Add specs for sessions controller including 2FA

This also contains specs for a bug described in #14900

spec/controllers/sessions_controller_spec.rb

+require 'spec_helper'
+
+describe SessionsController do
+  describe '#create' do
+    before do
+      @request.env['devise.mapping'] = Devise.mappings[:user]
+    end
+
+    context 'standard authentications' do
+      context 'invalid password' do
+        it 'does not authenticate user' do
+          post(:create, user: { login: 'invalid', password: 'invalid' })
+
+          expect(response)
+            .to set_flash.now[:alert].to /Invalid login or password/
+        end
+      end
+
+      context 'valid password' do
+        let(:user) { create(:user) }
+
+        it 'authenticates user correctly' do
+          post(:create, user: { login: user.username, password: user.password })
+
+          expect(response).to set_flash.to /Signed in successfully/
+          expect(subject.current_user). to eq user
+        end
+      end
+    end
+
+    context 'two-factor authentication' do
+      let(:user) { create(:user, :two_factor) }
+
+      def authenticate_2fa(user_params)
+        post(:create, { user: user_params }, { otp_user_id: user.id })
+      end
+
+      ##
+      # See #14900 issue
+      #
+      context 'authenticating with login and OTP belonging to another user' do
+        let(:another_user) { create(:user, :two_factor) }
+
+
+        context 'OTP valid for another user' do
+          it 'does not authenticate' do
+            authenticate_2fa(login: another_user.username,
+                             otp_attempt: another_user.current_otp)
+
+            expect(subject.current_user).to_not eq another_user
+          end
+        end
+
+        context 'OTP invalid for another user' do
+          before do
+            authenticate_2fa(login: another_user.username,
+                             otp_attempt: 'invalid')
+          end
+
+          it 'does not authenticate' do
+            expect(subject.current_user).to_not eq another_user
+          end
+
+          it 'does not leak information about 2FA enabled' do
+            expect(response).to_not set_flash.now[:alert].to /Invalid two-factor code/
+          end
+        end
+
+        context 'authenticating with OTP' do
+          context 'valid OTP' do
+            it 'authenticates correctly' do
+              authenticate_2fa(otp_attempt: user.current_otp)
+
+              expect(subject.current_user).to eq user
+            end
+          end
+
+          context 'invalid OTP' do
+            before { authenticate_2fa(otp_attempt: 'invalid') }
+
+            it 'does not authenticate' do
+              expect(subject.current_user).to_not eq user
+            end
+
+            it 'warns about invalid OTP code' do
+              expect(response).to set_flash.now[:alert].to /Invalid two-factor code/
+            end
+          end
+        end
+      end
+    end
+  end
+end