Cache letter page count on send yourself test

Calculating the number of pages in a letter is quite slow. And the send
yourself a test pages need to load _fast_. Since filling in placeholders
is very unlikely to change the number of pages in the resultant letter,
it’s pretty safe to cache that count, and makes the subsequent pages
load a lot faster.

app/main/views/send.py

@@ -162,6 +162,7 @@ def get_example_csv(service_id, template_id):
 @user_has_permissions('send_texts', 'send_emails', 'send_letters')
 def send_test(service_id, template_id):
     session['send_test_values'] = dict()
+    session['send_test_letter_page_count'] = None
     return redirect(url_for(
         '.send_test_step',
         service_id=service_id,
@@ -178,6 +179,9 @@ def send_test_step(service_id, template_id, step_index):
 
     template = service_api_client.get_service_template(service_id, template_id)['data']
 
+    if not session.get('send_test_letter_page_count'):
+        session['send_test_letter_page_count'] = get_page_count_for_letter(template)
+
     template = get_template(
         template,
         current_service,
@@ -189,7 +193,7 @@ def send_test_step(service_id, template_id, step_index):
             template_id=template_id,
             filetype='png',
         ),
-        page_count=get_page_count_for_letter(template),
+        page_count=session['send_test_letter_page_count']
     )
 
     placeholders = fields_to_fill_in(template)
@@ -271,7 +275,6 @@ def send_test_preview(service_id, template_id, filetype):
             template_id=template_id,
             filetype='png',
         ),
-        page_count=get_page_count_for_letter(template),
     )
 
     template.values = get_normalised_send_test_values_from_session()

tests/app/main/views/test_send.py

@@ -421,6 +421,30 @@ def test_send_test_sms_message_with_placeholders_shows_first_field(
         assert session['send_test_values']['phone number'] == '07700 900762'
 
 
+def test_send_test_letter_clears_previous_page_cache(
+    logged_in_platform_admin_client,
+    mocker,
+    service_one,
+    mock_login,
+    mock_get_service,
+    mock_get_service_letter_template,
+    fake_uuid,
+):
+
+    with logged_in_platform_admin_client.session_transaction() as session:
+        session['send_test_letter_page_count'] = 'WRONG'
+
+    response = logged_in_platform_admin_client.get(url_for(
+        'main.send_test',
+        service_id=service_one['id'],
+        template_id=fake_uuid,
+    ))
+    assert response.status_code == 302
+
+    with logged_in_platform_admin_client.session_transaction() as session:
+        assert session['send_test_letter_page_count'] is None
+
+
 def test_send_test_populates_field_from_session(
     logged_in_client,
     mocker,
@@ -447,6 +471,30 @@ def test_send_test_populates_field_from_session(
     assert page.select('input')[0]['value'] == 'Jo'
 
 
+def test_send_test_caches_page_count(
+    logged_in_client,
+    mocker,
+    service_one,
+    mock_login,
+    mock_get_service,
+    mock_get_service_letter_template,
+    fake_uuid,
+):
+
+    mocker.patch('app.main.views.send.get_page_count_for_letter', return_value=99)
+
+    response = logged_in_client.get(
+        url_for(
+            'main.send_test',
+            service_id=service_one['id'],
+            template_id=fake_uuid,
+        ),
+        follow_redirects=True,
+    )
+    with logged_in_client.session_transaction() as session:
+        assert session['send_test_letter_page_count'] == 99
+
+
 def test_send_test_indicates_optional_address_columns(
     logged_in_client,
     mocker,