Process login with OAuth.

        :param client_name: A name one of configured clients
        :param request: Web request
        :param redirect_uri: An URI for authorization redirect

async def login(self, client_name, request, redirect_uri=None, **params):
        """Process login with OAuth.

        :param client_name: A name one of configured clients
        :param request: Web request
        :param redirect_uri: An URI for authorization redirect
        """
        client = self.client(client_name, logger=self.app.logger)

        redirect_uri = redirect_uri or self.cfg.redirect_uri or '%s://%s%s' % (
            request.scheme, request.host, request.path)
        session = await self.app.ps.session(request)

        if isinstance(client, OAuth1Client):
            oauth_verifier = request.query.get('oauth_verifier')
            if not oauth_verifier:

                # Get request credentials
                token, secret = await client.get_request_token(
                    oauth_callback=redirect_uri)

                # Save the credentials in current user session
                session['oauth_token'] = token
                session['oauth_token_secret'] = secret

                url = client.get_authorize_url()
                raise muffin.HTTPFound(url)

            # Check request_token
            oauth_token = request.query.get('oauth_token')
            if session['oauth_token'] != oauth_token:
                raise muffin.HTTPForbidden(reason='Invalid token.')

            client.oauth_token = oauth_token
            client.oauth_token_secret = session.get('oauth_token_secret')

            # Get access tokens
            return client, await client.get_access_token(oauth_verifier)

        if isinstance(client, OAuth2Client):
            code = request.query.get('code')
            if not code:

                # Authorize an user
                state = sha1(str(random()).encode('ascii')).hexdigest()
                session['oauth_secret'] = state
                url = client.get_authorize_url(
                    redirect_uri=redirect_uri, state=state, **params)
                raise muffin.HTTPFound(url)

            # Check state
            state = request.query.get('state')
            oauth_secret = session.pop('oauth_secret', '')
            if oauth_secret != state:
                raise muffin.HTTPForbidden(reason='Invalid token "%s".' % oauth_secret)

            # Get access token
            return client, await client.get_access_token(code, redirect_uri=redirect_uri)

        return client