Make get_e2e_cross_signing_key delegate to get_e2e_cross_signing_keys_bulk

... mostly because the latter has a cache.

changelog.d/7428.misc

@@ -0,0 +1 @@
+Improve performance of `get_e2e_cross_signing_key`.

synapse/storage/data_stores/main/end_to_end_keys.py

@@ -270,53 +270,7 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
             "count_e2e_one_time_keys", _count_e2e_one_time_keys
         )
 
-    def _get_e2e_cross_signing_key_txn(self, txn, user_id, key_type, from_user_id=None):
-        """Returns a user's cross-signing key.
-
-        Args:
-            txn (twisted.enterprise.adbapi.Connection): db connection
-            user_id (str): the user whose key is being requested
-            key_type (str): the type of key that is being requested: either 'master'
-                for a master key, 'self_signing' for a self-signing key, or
-                'user_signing' for a user-signing key
-            from_user_id (str): if specified, signatures made by this user on
-                the key will be included in the result
-
-        Returns:
-            dict of the key data or None if not found
-        """
-        sql = (
-            "SELECT keydata "
-            "  FROM e2e_cross_signing_keys "
-            " WHERE user_id = ? AND keytype = ? ORDER BY stream_id DESC LIMIT 1"
-        )
-        txn.execute(sql, (user_id, key_type))
-        row = txn.fetchone()
-        if not row:
-            return None
-        key = json.loads(row[0])
-
-        device_id = None
-        for k in key["keys"].values():
-            device_id = k
-
-        if from_user_id is not None:
-            sql = (
-                "SELECT key_id, signature "
-                "  FROM e2e_cross_signing_signatures "
-                " WHERE user_id = ? "
-                "   AND target_user_id = ? "
-                "   AND target_device_id = ? "
-            )
-            txn.execute(sql, (from_user_id, user_id, device_id))
-            row = txn.fetchone()
-            if row:
-                key.setdefault("signatures", {}).setdefault(from_user_id, {})[
-                    row[0]
-                ] = row[1]
-
-        return key
-
+    @defer.inlineCallbacks
     def get_e2e_cross_signing_key(self, user_id, key_type, from_user_id=None):
         """Returns a user's cross-signing key.
 
@@ -331,13 +285,11 @@ class EndToEndKeyWorkerStore(SQLBaseStore):
         Returns:
             dict of the key data or None if not found
         """
-        return self.db.runInteraction(
-            "get_e2e_cross_signing_key",
-            self._get_e2e_cross_signing_key_txn,
-            user_id,
-            key_type,
-            from_user_id,
-        )
+        res = yield self.get_e2e_cross_signing_keys_bulk([user_id], from_user_id)
+        user_keys = res.get(user_id)
+        if not user_keys:
+            return None
+        return user_keys.get(key_type)
 
     @cached(num_args=1)
     def _get_bare_e2e_cross_signing_keys(self, user_id):