError, s_('TransferProject|Project is already in this namespace.')
+      end
+
+      unless allowed_transfer_project?(current_user, project)
+        raise TransferError, s_("TransferProject|You don't have permission to transfer this project.")
+      end
+
+      unless allowed_to_transfer_to_namespace?(current_user, @new_namespace)
+        raise TransferError, s_("TransferProject|You don't have permission to transfer projects into that namespace.")
       end
 
       transfer(project)
@@ -121,11 +129,12 @@ module Projects
       Milestones::TransferService.new(current_user, group, project).execute
     end
 
-    def allowed_transfer?(current_user, project)
-      @new_namespace &&
-        can?(current_user, :change_namespace, project) &&
-        @new_namespace.id != project.namespace_id &&
-        current_user.can?(:transfer_projects, @new_namespace)
+    def allowed_transfer_project?(current_user, project)
+      current_user.can?(:change_namespace, project)
+    end
+
+    def allowed_to_transfer_to_namespace?(current_user, namespace)
+      current_user.can?(:transfer_projects, namespace)
     end
 
     def update_namespace_and_visibility(to_namespace)

locale/gitlab.pot

@@ -35180,13 +35180,19 @@ msgstr ""
 msgid "TransferProject|Project cannot be transferred, because tags are present in its container registry"
 msgstr ""
 
+msgid "TransferProject|Project is already in this namespace."
+msgstr ""
+
 msgid "TransferProject|Project with same name or path in target namespace already exists"
 msgstr ""
 
 msgid "TransferProject|Root namespace can't be updated if project has NPM packages"
 msgstr ""
 
-msgid "TransferProject|Transfer failed, please contact an admin."
+msgid "TransferProject|You don't have permission to transfer projects into that namespace."
+msgstr ""
+
+msgid "TransferProject|You don't have permission to transfer this project."
 msgstr ""
 
 msgid "Tree view"

spec/services/projects/transfer_service_spec.rb

@@ -292,10 +292,37 @@ RSpec.describe Projects::TransferService do
     end
   end
 
-  context 'target namespace allows developers to create projects' do
+  context 'target namespace matches current namespace' do
+    let(:group) { user.namespace }
+
+    it 'does not allow project transfer' do
+      transfer_result = execute_transfer
+
+      expect(transfer_result).to eq false
+      expect(project.namespace).to eq(user.namespace)
+      expect(project.errors[:new_namespace]).to include('Project is already in this namespace.')
+    end
+  end
+
+  context 'when user does not own the project' do
+    let(:project) { create(:project, :repository, :legacy_storage) }
+
+    before do
+      project.add_developer(user)
+    end
+
+    it 'does not allow project transfer to the target namespace' do
+      transfer_result = execute_transfer
+
+      expect(transfer_result).to eq false
+      expect(project.errors[:new_namespace]).to include("You don't have permission to transfer this project.")
+    end
+  end
+
+  context 'when user can create projects in the target namespace' do
     let(:group) { create(:group, project_creation_level: ::Gitlab::Access::DEVELOPER_MAINTAINER_PROJECT_ACCESS) }
 
-    context 'the user is a member of the target namespace with developer permissions' do
+    context 'but has only developer permissions in the target namespace' do
       before do
         group.add_developer(user)
       end
@@ -305,7 +332,7 @@ RSpec.describe Projects::TransferService do
 
         expect(transfer_result).to eq false
         expect(project.namespace).to eq(user.namespace)
-        expect(project.errors[:new_namespace]).to include('Transfer failed, please contact an admin.')
+        expect(project.errors[:new_namespace]).to include("You don't have permission to transfer projects into that namespace.")
       end
     end
   end