When requesting a png for a single page, show overlay as appropriate

If the png page is invalid then show the overlay for that page
If the png page is not invalid, even if other pages are invalid, then
don't shown the overlay for that page

These keeps the behaviour were if you get the pdf for a pdf which has
invalid pages then the whole PDF is requested (not just a single page)
and all of the pages include the overlay

Tests have been improved to be more explicit about what we are testing

app/template/rest.py

@@ -245,11 +245,15 @@ def preview_letter_template_by_notification_id(service_id, notification_id, file
                 status_code=500
             )
 
-        content = base64.b64encode(pdf_file).decode('utf-8')
-        overlay = metadata.get("message") == "content-outside-printable-area"
         page_number = page if page else "1"
+        content = base64.b64encode(pdf_file).decode('utf-8')
+        content_outside_printable_area = metadata.get("message") == "content-outside-printable-area"
 
-        if overlay:
+        show_overlay_for_page = False
+        if content_outside_printable_area and page_number in metadata.get('invalid_pages', '[]'):
+            show_overlay_for_page = True
+
+        if show_overlay_for_page or (content_outside_printable_area and file_type == "pdf"):
             path = '/precompiled/overlay.{}'.format(file_type)
             query_string = '?page_number={}'.format(page_number) if file_type == 'png' else ''
             content = pdf_file
@@ -262,7 +266,7 @@ def preview_letter_template_by_notification_id(service_id, notification_id, file
         if file_type == 'png':
             try:
                 pdf_page = extract_page_from_pdf(BytesIO(pdf_file), int(page_number) - 1)
-                content = pdf_page if overlay else base64.b64encode(pdf_page).decode('utf-8')
+                content = pdf_page if show_overlay_for_page else base64.b64encode(pdf_page).decode('utf-8')
             except PdfReadError as e:
                 raise InvalidRequest(
                     'Error extracting requested page from PDF file for notification_id {} type {} {}'.format(

tests/app/template/test_rest.py

@@ -1143,23 +1143,27 @@ def test_preview_letter_template_precompiled_s3_error(
 
 
 @pytest.mark.parametrize(
-    "requested_page, filetype, message, expected_post_url",
+    "requested_page, message, expected_post_url",
     [
-        ("", 'png', "", 'precompiled-preview.png'),
+        # page defaults to 1, page is valid, no overlay shown
-        ("1", 'png', "content-outside-printable-area", 'precompiled/overlay.png?page_number=1'),
+        ("", "", 'precompiled-preview.png'),
-        ("2", 'png', "content-outside-printable-area", 'precompiled/overlay.png?page_number=2'),
+        # page is valid, no overlay shown
-        ("3", 'png', "content-outside-printable-area", 'precompiled/overlay.png?page_number=3'),
+        ("1", "", 'precompiled-preview.png'),
-        ("", 'pdf', "content-outside-printable-area", 'precompiled/overlay.pdf')
+        # page is invalid, overlay shown
+        ("1", "content-outside-printable-area", 'precompiled/overlay.png?page_number=1'),
+        # page is valid, no overlay shown
+        ("2", "content-outside-printable-area", 'precompiled-preview.png'),
+        # page is invalid, overlay shown
+        ("3", "content-outside-printable-area", 'precompiled/overlay.png?page_number=3'),
     ]
 )
-def test_preview_letter_template_precompiled_png_file_type_or_pdf_with_overlay(
+def test_preview_letter_template_precompiled_for_png_file_shows_overlay_where_appropriate(
         notify_api,
         client,
         admin_request,
         sample_service,
         mocker,
         requested_page,
-        filetype,
         message,
         expected_post_url,
 ):
@@ -1206,7 +1210,74 @@ def test_preview_letter_template_precompiled_png_file_type_or_pdf_with_overlay(
                 page=requested_page,
                 service_id=notification.service_id,
                 notification_id=notification.id,
-                file_type=filetype,
+                file_type="png",
+            )
+
+            with pytest.raises(ValueError):
+                mock_post.last_request.json()
+            assert mock_get_letter_pdf.called_once_with(notification)
+            assert base64.b64decode(response['content']) == expected_returned_content
+            assert response["metadata"] == metadata
+
+
+@pytest.mark.parametrize(
+    "invalid_pages",
+    [
+        "[1,3]",
+        "[2,4]",  # it shouldn't make a difference if the error was on the first page or not
+    ]
+)
+def test_preview_letter_template_precompiled_for_pdf_file_shows_overlay_if_content_outside_printable_area(
+        notify_api,
+        client,
+        admin_request,
+        sample_service,
+        mocker,
+        invalid_pages,
+):
+
+    template = create_template(sample_service,
+                               template_type='letter',
+                               template_name='Pre-compiled PDF',
+                               subject='Pre-compiled PDF',
+                               hidden=True)
+
+    notification = create_notification(template)
+
+    with set_config_values(notify_api, {
+        'TEMPLATE_PREVIEW_API_HOST': 'http://localhost/notifications-template-preview',
+        'TEMPLATE_PREVIEW_API_KEY': 'test-key'
+    }):
+        with requests_mock.Mocker() as request_mock:
+
+            pdf_content = b'\x00\x01'
+            expected_returned_content = b'\x00\x02'
+
+            metadata = {
+                "message": "content-outside-printable-area",
+                "invalid_pages": invalid_pages,
+                "page_count": "4"
+            }
+
+            mock_get_letter_pdf = mocker.patch(
+                'app.template.rest.get_letter_pdf_and_metadata',
+                return_value=(pdf_content, metadata)
+            )
+
+            mocker.patch('app.template.rest.extract_page_from_pdf', return_value=pdf_content)
+
+            mock_post = request_mock.post(
+                'http://localhost/notifications-template-preview/precompiled/overlay.pdf',
+                content=expected_returned_content,
+                headers={'X-pdf-page-count': '4'},
+                status_code=200
+            )
+
+            response = admin_request.get(
+                'template.preview_letter_template_by_notification_id',
+                service_id=notification.service_id,
+                notification_id=notification.id,
+                file_type="pdf",
             )
 
             with pytest.raises(ValueError):