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, 5 insertions, 4 deletions

diff --git a/synapse/storage/databases/main/end_to_end_keys.py b/synapse/storage/databases/main/end_to_end_keys.py
index dc7768c50c..1a4ae55304 100644
--- a/synapse/storage/databases/main/end_to_end_keys.py
+++ b/synapse/storage/databases/main/end_to_end_keys.py
@@ -1149,18 +1149,19 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore, CacheInvalidationWorker
         return results, missing
 
     async def claim_e2e_fallback_keys(
-        self, query_list: Iterable[Tuple[str, str, str]]
+        self, query_list: Iterable[Tuple[str, str, str, bool]]
     ) -> Dict[str, Dict[str, Dict[str, JsonDict]]]:
         """Take a list of fallback keys out of the database.
 
         Args:
-            query_list: An iterable of tuples of (user ID, device ID, algorithm).
+            query_list: An iterable of tuples of
+                (user ID, device ID, algorithm, whether the key should be marked as used).
 
         Returns:
             A map of user ID -> a map device ID -> a map of key ID -> JSON.
         """
         results: Dict[str, Dict[str, Dict[str, JsonDict]]] = {}
-        for user_id, device_id, algorithm in query_list:
+        for user_id, device_id, algorithm, mark_as_used in query_list:
             row = await self.db_pool.simple_select_one(
                 table="e2e_fallback_keys_json",
                 keyvalues={
@@ -1180,7 +1181,7 @@ class EndToEndKeyWorkerStore(EndToEndKeyBackgroundStore, CacheInvalidationWorker
             used = row["used"]
 
             # Mark fallback key as used if not already.
-            if not used:
+            if not used and mark_as_used:
                 await self.db_pool.simple_update_one(
                     table="e2e_fallback_keys_json",
                     keyvalues={