Add unstable /keys/claim endpoint which always returns fallback keys. (#15462)

It can be useful to always return the fallback key when attempting to
claim keys. This adds an unstable endpoint for `/keys/claim` which
always returns fallback keys in addition to one-time-keys.

The fallback key(s) are not marked as "used" unless there are no
corresponding OTKs.

This is currently defined in MSC3983 (although likely to be split out
to a separate MSC). The endpoint shape may change or be requested
differently (i.e. a keyword parameter on the current endpoint), but the
core logic should be reasonable.

2 files changed, 28 insertions, 1 deletions

diff --git a/synapse/federation/transport/server/__init__.py b/synapse/federation/transport/server/__init__.py
index 753372fc54..55d2cd0a9a 100644
--- a/synapse/federation/transport/server/__init__.py
+++ b/synapse/federation/transport/server/__init__.py
@@ -25,6 +25,7 @@ from synapse.federation.transport.server._base import (
 from synapse.federation.transport.server.federation import (
     FEDERATION_SERVLET_CLASSES,
     FederationAccountStatusServlet,
+    FederationUnstableClientKeysClaimServlet,
 )
 from synapse.http.server import HttpServer, JsonResource
 from synapse.http.servlet import (
@@ -298,6 +299,11 @@ def register_servlets(
                 and not hs.config.experimental.msc3720_enabled
             ):
                 continue
+            if (
+                servletclass == FederationUnstableClientKeysClaimServlet
+                and not hs.config.experimental.msc3983_appservice_otk_claims
+            ):
+                continue
 
             servletclass(
                 hs=hs,
diff --git a/synapse/federation/transport/server/federation.py b/synapse/federation/transport/server/federation.py
index ec5b5eeafa..e2340d70d5 100644
--- a/synapse/federation/transport/server/federation.py
+++ b/synapse/federation/transport/server/federation.py
@@ -577,7 +577,28 @@ class FederationClientKeysClaimServlet(BaseFederationServerServlet):
     async def on_POST(
         self, origin: str, content: JsonDict, query: Dict[bytes, List[bytes]]
     ) -> Tuple[int, JsonDict]:
-        response = await self.handler.on_claim_client_keys(origin, content)
+        response = await self.handler.on_claim_client_keys(
+            origin, content, always_include_fallback_keys=False
+        )
+        return 200, response
+
+
+class FederationUnstableClientKeysClaimServlet(BaseFederationServerServlet):
+    """
+    Identical to the stable endpoint (FederationClientKeysClaimServlet) except it
+    always includes fallback keys in the response.
+    """
+
+    PREFIX = FEDERATION_UNSTABLE_PREFIX
+    PATH = "/user/keys/claim"
+    CATEGORY = "Federation requests"
+
+    async def on_POST(
+        self, origin: str, content: JsonDict, query: Dict[bytes, List[bytes]]
+    ) -> Tuple[int, JsonDict]:
+        response = await self.handler.on_claim_client_keys(
+            origin, content, always_include_fallback_keys=True
+        )
         return 200, response