add MCP OAuth discovery, DCR, and token exchange

surfsense_backend/app/services/mcp_oauth/discovery.py

@@ -0,0 +1,111 @@
+"""MCP OAuth 2.1 metadata discovery, Dynamic Client Registration, and token exchange."""
+
+from __future__ import annotations
+
+import base64
+import logging
+from urllib.parse import urlparse
+
+import httpx
+
+logger = logging.getLogger(__name__)
+
+
+async def discover_oauth_metadata(mcp_url: str, *, timeout: float = 15.0) -> dict:
+    """Fetch OAuth 2.1 metadata from the MCP server's well-known endpoint.
+
+    Per the MCP spec the discovery document lives at the *origin* of the
+    MCP server URL, not at the MCP endpoint path.
+    """
+    parsed = urlparse(mcp_url)
+    origin = f"{parsed.scheme}://{parsed.netloc}"
+    discovery_url = f"{origin}/.well-known/oauth-authorization-server"
+
+    async with httpx.AsyncClient(follow_redirects=True) as client:
+        resp = await client.get(discovery_url, timeout=timeout)
+        resp.raise_for_status()
+        return resp.json()
+
+
+async def register_client(
+    registration_endpoint: str,
+    redirect_uri: str,
+    *,
+    client_name: str = "SurfSense",
+    timeout: float = 15.0,
+) -> dict:
+    """Perform Dynamic Client Registration (RFC 7591)."""
+    payload = {
+        "client_name": client_name,
+        "redirect_uris": [redirect_uri],
+        "grant_types": ["authorization_code", "refresh_token"],
+        "response_types": ["code"],
+        "token_endpoint_auth_method": "client_secret_basic",
+    }
+
+    async with httpx.AsyncClient(follow_redirects=True) as client:
+        resp = await client.post(
+            registration_endpoint, json=payload, timeout=timeout,
+        )
+        resp.raise_for_status()
+        return resp.json()
+
+
+async def exchange_code_for_tokens(
+    token_endpoint: str,
+    code: str,
+    redirect_uri: str,
+    client_id: str,
+    client_secret: str,
+    code_verifier: str,
+    *,
+    timeout: float = 30.0,
+) -> dict:
+    """Exchange an authorization code for access + refresh tokens."""
+    creds = base64.b64encode(f"{client_id}:{client_secret}".encode()).decode()
+
+    async with httpx.AsyncClient(follow_redirects=True) as client:
+        resp = await client.post(
+            token_endpoint,
+            data={
+                "grant_type": "authorization_code",
+                "code": code,
+                "redirect_uri": redirect_uri,
+                "code_verifier": code_verifier,
+            },
+            headers={
+                "Content-Type": "application/x-www-form-urlencoded",
+                "Authorization": f"Basic {creds}",
+            },
+            timeout=timeout,
+        )
+        resp.raise_for_status()
+        return resp.json()
+
+
+async def refresh_access_token(
+    token_endpoint: str,
+    refresh_token: str,
+    client_id: str,
+    client_secret: str,
+    *,
+    timeout: float = 30.0,
+) -> dict:
+    """Refresh an expired access token."""
+    creds = base64.b64encode(f"{client_id}:{client_secret}".encode()).decode()
+
+    async with httpx.AsyncClient(follow_redirects=True) as client:
+        resp = await client.post(
+            token_endpoint,
+            data={
+                "grant_type": "refresh_token",
+                "refresh_token": refresh_token,
+            },
+            headers={
+                "Content-Type": "application/x-www-form-urlencoded",
+                "Authorization": f"Basic {creds}",
+            },
+            timeout=timeout,
+        )
+        resp.raise_for_status()
+        return resp.json()