Add transfer project endpoint to the Projects API

changelogs/unreleased/transfer_project_api_endpoint.yml

+---
+title: Add transfer project API endpoint
+merge_request: 20122
+author: Aram Visser
+type: added

doc/api/projects.md

@@ -1390,6 +1390,16 @@ POST /projects/:id/housekeeping
 | --------- | ---- | -------- | ----------- |
 | `id` | integer/string | yes | The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) |
 
+### Transfer a project to a new namespace
+
+```
+PUT /projects/:id/transfer
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `namespace` | integer/string | yes | The ID or path of the namespace to transfer to project to |
+
 ## Branches
 
 Read more in the [Branches](branches.md) documentation.

lib/api/projects.rb

@@ -459,6 +459,23 @@ module API
           conflict!(error.message)
         end
       end
+
+      desc 'Transfer a project to a new namespace'
+      params do
+        requires :namespace, type: String, desc: 'The ID or path of the new namespace'
+      end
+      put ":id/transfer" do
+        authorize! :change_namespace, user_project
+
+        namespace = find_namespace!(params[:namespace])
+        result = ::Projects::TransferService.new(user_project, current_user).execute(namespace)
+
+        if result
+          present user_project, with: Entities::Project
+        else
+          render_api_error!("Failed to transfer project #{user_project.errors.messages}", 400)
+        end
+      end
     end
   end
 end

spec/requests/api/projects_spec.rb

@@ -1990,6 +1990,38 @@ describe API::Projects do
     end
   end
 
+  describe 'PUT /projects/:id/transfer' do
+    context 'when authenticated as owner' do
+      let(:group) { create :group }
+
+      it 'transfers the project to the new namespace' do
+        group.add_owner(user)
+
+        put api("/projects/#{project.id}/transfer", user), namespace: group.id
+
+        expect(response).to have_gitlab_http_status(200)
+      end
+
+      it 'fails when transferring to a non owned namespace' do
+        put api("/projects/#{project.id}/transfer", user), namespace: group.id
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+
+      it 'fails when transferring to an unknown namespace' do
+        put api("/projects/#{project.id}/transfer", user), namespace: 'unknown'
+
+        expect(response).to have_gitlab_http_status(404)
+      end
+
+      it 'fails on missing namespace' do
+        put api("/projects/#{project.id}/transfer", user)
+
+        expect(response).to have_gitlab_http_status(400)
+      end
+    end
+  end
+
   it_behaves_like 'custom attributes endpoints', 'projects' do
     let(:attributable) { project }
     let(:other_attributable) { project2 }