Show error message if precompiled PDF cannot be opened

If PDF files have a validation error which means that they can't be
opened by PyPDF2 we would previously show the 500 status error page. We
now catch PyPDF2.utils.PdfReadErrors so that we can display a custom
error message on the notification page instead.

app/main/views/notifications.py

@@ -24,6 +24,7 @@ from notifications_utils.letter_timings import (
 )
 from notifications_utils.pdf import pdf_page_count
 from notifications_utils.timezones import utc_string_to_aware_gmt_datetime
+from PyPDF2.utils import PdfReadError
 
 from app import (
     _format_datetime_short,
@@ -58,8 +59,14 @@ def view_notification(service_id, notification_id):
     personalisation = get_all_personalisation_from_notification(notification)
 
     if notification['template']['is_precompiled_letter']:
-        file_contents = view_letter_notification_as_preview(service_id, notification_id, "pdf")
-        page_count = pdf_page_count(io.BytesIO(file_contents))
+        try:
+            file_contents = view_letter_notification_as_preview(service_id, notification_id, "pdf")
+            page_count = pdf_page_count(io.BytesIO(file_contents))
+        except PdfReadError:
+            return render_template(
+                'views/notifications/invalid_precompiled_letter.html',
+                created_at=notification['created_at']
+            )
     else:
         page_count = get_page_count_for_letter(notification['template'], values=personalisation)
 

app/templates/views/notifications/invalid_precompiled_letter.html

@@ -0,0 +1,17 @@
+{% extends "withnav_template.html" %}
+
+{% block service_page_title %}
+  Letter
+{% endblock %}
+
+{% block maincolumn_content %}
+
+    <h1 class="heading-large">Letter</h1>
+
+    <p>
+      Provided as PDF on {{ created_at|format_datetime_short }}
+    </p>
+    <p class="notification-status-cancelled">
+      Couldn’t read this file
+    </p>
+{% endblock %}

tests/app/main/views/test_notifications.py

@@ -7,6 +7,7 @@ import pytest
 from flask import url_for
 from freezegun import freeze_time
 from notifications_python_client.errors import APIError
+from PyPDF2.utils import PdfReadError
 
 from app.main.views.notifications import get_letter_printing_statement
 from tests.conftest import (
@@ -398,6 +399,36 @@ def test_should_show_preview_error_image_letter_notification_on_preview_error(
     assert response.get_data(as_text=True) == 'preview error image'
 
 
+def test_notifification_page_shows_error_message_if_precompiled_letter_cannot_be_opened(
+    client_request,
+    mocker,
+    fake_uuid,
+):
+    mock_get_notification(
+        mocker,
+        fake_uuid,
+        notification_status='validation-failed',
+        template_type='letter',
+        is_precompiled_letter=True,
+    )
+    mocker.patch(
+        'app.main.views.notifications.view_letter_notification_as_preview',
+        side_effect=PdfReadError()
+    )
+    mocker.patch(
+        'app.main.views.notifications.pdf_page_count',
+        side_effect=PdfReadError()
+    )
+    page = client_request.get(
+        'main.view_notification',
+        service_id=SERVICE_ONE_ID,
+        notification_id=fake_uuid,
+    )
+
+    error_message = page.find('p', class_='notification-status-cancelled').text
+    assert normalize_spaces(error_message) == "Couldn’t read this file"
+
+
 def test_should_404_for_unknown_extension(
     client_request,
     fake_uuid,