Make sure 'code' returns None instead of crashing if key is missing.

[andersnauman]

DO NOT SEND ANY SECURITY FIX HERE. Please read "Security Reporting" section
on README.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Other, please describe:

---

• You consent that the copyright of your pull request source code belongs to Authlib's author.

---

Instead of having the library users keeping track of which requests.args that are needed, change the behavior to not crash if variables requirements are not met.

Example code before change:

authorize = Blueprint('authorize', __name__)

@authorize.route('/<client_name>')
def oauth(client_name):
    """ Verify oauth """
    args = request.args
    if None in (args.get("code"), args.get("state")):
        abort(500)
    client = _oauth.create_client(client_name)
    if not client:
        abort(404)
    token = client.authorize_access_token()

Example code after change:
Now the correct raise will occur which we can check for as expected:

from authlib.integrations.base_client.errors import MismatchingStateError
authorize = Blueprint('authorize', __name__)

@authorize.route('/<client_name>')
def oauth(client_name):
    """ Verify oauth """
    client = _oauth.create_client(client_name)
    if not client:
        abort(404)

    try:
        token = client.authorize_access_token()
    except MismatchingStateError:
        abort(500)

[lepture]

It will not crash, Flask will raise a 400 bad request for this case, unless Flask has changed this behavior.

[andersnauman]

You are correct that Flask will catch the crash and respond accordingly.
If we look into the context of the library, it still crashes. And while this might not pose a fatal error for the running process, it still makes the code behave unpredictable and stop further actions.

This can easily become a religious discussion, but I would suggest that any library should never crash but to return (or at least raise an error) a None state or any other way of giving a signal that something went wrong. The library user should not be forced to understand the underlying code just to run it in a safe matter.

Please also note that this code-change have already been made in other parts of your library. E.g Here