redirect from upload preview to notifications if it already exists

the upload preview page has a file_id - this corresponds to the file in
the transient pdf uploads bucket. However, if the user already hit send
(and then navigated back) the file's no longer in that bcuket, it's been
moved to the regular letters-pdf bucket. So the s3 get request fails. To
avoid this, simply redirect to the notifications page if the file isn't
in the transient bucket. This is better for the user as it'll stop them
trying to submit it twice, and will provide more clarity on the status
of the notification too.

app/main/views/uploads.py

@@ -7,6 +7,7 @@ from functools import partial
 from io import BytesIO
 from zipfile import BadZipFile
 
+from botocore.exceptions import ClientError
 from flask import (
     abort,
     current_app,
@@ -257,7 +258,20 @@ def _get_error_from_upload_form(form_errors):
 def uploaded_letter_preview(service_id, file_id):
     re_upload_form = PDFUploadForm()
 
-    metadata = get_letter_metadata(service_id, file_id)
+    try:
+        metadata = get_letter_metadata(service_id, file_id)
+    except ClientError as e:
+        # if the file's not there, it's probably because we've already created the notification and the letter has been
+        # moved to the normal letters-pdf bucket. So lets just bounce out to the notification page
+        if e.response['Error']['Code'] == 'NoSuchKey':
+            return redirect(url_for(
+                '.view_notification',
+                service_id=service_id,
+                notification_id=file_id,
+            ))
+        else:
+            raise
+
     original_filename = metadata.get('filename')
     page_count = metadata.get('page_count')
     status = metadata.get('status')

tests/app/main/views/uploads/test_upload_letter.py

@@ -1,6 +1,7 @@
 from unittest.mock import ANY, Mock
 
 import pytest
+from botocore.exceptions import ClientError
 from flask import make_response, url_for
 from requests import RequestException
 
@@ -441,6 +442,30 @@ def test_uploaded_letter_preview_does_not_show_send_button_if_service_in_trial_m
     assert len(page.select('main button[type=submit]')) == 0
 
 
+def test_uploaded_letter_preview_redirects_if_file_not_in_s3(
+    mocker,
+    client_request,
+    fake_uuid
+):
+    boto_error_json = {'Error': {'Code': 'NoSuchKey', 'Message': 'The specified key does not exist.'}}
+    mocker.patch(
+        'app.main.views.uploads.get_letter_metadata',
+        side_effect=ClientError(boto_error_json, 'operation_name')
+    )
+    client_request.get(
+        'main.uploaded_letter_preview',
+        service_id=SERVICE_ONE_ID,
+        file_id=fake_uuid,
+        _expected_status=302,
+        _expected_redirect=url_for(
+            'main.view_notification',
+            service_id=SERVICE_ONE_ID,
+            notification_id=fake_uuid,
+            _external=True
+        )
+    )
+
+
 @pytest.mark.parametrize('invalid_pages, page_requested, overlay_expected', (
     ('[1, 2]', 1, True),
     ('[1, 2]', 2, True),