Catch error if letter does not exist on send

This repeats the pattern we already have for previewing a letter,
where we assume the error is because the notification has already
been sent and redirect the user to see it.

I've improved the original pattern a bit:

- I've DRYed-up the low-level boto code and moved the error handler
there so it can be reused.

- I've introduced a custom exception, which the calling code can
choose to log.

- I've introduced the moto library, which we use elsewhere, to make
it easier to test S3 code.

I've used an error level log when sending a notification - now that
we have a more descriptive log, we can verify the assumption is true
and then make an informed decision to downgrade the log.

In future we may want to merge this handler with the similar code
in utils [1], but we'll need to be careful as the utils handler is
superficial - it doesn't check the reason for the error.

[1]: bce0f4e596/notifications_utils/s3.py (L52)

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

@@ -1,12 +1,14 @@
 from unittest.mock import ANY, Mock
 
 import pytest
-from botocore.exceptions import ClientError
 from flask import make_response, url_for
 from requests import RequestException
 
 from app.formatters import normalize_spaces
-from app.s3_client.s3_letter_upload_client import LetterMetadata
+from app.s3_client.s3_letter_upload_client import (
+    LetterMetadata,
+    LetterNotFoundError,
+)
 from tests.conftest import SERVICE_ONE_ID
 
 
@@ -465,11 +467,11 @@ def test_uploaded_letter_preview_redirects_if_file_not_in_s3(
     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')
+        side_effect=LetterNotFoundError
     )
+
     client_request.get(
         'main.uploaded_letter_preview',
         service_id=SERVICE_ONE_ID,
@@ -646,6 +648,33 @@ def test_send_uploaded_letter_sends_letter_and_redirects_to_notification_page(
     )
 
 
+def test_send_uploaded_letter_redirects_if_file_not_in_s3(
+    mocker,
+    client_request,
+    fake_uuid,
+    service_one,
+):
+    mocker.patch(
+        'app.main.views.uploads.get_letter_metadata',
+        side_effect=LetterNotFoundError
+    )
+
+    service_one['permissions'] = ['letter', 'upload_letters']
+
+    client_request.post(
+        'main.send_uploaded_letter',
+        service_id=SERVICE_ONE_ID,
+        file_id=fake_uuid,
+        _data={'filename': 'my_file.pdf'},
+        _expected_redirect=url_for(
+            'main.view_notification',
+            service_id=SERVICE_ONE_ID,
+            notification_id=fake_uuid,
+            _external=True
+        )
+    )
+
+
 @pytest.mark.parametrize('permissions', [
     ['email'],
     ['sms'],

tests/app/s3_client/test_s3_letter_upload_client.py

@@ -1,11 +1,17 @@
 import urllib
 import uuid
 
+import boto3
+import botocore
+import pytest
 from flask import current_app
+from moto import mock_s3
 
 from app.s3_client.s3_letter_upload_client import (
     LetterMetadata,
+    LetterNotFoundError,
     backup_original_letter_to_s3,
+    get_letter_metadata,
     upload_letter_to_s3,
 )
 
@@ -89,3 +95,26 @@ def test_lettermetadata_unquotes_special_keys():
     metadata = LetterMetadata({"filename": "%C2%A3hello", "recipient": "%C2%A3hi"})
     assert metadata.get("filename") == "£hello"
     assert metadata.get("recipient") == "£hi"
+
+
+@mock_s3
+@pytest.mark.parametrize('will_raise_custom_error,expected_exception', [
+    (True, LetterNotFoundError),
+    (False, botocore.exceptions.ClientError)
+])
+def test_get_letter_s3_object_raises_custom_error(
+    will_raise_custom_error,
+    expected_exception
+):
+    bucket_name = current_app.config['TRANSIENT_UPLOADED_LETTERS']
+    s3 = boto3.client('s3', region_name='eu-west-1')
+
+    # bucket not existing will trigger some other error
+    if will_raise_custom_error:
+        s3.create_bucket(
+            Bucket=bucket_name,
+            CreateBucketConfiguration={'LocationConstraint': 'eu-west-1'}
+        )
+
+    with pytest.raises(expected_exception):
+        get_letter_metadata('service', 'file')