Support for form_post in OIDC responses (#9376)

Apple want to POST the OIDC auth response back to us rather than using query-params; add the necessary support to make that work.

synapse/rest/synapse/client/oidc/callback_resource.py

@@ -12,19 +12,30 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 import logging
+from typing import TYPE_CHECKING
 
 from synapse.http.server import DirectServeHtmlResource
 
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
 logger = logging.getLogger(__name__)
 
 
 class OIDCCallbackResource(DirectServeHtmlResource):
     isLeaf = 1
 
-    def __init__(self, hs):
+    def __init__(self, hs: "HomeServer"):
         super().__init__()
         self._oidc_handler = hs.get_oidc_handler()
 
     async def _async_render_GET(self, request):
         await self._oidc_handler.handle_oidc_callback(request)
+
+    async def _async_render_POST(self, request):
+        # the auth response can be returned via an x-www-form-urlencoded form instead
+        # of GET params, as per
+        # https://openid.net/specs/oauth-v2-form-post-response-mode-1_0.html.
+        await self._oidc_handler.handle_oidc_callback(request)