feat: allow redirect_uri param to spotify login, for Lockstep Demo app flow

api.py

@@ -21,8 +21,9 @@ import sqlite3
 from base64 import b64encode
 from datetime import datetime, timedelta, timezone
 import json
+from urllib.parse import urlencode
 
-from flask import Flask, request, session, jsonify, make_response
+from flask import Flask, request, session, jsonify, make_response, redirect, url_for
 from werkzeug.middleware.proxy_fix import ProxyFix
 from authomatic import Authomatic
 from authomatic.adapters import WerkzeugAdapter
@@ -93,13 +94,22 @@ PROVIDER_ID_MAP = list(AUTHOMATIC_PROVIDER_ID_MAP) + [Spotify]
 SPOTIFY_CLIENT_ID = os.environ["SPOTIFY_CLIENT_ID"]
 SPOTIFY_CLIENT_SECRET = os.environ["SPOTIFY_CLIENT_SECRET"]
 
-# Must exactly match a Redirect URI configured in your Spotify app settings.
+# OAuth redirect registered in the Spotify Developer Dashboard (Authorization Code
+# flow). Spotify sends the browser here with ?code=&state= — not to the Android app scheme.
 REDIRECT_URI = os.environ.get(
     "SPOTIFY_REDIRECT_URI",
     #"https://api.lockstep.at/spotify/callback"
     "https://api.lockstep.at/login/spotify/"
 )
 
+# After server-side token exchange, the browser may be redirected to this URL with
+# tokens in the query string (mobile / Custom Tab). This is NOT registered with Spotify;
+# only REDIRECT_URI above goes in the dashboard for this flow.
+ALLOWED_SPOTIFY_APP_POST_LOGIN_REDIRECT = os.environ.get(
+    "SPOTIFY_APP_POST_LOGIN_REDIRECT_URI",
+    "at.lockstep.player://spotify/callback",
+)
+
 DB_PATH = os.environ.get("TOKEN_DB_PATH", "spotify_tokens.db")
 
 app = Flask(__name__)
@@ -391,6 +401,28 @@ def index():
 
 @app.route("/login/<provider_name>/", methods=["GET", "POST"])
 def login(provider_name):
+    # Authomatic 1.3.0 (oauth2.login) only runs "phase 1" — redirect to Spotify —
+    # when there are no query parameters, or only ``user_state``:
+    #   elif (not self.params or (len(self.params) == 1 and 'user_state' in self.params))
+    # A mobile client opening ``/login/spotify/?redirect_uri=...`` therefore matches
+    # no branch; login() returns without calling redirect() → empty body and HTTP 200
+    # (white page). Stash the app callback in the session and reload without query args.
+    if (
+        provider_name == "spotify"
+        and request.args.get("redirect_uri")
+        and "code" not in request.args
+        and "error" not in request.args
+    ):
+        requested = request.args.get("redirect_uri", "")
+        if requested != ALLOWED_SPOTIFY_APP_POST_LOGIN_REDIRECT:
+            return jsonify({
+                "ok": False,
+                "error": "redirect_uri not allowed for this client",
+            }), 400
+        session["spotify_oauth_app_redirect_uri"] = requested
+        session.modified = True
+        return redirect(url_for("login", provider_name=provider_name), code=302)
+
     response = make_response()
 
     # Let Authomatic handle the OAuth2 handshake.
@@ -398,7 +430,7 @@ def login(provider_name):
         WerkzeugAdapter(request, response),
         provider_name,
         session=session,
-        session_saver=lambda: session.modified
+        session_saver=lambda: setattr(session, "modified", True),
     )
 
     # If result is None, Authomatic is still redirecting/processing.
@@ -469,6 +501,27 @@ def login(provider_name):
     # keep the Spotify user id in session
     session["spotify_user_id"] = result.user.id
 
+    app_redirect = session.pop("spotify_oauth_app_redirect_uri", None)
+    if app_redirect:
+        if app_redirect != ALLOWED_SPOTIFY_APP_POST_LOGIN_REDIRECT:
+            return jsonify({
+                "ok": False,
+                "error": "Stored app redirect_uri does not match allowlist",
+            }), 400
+        sep = "&" if ("?" in app_redirect) else "?"
+        target = (
+            f"{app_redirect}{sep}"
+            + urlencode(
+                {
+                    "access_token": access_token,
+                    "refresh_token": refresh_token,
+                    "expires_in": str(expires_in),
+                    "token_type": token_type or "",
+                },
+            )
+        )
+        return redirect(target, code=302)
+
     return jsonify({
         "ok": True,
         "spotify_user_id": result.user.id,