Added fork API paremeter for a specific namespace

Browser interface allows forking to an owned grup.
This commit brings API up to speed by providing optional namespace
parameter to fork API. This allows forking to users and groups under
forker's control using their id or unique name.

Fixes #21591

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.12.0 (unreleased)
+  - Add abillity to fork to a specific namespace using API. @ritave <olaf@tomalka.me>
   - Prepend blank line to `Closes` message on merge request linked to issue (lukehowell)
   - Filter tags by name !6121
   - Make push events have equal vertical spacing.

doc/api/projects.md

@@ -514,7 +514,7 @@ invalid, 400 is returned.
 
 ### Fork project
 
-Forks a project into the user namespace of the authenticated user.
+Forks a project into the user namespace of the authenticated user or the one provided.
 
 ```
 POST /projects/fork/:id
@@ -523,6 +523,7 @@ POST /projects/fork/:id
 Parameters:
 
 - `id` (required) - The ID or NAMESPACE/PROJECT_NAME of the project to be forked
+- `namespace` (optional) - The ID or path of the namespace that the project will be forked to
 
 ### Star a project
 

lib/api/projects.rb

@@ -189,16 +189,28 @@ module API
         end
       end
 
-      # Fork new project for the current user.
+      # Fork new project for the current user or provided namespace.
       #
       # Parameters:
       #   id (required) - The ID of a project
+      #   namespace (optional) - The ID or name of the namespace that the project will be forked into.
       # Example Request
       #   POST /projects/fork/:id
       post 'fork/:id' do
+        attrs = {}
+        namespace_id = params[:namespace]
+        if namespace_id.present?
+          namespace = Namespace.find_by(id: namespace_id) || Namespace.find_by_path_or_name(namespace_id)
+          if namespace.nil?
+            not_found!('Target Namespace')
+          end
+          authorize! :create_projects, namespace
+          attrs[:namespace] = namespace
+        end
         @forked_project =
           ::Projects::ForkService.new(user_project,
-                                      current_user).execute
+                                      current_user,
+                                      attrs).execute
         if @forked_project.errors.any?
           conflict!(@forked_project.errors.messages)
         else

spec/requests/api/fork_spec.rb

@@ -3,9 +3,15 @@ require 'spec_helper'
 describe API::API, api: true  do
   include ApiHelpers
   let(:user)  { create(:user) }
-  let(:user2) { create(:user) }
+  let(:user2) { create(:user, username: 'user2_name') }
   let(:user3) { create(:user) }
   let(:admin) { create(:admin) }
+  let(:group) { create(:group, name: 'group_name') }
+  let(:group2) do
+    group = create(:group, name: 'group2_name')
+    group.add_owner(user2)
+    group
+  end
 
   let(:project) do
     create(:project, creator_id: user.id, namespace: user.namespace)
@@ -58,6 +64,52 @@ describe API::API, api: true  do
         expect(response).to have_http_status(404)
         expect(json_response['message']).to eq('404 Project Not Found')
       end
+
+      it 'forks with explicit own user namespace id' do
+        post api("/projects/fork/#{project.id}?namespace=#{user2.namespace.id}", user2)
+        expect(response).to have_http_status(201)
+        expect(json_response['owner']['id']).to eq(user2.id)
+      end
+
+      it 'forks with explicit own user name as namespace' do
+        post api("/projects/fork/#{project.id}?namespace=#{user2.username}", user2)
+        expect(response).to have_http_status(201)
+        expect(json_response['owner']['id']).to eq(user2.id)
+      end
+
+      it 'forks to another user when admin' do
+        post api("/projects/fork/#{project.id}?namespace=#{user2.username}", admin)
+        expect(response).to have_http_status(201)
+        expect(json_response['owner']['id']).to eq(user2.id)
+      end
+
+      it 'fails if trying to fork to another user when not admin' do
+        post api("/projects/fork/#{project.id}?namespace=#{admin.namespace.id}", user2)
+        expect(response).to have_http_status(403)
+      end
+
+      it 'fails if trying to fork to non-existent namespace' do
+        post api("/projects/fork/#{project.id}?namespace=42424242", user2)
+        expect(response).to have_http_status(404)
+        expect(json_response['message']).to eq('404 Target Namespace Not Found')
+      end
+
+      it 'forks to owned group' do
+        post api("/projects/fork/#{project.id}?namespace=#{group2.name}", user2)
+        expect(response).to have_http_status(201)
+        expect(json_response['namespace']['name']).to eq(group2.name)
+      end
+
+      it 'fails to fork to not owned group' do
+        post api("/projects/fork/#{project.id}?namespace=#{group.name}", user2)
+        expect(response).to have_http_status(403)
+      end
+
+      it 'forks to not owned group when admin' do
+        post api("/projects/fork/#{project.id}?namespace=#{group.name}", admin)
+        expect(response).to have_http_status(201)
+        expect(json_response['namespace']['name']).to eq(group.name)
+      end
     end
 
     context 'when unauthenticated' do