Raise HTTPError in case of HTTP 5XX responses.

It is misleading to raise a MissingTokenError when the server has
returned an HTTP server error.

Instead, raise requests.exceptions.HTTPError if the server has returned
an HTTP 5XX server error.

Prior PR conversation in PR #217 indicates that the maintainers do not
want to raise on 4XX errors due to certain providers using those
responses to send data. So we need a custom handler with a slight
variation on the built-in requests.models.Response.raise_for_status().

Author: ab

requests_oauthlib/oauth2_session.py

@@ -7,6 +7,7 @@
 from oauthlib.oauth2 import LegacyApplicationClient
 from oauthlib.oauth2 import TokenExpiredError, is_secure_transport
 import requests
+from requests.exceptions import HTTPError
 
 log = logging.getLogger(__name__)
 
@@ -363,6 +364,8 @@ def fetch_token(
             log.debug("Invoking hook %s.", hook)
             r = hook(r)
 
+        self._raise_for_5xx(response=r)
+
         self._client.parse_request_body_response(r.text, scope=self.scope)
         self.token = self._client.token
         log.debug("Obtained token %s.", self.token)
@@ -449,6 +452,8 @@ def refresh_token(
             log.debug("Invoking hook %s.", hook)
             r = hook(r)
 
+        self._raise_for_5xx(response=r)
+
         self.token = self._client.parse_request_body_response(r.text, scope=self.scope)
         if not "refresh_token" in self.token:
             log.debug("No new refresh token given. Re-using old.")
@@ -538,3 +543,37 @@ def register_compliance_hook(self, hook_type, hook):
                 "Hook type %s is not in %s.", hook_type, self.compliance_hook
             )
         self.compliance_hook[hook_type].add(hook)
+
+    def _raise_for_5xx(self, response: requests.models.Response) -> None:
+        """
+        Raise requests.HTTPError if response is an HTTP 5XX error.
+
+        Just like the existing Response.raise_for_status() but ignores 4XX
+        errors.
+
+        :param response: HTTP response object from requests
+        Raises :class:`requests.exceptions.HTTPError`, if a 5XX error occurred.
+        """
+        http_error_msg = ''
+        if isinstance(response.reason, bytes):
+            # We attempt to decode utf-8 first because some servers
+            # choose to localize their reason strings. If the string
+            # isn't utf-8, we fall back to iso-8859-1 for all other
+            # encodings. (See psf/requests PR #3538)
+            try:
+                reason = response.reason.decode('utf-8')
+            except UnicodeDecodeError:
+                reason = response.reason.decode('iso-8859-1')
+        else:
+            reason = response.reason
+
+        if 400 <= response.status_code < 500:
+            pass  # ignored
+
+        elif 500 <= response.status_code < 600:
+            http_error_msg = u'%s Server Error: %s for url: %s' % (
+                response.status_code, reason, response.url
+            )
+
+        if http_error_msg:
+            raise HTTPError(http_error_msg, response=response)