check users' session id.

when a user enters their 2FA code, the API will store a random UUID
against them in the database - this code is then stored on the cookie
on the front end.

At the beginning of each authenticated request, we do the following
steps:
  * Retrieve the user's cookie, and get the user_id from it
  * Request that user's details from the database
  * populate current_user with the DB model
  * run the login_required decorator, which calls
    current_user.is_authenticated

is_authenticated now also checks that the database model matches the
cookie for session_id. The potential states and meanings are as follows:

 database | cookie | meaning
----------+--------+---------
 None     | None   | New user, or system just been deployed.
          |        | Redirect to start page.
----------+--------+---------
 'abc'    | None   | New browser (or cleared cookies). Redirect to
          |        | start page.
----------+--------+---------
 None     | 'abc'  | Invalid state (cookie is set from user obj, so
          |        | would only happen if DB is cleared)
----------+--------+---------
 'abc'    | 'abc'  | Same browser. Business as usual
----------+--------+---------
 'abc'    | 'def'  | Different browser in cookie - db has been changed
          |        | since then. Redirect to start

tests/app/main/views/test_accept_invite.py

@@ -80,7 +80,7 @@ def test_if_existing_user_accepts_twice_they_redirect_to_sign_in(
         page.select('main p')[0].text.strip(),
     ) == (
         'You need to sign in again',
-        'We sign you out if you haven’t used Notify for a while.',
+        'We signed you out because you haven’t used Notify for a while.',
     )
 
 
@@ -106,7 +106,7 @@ def test_existing_user_of_service_get_redirected_to_signin(
         page.select('main p')[0].text.strip(),
     ) == (
         'You need to sign in again',
-        'We sign you out if you haven’t used Notify for a while.',
+        'We signed you out because you haven’t used Notify for a while.',
     )
     assert mock_accept_invite.call_count == 1
 
@@ -141,7 +141,7 @@ def test_existing_signed_out_user_accept_invite_redirects_to_sign_in(
         page.select('main p')[0].text.strip(),
     ) == (
         'You need to sign in again',
-        'We sign you out if you haven’t used Notify for a while.',
+        'We signed you out because you haven’t used Notify for a while.',
     )
 
 

tests/app/main/views/test_sign_in.py

@@ -1,16 +1,73 @@
+import uuid
+
+import pytest
 from flask import url_for
 from bs4 import BeautifulSoup
 
 
-def test_render_sign_in_returns_sign_in_template(
+def test_render_sign_in_template_for_new_user(
     client
 ):
-    response = client.get(url_for('main.sign_in'))
+    response = client.get(url_for('main.sign_in', next=None))
     assert response.status_code == 200
-    assert 'Sign in' in response.get_data(as_text=True)
-    assert 'Email address' in response.get_data(as_text=True)
-    assert 'Password' in response.get_data(as_text=True)
-    assert 'Forgot your password?' in response.get_data(as_text=True)
+    resp = response.get_data(as_text=True)
+    assert 'Sign in' in resp
+    assert 'Email address' in resp
+    assert 'Password' in resp
+    assert 'Forgot your password?' in resp
+    assert 'If you do not have an account, you can' in resp
+    assert 'Sign in again' not in resp
+
+
+def test_sign_in_explains_session_timeout(client):
+    response = client.get(url_for('main.sign_in', next='/foo'))
+    assert response.status_code == 200
+    assert 'We signed you out because you haven’t used Notify for a while.' in response.get_data(as_text=True)
+
+
+def test_sign_in_explains_other_browser(logged_in_client, api_user_active, mocker):
+    api_user_active.current_session_id = str(uuid.UUID(int=1))
+    mocker.patch('app.user_api_client.get_user', return_value=api_user_active)
+
+    with logged_in_client.session_transaction() as session:
+        session['current_session_id'] = str(uuid.UUID(int=2))
+
+    response = logged_in_client.get(url_for('main.sign_in', next='/foo'))
+
+    assert response.status_code == 200
+    assert 'We signed you out because you logged in to Notify on another device' in response.get_data(as_text=True)
+
+
+def test_doesnt_redirect_to_sign_in_if_no_session_info(logged_in_client, api_user_active):
+    assert api_user_active.current_session_id is None
+    with logged_in_client.session_transaction() as session:
+        session['current_session_id'] = None
+
+    response = logged_in_client.get(url_for('main.choose_service'))
+    assert response.status_code == 200
+
+
+@pytest.mark.parametrize('db_sess_id, cookie_sess_id', [
+    pytest.mark.xfail((None, None)),  # OK - not used notify since browser signout was implemented
+
+    (uuid.UUID(int=1), None),  # BAD - has used other browsers before but this is a brand new browser with no cookie
+    (uuid.UUID(int=1), uuid.UUID(int=2)),  # BAD - this person has just signed in on a different browser
+])
+def test_redirect_to_sign_in_if_logged_in_from_other_browser(
+    logged_in_client,
+    api_user_active,
+    mocker,
+    db_sess_id,
+    cookie_sess_id
+):
+    api_user_active.current_session_id = db_sess_id
+    mocker.patch('app.user_api_client.get_user', return_value=api_user_active)
+    with logged_in_client.session_transaction() as session:
+        session['current_session_id'] = str(cookie_sess_id)
+
+    response = logged_in_client.get(url_for('main.choose_service'))
+    assert response.status_code == 302
+    assert response.location == url_for('main.sign_in', next='/services', _external=True)
 
 
 def test_logged_in_user_redirects_to_choose_service(