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.

1 files changed, 4 insertions, 2 deletions

diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index d7740eb3b4..c618f3d7a6 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -1005,7 +1005,7 @@ class FederationServer(FederationBase):
 
     @trace
     async def on_claim_client_keys(
-        self, origin: str, content: JsonDict
+        self, origin: str, content: JsonDict, always_include_fallback_keys: bool
     ) -> Dict[str, Any]:
         query = []
         for user_id, device_keys in content.get("one_time_keys", {}).items():
@@ -1013,7 +1013,9 @@ class FederationServer(FederationBase):
                 query.append((user_id, device_id, algorithm))
 
         log_kv({"message": "Claiming one time keys.", "user, device pairs": query})
-        results = await self._e2e_keys_handler.claim_local_one_time_keys(query)
+        results = await self._e2e_keys_handler.claim_local_one_time_keys(
+            query, always_include_fallback_keys=always_include_fallback_keys
+        )
 
         json_result: Dict[str, Dict[str, Dict[str, JsonDict]]] = {}
         for result in results: