Add controllers for user contribution mapping

This MR introduces the basic controller functionality to support the
assignee-facing side of #443555.

There is more work to be done on the interface, but this MR enables the
basic flow of accepting/rejecting assignment, and introduces the flag to
be used across the feature.

app/controllers/import/source_users_controller.rb

+# frozen_string_literal: true
+
+module Import
+  class SourceUsersController < ApplicationController
+    prepend_before_action :check_feature_flag!
+
+    before_action :source_user
+    before_action :check_current_user_matches_invite!
+    before_action :load_invite_details
+
+    respond_to :html
+    feature_category :importers
+
+    def accept
+      if current_user_matches_invite? && source_user.accept
+        # TODO: This is where we enqueue the job to assign the contributions.
+
+        # OTHER TODO: This is a placeholder for the proper UI to be provided
+        # in a follow-up MR.
+        notice = "You have approved the reassignment of contributions from "
+        notice += "%{source_username} on %{source_hostname} "
+        notice += "to yourself on %{destination_group}."
+
+        redirect_to(root_path, notice: format(notice, @invite_details))
+      else
+        redirect_back_or_default(options: { alert: _("The invitation could not be accepted.") })
+      end
+    end
+
+    def decline
+      if current_user_matches_invite? && source_user.reject
+        # TODO: This is a placeholder for the proper UI to be provided
+        # in a follow-up MR.
+        notice = "You have rejected the reassignment of contributions from "
+        notice += "%{source_username} on %{source_hostname} to yourself on "
+        notice += "%{destination_group}."
+
+        redirect_to(root_path, notice: format(notice, @invite_details))
+      else
+        redirect_back_or_default(options: { alert: _("The invitation could not be declined.") })
+      end
+    end
+
+    private
+
+    def check_current_user_matches_invite!
+      not_found unless current_user_matches_invite?
+    end
+
+    def current_user_matches_invite?
+      current_user.id == @source_user.reassign_to_user_id
+    end
+
+    def source_user
+      Import::SourceUser.find(params[:id])
+    end
+    strong_memoize_attr :source_user
+
+    def authenticate_user!
+      return if current_user
+
+      redirect_to new_user_session_path, notice: sign_in_notice
+    end
+
+    def sign_in_notice
+      if Gitlab::CurrentSettings.allow_signup?
+        _("To accept this invitation, sign in or create an account.")
+      else
+        _("To accept this invitation, sign in.")
+      end
+    end
+
+    def load_invite_details
+      @invite_details ||= {
+        source_username: source_user.source_username,
+        source_hostname: source_user.source_hostname,
+        destination_group: source_user.namespace.name
+      }
+    end
+
+    def check_feature_flag!
+      not_found unless Feature.enabled?(:improved_user_mapping, current_user)
+    end
+  end
+end

app/models/import/source_user.rb

@@ -18,6 +18,14 @@ class SourceUser < ApplicationRecord
       state :rejected, value: 2
       state :failed, value: 3
       state :completed, value: 4
+
+      event :accept do
+        transition awaiting_approval: :completed
+      end
+
+      event :reject do
+        transition awaiting_approval: :rejected
+      end
     end
 
     def self.find_source_user(source_user_identifier:, namespace:, source_hostname:, import_type:)

config/feature_flags/wip/improved_user_mapping.yml

+---
+name: improved_user_mapping
+feature_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/443555
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/150736
+rollout_issue_url: n/a
+milestone: '17.0'
+group: group::import and integrate
+type: wip
+default_enabled: false

config/routes/import.rb

@@ -88,4 +88,11 @@
     get :realtime_changes
     post :upload
   end
+
+  resources :source_users, only: [] do
+    member do
+      get :accept
+      get :decline
+    end
+  end
 end

spec/controllers/import/source_users_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Import::SourceUsersController, feature_category: :importers do
+  shared_examples 'it requires feature flag' do |action|
+    context 'when :improved_user_mapping is disabled' do
+      it 'returns 404' do
+        stub_feature_flags(improved_user_mapping: false)
+
+        get action, params: { id: source_user.to_param }
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+  end
+
+  shared_examples 'it requires the user to be signed in' do |action|
+    context 'when the user is not signed in' do
+      it 'redirects to the login screen' do
+        get action, params: { id: source_user.to_param }
+
+        expect(response).to redirect_to(new_user_session_path)
+        expect(flash[:notice]).to match(/To accept this invitation/)
+      end
+    end
+  end
+
+  let_it_be_with_reload(:source_user) { create(:import_source_user, :with_reassign_to_user, :awaiting_approval) }
+
+  describe 'GET /accept' do
+    context 'when signed in' do
+      before do
+        sign_in(source_user.reassign_to_user)
+      end
+
+      subject(:accept_invite) { get :accept, params: { id: source_user.to_param } }
+
+      it { expect { accept_invite }.to change { source_user.reload.completed? }.from(false).to(true) }
+
+      it 'redirects with a notice when accepted' do
+        accept_invite
+
+        expect(response).to redirect_to(root_path)
+        expect(flash[:notice]).to match(/You have approved the reassignment/)
+      end
+
+      it 'can only be accepted by the reassign_to_user' do
+        source_user.update!(reassign_to_user: create(:user))
+
+        expect { accept_invite }.not_to change { source_user.reload.status }
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+
+    it_behaves_like 'it requires feature flag', :accept
+    it_behaves_like 'it requires the user to be signed in', :accept
+  end
+
+  describe 'GET /decline' do
+    context 'when signed in' do
+      before do
+        sign_in(source_user.reassign_to_user)
+      end
+
+      subject(:accept_invite) { get :decline, params: { id: source_user.to_param } }
+
+      it { expect { accept_invite }.to change { source_user.reload.rejected? }.from(false).to(true) }
+
+      it 'redirects with a notice' do
+        accept_invite
+
+        expect(response).to redirect_to(root_path)
+        expect(flash[:notice]).to match(/You have rejected the reassignment/)
+      end
+    end
+
+    it_behaves_like 'it requires feature flag', :decline
+    it_behaves_like 'it requires the user to be signed in', :decline
+  end
+end

spec/factories/import_source_users.rb

@@ -14,5 +14,9 @@
     trait :with_reassign_to_user do
       reassign_to_user factory: :user
     end
+
+    trait :awaiting_approval do
+      status { 1 }
+    end
   end
 end