Relax lookup of letter PDFs in S3 buckets

Previously we generated the filename we expected a letter PDF to be
stored at in S3, and used that to retrieve it. However, the generated
filename can change over the course of a notification's lifetime e.g.
if the service changes from crown ('.C.') to non-crown ('.N.').

The prefix of the filename is stable: it's based on properties of the
notification - reference and creation - that don't change. This commit
changes the way we interact with letter PDFs in S3:

- Uploading uses the original method to generate the full file name.
The method is renamed to 'generate_' to distinguish it from the new one.

- Downloading uses a new 'find_' method to get the filename using just
its prefix, which makes it agnostic to changes in the filename suffix.

Making this change helps to decouple our code from the requirements DVLA
have on the filenames. While it means more traffic to S3, we rely on S3
in any case to download the files. From experience, we know S3 is highly
reliable and performant, so don't anticipate any issues.

In the tests we favour using moto to mock S3, so that the behaviour is
realistic. There are a couple of places where we just mock the method,
since what it returns isn't important for the test.

Note that, since the new method requires a notification object, we need
to change a query in one place, the columns of which were only selected
to appease the original method to generate a filename.

tests/app/letters/test_letter_utils.py

@@ -9,10 +9,11 @@ from moto import mock_s3
 
 from app.letters.utils import (
     ScanErrorType,
+    find_letter_pdf_filename,
+    generate_letter_pdf_filename,
     get_bucket_name_and_prefix_for_notification,
     get_folder_name,
     get_letter_pdf_and_metadata,
-    get_letter_pdf_filename,
     letter_print_day,
     move_failed_pdf,
     move_sanitised_letter_to_test_or_live_pdf_bucket,
@@ -46,6 +47,21 @@ def _sample_precompiled_letter_notification_using_test_key(sample_precompiled_le
     return sample_precompiled_letter_notification
 
 
+@mock_s3
+def test_find_letter_pdf_filename_returns_filename(sample_notification):
+    bucket_name = current_app.config['LETTERS_PDF_BUCKET_NAME']
+    s3 = boto3.client('s3', region_name='eu-west-1')
+    s3.create_bucket(
+        Bucket=bucket_name,
+        CreateBucketConfiguration={'LocationConstraint': 'eu-west-1'}
+    )
+
+    _, prefix = get_bucket_name_and_prefix_for_notification(sample_notification)
+    s3.put_object(Bucket=bucket_name, Key=f'{prefix}-and-then-some', Body=b'f')
+
+    assert find_letter_pdf_filename(sample_notification) == f'{prefix}-and-then-some'
+
+
 @pytest.mark.parametrize('created_at,folder', [
     (datetime(2017, 1, 1, 17, 29), '2017-01-01'),
     (datetime(2017, 1, 1, 17, 31), '2017-01-02'),
@@ -145,10 +161,10 @@ def test_get_bucket_name_and_prefix_for_notification_invalid_notification():
     (True, 'C'),
     (False, 'N'),
 ])
-def test_get_letter_pdf_filename_returns_correct_filename(
+def test_generate_letter_pdf_filename_returns_correct_filename(
         notify_api, mocker, crown_flag, expected_crown_text):
     created_at = datetime(2017, 12, 4, 17, 29)
-    filename = get_letter_pdf_filename(reference='foo', crown=crown_flag, created_at=created_at)
+    filename = generate_letter_pdf_filename(reference='foo', crown=crown_flag, created_at=created_at)
 
     assert filename == '2017-12-04/NOTIFY.FOO.D.2.C.{}.20171204172900.PDF'.format(expected_crown_text)
 
@@ -157,26 +173,30 @@ def test_get_letter_pdf_filename_returns_correct_filename(
     ('second', 2),
     ('first', 1),
 ])
-def test_get_letter_pdf_filename_returns_correct_postage_for_filename(
+def test_generate_letter_pdf_filename_returns_correct_postage_for_filename(
         notify_api, postage, expected_postage):
     created_at = datetime(2017, 12, 4, 17, 29)
-    filename = get_letter_pdf_filename(reference='foo', crown=True, created_at=created_at, postage=postage)
+    filename = generate_letter_pdf_filename(reference='foo', crown=True, created_at=created_at, postage=postage)
 
     assert filename == '2017-12-04/NOTIFY.FOO.D.{}.C.C.20171204172900.PDF'.format(expected_postage)
 
 
-def test_get_letter_pdf_filename_returns_correct_filename_for_test_letters(
+def test_generate_letter_pdf_filename_returns_correct_filename_for_test_letters(
         notify_api, mocker):
     created_at = datetime(2017, 12, 4, 17, 29)
-    filename = get_letter_pdf_filename(reference='foo', crown='C',
-                                       created_at=created_at, ignore_folder=True)
+    filename = generate_letter_pdf_filename(
+        reference='foo',
+        crown='C',
+        created_at=created_at,
+        ignore_folder=True
+    )
 
     assert filename == 'NOTIFY.FOO.D.2.C.C.20171204172900.PDF'
 
 
-def test_get_letter_pdf_filename_returns_tomorrows_filename(notify_api, mocker):
+def test_generate_letter_pdf_filename_returns_tomorrows_filename(notify_api, mocker):
     created_at = datetime(2017, 12, 4, 17, 31)
-    filename = get_letter_pdf_filename(reference='foo', crown=True, created_at=created_at)
+    filename = generate_letter_pdf_filename(reference='foo', crown=True, created_at=created_at)
 
     assert filename == '2017-12-05/NOTIFY.FOO.D.2.C.C.20171204173100.PDF'
 
@@ -223,7 +243,7 @@ def test_upload_letter_pdf_to_correct_bucket(
 
     mock_s3 = mocker.patch('app.letters.utils.s3upload')
 
-    filename = get_letter_pdf_filename(
+    filename = generate_letter_pdf_filename(
         reference=sample_letter_notification.reference,
         crown=sample_letter_notification.service.crown,
         created_at=sample_letter_notification.created_at,
@@ -250,7 +270,7 @@ def test_upload_letter_pdf_uses_postage_from_notification(
     letter_notification = create_notification(template=sample_letter_template, postage=postage)
     mock_s3 = mocker.patch('app.letters.utils.s3upload')
 
-    filename = get_letter_pdf_filename(
+    filename = generate_letter_pdf_filename(
         reference=letter_notification.reference,
         crown=letter_notification.service.crown,
         created_at=letter_notification.created_at,