Support for MSC4108 via delegation (#17086)

This adds support for MSC4108 via delegation, similar to what has been done for MSC3886

---------

Co-authored-by: Hugh Nimmo-Smith <hughns@element.io>

2 files changed, 30 insertions, 3 deletions

diff --git a/synapse/rest/client/rendezvous.py b/synapse/rest/client/rendezvous.py
index dee7c37ec5..ed06a29987 100644
--- a/synapse/rest/client/rendezvous.py
+++ b/synapse/rest/client/rendezvous.py
@@ -2,7 +2,7 @@
 # This file is licensed under the Affero General Public License (AGPL) version 3.
 #
 # Copyright 2022 The Matrix.org Foundation C.I.C.
-# Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2023-2024 New Vector, Ltd
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -34,7 +34,7 @@ if TYPE_CHECKING:
 logger = logging.getLogger(__name__)
 
 
-class RendezvousServlet(RestServlet):
+class MSC3886RendezvousServlet(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.
@@ -76,6 +76,30 @@ class RendezvousServlet(RestServlet):
     # PUT, GET and DELETE are not implemented as they should be fulfilled by the redirect target.
 
 
+class MSC4108DelegationRendezvousServlet(RestServlet):
+    PATTERNS = client_patterns(
+        "/org.matrix.msc4108/rendezvous$", releases=[], v1=False, unstable=True
+    )
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        redirection_target: Optional[str] = (
+            hs.config.experimental.msc4108_delegation_endpoint
+        )
+        assert (
+            redirection_target is not None
+        ), "Servlet is only registered if there is a delegation 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
+        )
+
+
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     if hs.config.experimental.msc3886_endpoint is not None:
-        RendezvousServlet(hs).register(http_server)
+        MSC3886RendezvousServlet(hs).register(http_server)
+
+    if hs.config.experimental.msc4108_delegation_endpoint is not None:
+        MSC4108DelegationRendezvousServlet(hs).register(http_server)
diff --git a/synapse/rest/client/versions.py b/synapse/rest/client/versions.py
index c46d4fe8cf..638d4c45ae 100644
--- a/synapse/rest/client/versions.py
+++ b/synapse/rest/client/versions.py
@@ -140,6 +140,9 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.msc4069": self.config.experimental.msc4069_profile_inhibit_propagation,
                     # Allows clients to handle push for encrypted events.
                     "org.matrix.msc4028": self.config.experimental.msc4028_push_encrypted_events,
+                    # MSC4108: Mechanism to allow OIDC sign in and E2EE set up via QR code
+                    "org.matrix.msc4108": self.config.experimental.msc4108_delegation_endpoint
+                    is not None,
                 },
             },
         )