Allow subpaths in MAS endpoints (#19186)

Fixes #19184

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- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
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changelog.d/19186.bugfix

@@ -0,0 +1 @@
+Fix regression preventing subpaths in MAS endpoints.

synapse/api/auth/mas.py

@@ -17,7 +17,6 @@ from typing import TYPE_CHECKING, Optional
 from urllib.parse import urlencode
 
 from pydantic import (
-    AnyHttpUrl,
     BaseModel,
     ConfigDict,
     StrictBool,
@@ -147,33 +146,13 @@ class MasDelegatedAuth(BaseAuth):
 
     @property
     def _metadata_url(self) -> str:
-        return str(
-            AnyHttpUrl.build(
-                scheme=self._config.endpoint.scheme,
-                username=self._config.endpoint.username,
-                password=self._config.endpoint.password,
-                host=self._config.endpoint.host or "",
-                port=self._config.endpoint.port,
-                path=".well-known/openid-configuration",
-                query=None,
-                fragment=None,
-            )
+        return (
+            f"{str(self._config.endpoint).rstrip('/')}/.well-known/openid-configuration"
         )
 
     @property
     def _introspection_endpoint(self) -> str:
-        return str(
-            AnyHttpUrl.build(
-                scheme=self._config.endpoint.scheme,
-                username=self._config.endpoint.username,
-                password=self._config.endpoint.password,
-                host=self._config.endpoint.host or "",
-                port=self._config.endpoint.port,
-                path="oauth2/introspect",
-                query=None,
-                fragment=None,
-            )
-        )
+        return f"{str(self._config.endpoint).rstrip('/')}/oauth2/introspect"
 
     async def _load_metadata(self) -> ServerMetadata:
         response = await self._http_client.get_json(self._metadata_url)

tests/handlers/test_oauth_delegation.py

@@ -1057,6 +1057,32 @@ class MasAuthDelegation(HomeserverTestCase):
         self.assertEqual(self.server.calls, 1)
 
 
+class MasAuthDelegationWithSubpath(MasAuthDelegation):
+    """Test MAS delegation when the MAS server is hosted on a subpath."""
+
+    def default_config(self) -> dict[str, Any]:
+        config = super().default_config()
+        # Override the endpoint to include a subpath
+        config["matrix_authentication_service"]["endpoint"] = (
+            self.server.endpoint + "auth/path/"
+        )
+        return config
+
+    def test_introspection_endpoint_uses_subpath(self) -> None:
+        """Test that the introspection endpoint correctly uses the configured subpath."""
+        expected_introspection_url = (
+            self.server.endpoint + "auth/path/oauth2/introspect"
+        )
+        self.assertEqual(self._auth._introspection_endpoint, expected_introspection_url)
+
+    def test_metadata_url_uses_subpath(self) -> None:
+        """Test that the metadata URL correctly uses the configured subpath."""
+        expected_metadata_url = (
+            self.server.endpoint + "auth/path/.well-known/openid-configuration"
+        )
+        self.assertEqual(self._auth._metadata_url, expected_metadata_url)
+
+
 @parameterized_class(
     ("config",),
     [