OAuth login includes prefix twice in the login url

[kuykendall-ben]

Minor bug related to #5422. The login endpoint configured with OAuth is now /<prefix>/<prefix>/login which appears to work, but can't possibly be the intended path.

ALL software version info

Panel: 1.2.2
Bokeh: 3.2.0
Tornado: 6.2
Python: 3.10.13
OAuth provider: GitHub
Platform: macOS Ventura 13.5.1

Description of expected behavior and the observed behavior

Visiting http://localhost:5006/prefix/app should redirect to http://localhost:5006/prefix/login to start the authentication flow. Instead it redirects to http://localhost:5006/prefix/prefix//login. Login does proceed correctly.

Complete, minimal, self-contained example code that reproduces the issue

Create an OAuth app in GitHub per https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/creating-an-oauth-app. Set the "authorization callback URL" to http://localhost:5006/prefix/. Generate a "client secret", and save the key and secret for use below.

Write a minimal app.py

import panel as pn
pn.Column('test').servable()

Start the panel server with OAuth parameters and a prefix. Make sure to include the prefix (including the trailing /) in the OAuth redirect uri

panel serve app.py --port 5006  --cookie-secret `panel secret` \
  --oauth-provider github --oauth-key $KEY --oauth-secret $SECRET \
  --oauth-redirect-uri http://localhost:5006/prefix/ \
  --prefix prefix

Visit http://localhost:5006/prefix/app. It will redirect to http://localhost:5006/prefix/prefix//login?next=/prefix/app. The authentication flow will start; optionally sign in with GitHub and eventually get redirected to http://localhost:5006/prefix/app. Verify the page renders.

Screenshots or screencasts of the bug in action

Included are screenshots of the first two requests in the authentication flow.

The initial GET /prefix/app request returns with a redirect to ./prefix//login?next=/prefix/app

This resolves to GET /prefix/prefix//login (?!) which redirects to the OAuth provider as desired