`twitterv2` provider uses OAuth 1.0a internally — fails with X Free plan (401 Unauthorized)

Summary

The twitterv2 provider does not actually use OAuth 2.0 for its authentication flow. It uses OAuth 1.0a via mrjones/oauth, and only calls the Twitter API v2 endpoints (/2/users/me). This causes a 401 Unauthorized error on X's Free plan, which does not support OAuth 1.0a.

Details

The twitterv2 provider was added in #460 to address #440, but the underlying auth mechanism was not changed to OAuth 2.0 — only the API endpoints were updated to v2.

Internal dependency chain:

goth/providers/twitterv2
  → github.com/mrjones/oauth (OAuth 1.0a, HMAC-SHA1 signatures)

Error seen at runtime:

HTTP response is not 200/OK as expected.
  Response Status: '401 Unauthorized'
  Request Headers: Authorization: OAuth oauth_signature_method="HMAC-SHA1" ...

X's Free plan (the only plan available for new developer accounts) requires OAuth 2.0 with PKCE. The OAuth 1.0a request token endpoint returns 401 because it is disabled for Free-tier apps.

Expected behavior

The twitterv2 provider should use OAuth 2.0 + PKCE (code_challenge_method=S256) as documented in X's OAuth 2.0 documentation.

Workaround

I worked around this by implementing X OAuth 2.0 + PKCE outside of goth using golang.org/x/oauth2, while keeping goth for Google login.

Implementation approach:

1. Created a standalone X OAuth 2.0 handler that wraps golang.org/x/oauth2.Config
2. In the HTTP handler, routes for "x" are dispatched to the custom OAuth 2.0 flow; all other providers continue to use goth
3. State and PKCE code verifier are stored in a dedicated gorilla/sessions cookie (separate from goth's session)
4. On callback: validate state, exchange code with PKCE verifier, then call /2/users/me to fetch user info
5. The result is mapped to the same internal user struct used by goth providers, so downstream session/account logic is unchanged

Key golang.org/x/oauth2 APIs used:

• oauth2.GenerateVerifier() — generate PKCE code verifier
• oauth2.S256ChallengeOption(verifier) — attach S256 challenge to auth URL
• oauth2.VerifierOption(verifier) — attach verifier to token exchange
• Config.AuthCodeURL() / Config.Exchange() / Config.Client() — standard OAuth 2.0 flow

X OAuth 2.0 endpoints:

• Auth: https://twitter.com/i/oauth2/authorize
• Token: https://api.twitter.com/2/oauth2/token
• User info: https://api.twitter.com/2/users/me?user.fields=id,name,username,profile_image_url
• Scopes: users.read tweet.read

This works correctly with X's Free plan. Login flow completes successfully and returns user profile data.

Related issues

• #440 — Original request for Twitter OAuth 2.0 support (closed by #460)
• #460 — Added twitterv2 provider (API v2 endpoints, but auth flow remained OAuth 1.0a)

Environment

• goth v1.82.0
• Go 1.26
• X Developer Portal: Free plan

Start in goth/providers/twitterv2 and trace its current mrjones/oauth authentication flow and /2/users/me request. Compare the provider with X's documented OAuth 2.0 + PKCE endpoints and the listed golang.org/x/oauth2 APIs. Done means the provider authenticates on X's Free plan and still returns the existing internal user data.