[TOOLCM-2168] Catch missing link header (#45)

* Catch missing link header

In Python 2, `response.headers["header-key"]` would return a `KeyError` if the header wasn't present, but in Python 3 it returns `None`. This is currently the top source of errors in Sentry.

* Add comment

* Update comment

* Fix if-else indentation

packages/github_backup.py

@@ -102,11 +102,16 @@ def get_repos(org, repo_type, access_token=None, username=None, password=None, p
         raise ValueError('unworkable combination of authentication inputs')
 
     response = urlopen(request)
-    try:
-        pagination = get_pagination(response.headers['Link'])
-    except KeyError:
+    # response.headers is an instance of http.client.HTTPMessage, which returns `None`
+    # for missing keys. Refer to email.message.Message.__getitem__ (currently at
+    # https://github.com/python/cpython/blob/3.11/Lib/email/message.py#L410-L419)
+    # for the specific implementation.
+    raw_link_header = response.headers['Link']
+    if raw_link_header is None:
         logging.debug('no Link header, nothing to paginate through.')
         pagination = Pagination(None, None, None, None)
+    else:
+        pagination = get_pagination(raw_link_header)
 
     repos = json.loads(response.read())
     for r in repos: