Implementation of HTTP 307 response for MSC3886 POST endpoint (#14018)

Co-authored-by: reivilibre <olivier@librepush.net>
Co-authored-by: Andrew Morgan <andrewm@element.io>

synapse/rest/client/rendezvous.py

@@ -0,0 +1,74 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# 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 http.client import TEMPORARY_REDIRECT
+from typing import TYPE_CHECKING, Optional
+
+from synapse.http.server import HttpServer, respond_with_redirect
+from synapse.http.servlet import RestServlet
+from synapse.http.site import SynapseRequest
+from synapse.rest.client._base import client_patterns
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+
+class RendezvousServlet(RestServlet):
+    """
+    This is a placeholder implementation of [MSC3886](https://github.com/matrix-org/matrix-spec-proposals/pull/3886)
+    simple client rendezvous capability that is used by the "Sign in with QR" functionality.
+
+    This implementation only serves as a 307 redirect to a configured server rather than being a full implementation.
+
+    A module that implements the full functionality is available at: https://pypi.org/project/matrix-http-rendezvous-synapse/.
+
+    Request:
+
+    POST /rendezvous HTTP/1.1
+    Content-Type: ...
+
+    ...
+
+    Response:
+
+    HTTP/1.1 307
+    Location: <configured endpoint>
+    """
+
+    PATTERNS = client_patterns(
+        "/org.matrix.msc3886/rendezvous$", releases=[], v1=False, unstable=True
+    )
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        redirection_target: Optional[str] = hs.config.experimental.msc3886_endpoint
+        assert (
+            redirection_target is not None
+        ), "Servlet is only registered if there is a redirection target"
+        self.endpoint = redirection_target.encode("utf-8")
+
+    async def on_POST(self, request: SynapseRequest) -> None:
+        respond_with_redirect(
+            request, self.endpoint, statusCode=TEMPORARY_REDIRECT, cors=True
+        )
+
+    # PUT, GET and DELETE are not implemented as they should be fulfilled by the redirect target.
+
+
+def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
+    if hs.config.experimental.msc3886_endpoint is not None:
+        RendezvousServlet(hs).register(http_server)