Check that an appservice protocol has at least one service providing it

changelog.d/10532.bugfix

@@ -0,0 +1 @@
+Only report protocols that are implemented by at least one service, as required by the spec when handling `GET /_matrix/client/r0/thirdparty/protocols`.

synapse/handlers/appservice.py

@@ -392,9 +392,6 @@ async def get_3pe_protocols(
                     protocols[p].append(info)
 
         def _merge_instances(infos: List[JsonDict]) -> JsonDict:
-            if not infos:
-                return {}
-
             # Merge the 'instances' lists of multiple results, but just take
             # the other fields from the first as they ought to be identical
             # copy the result so as not to corrupt the cached one
@@ -406,7 +403,11 @@ def _merge_instances(infos: List[JsonDict]) -> JsonDict:
 
             return combined
 
-        return {p: _merge_instances(protocols[p]) for p in protocols.keys()}
+        return {
+            p: _merge_instances(protocols[p])
+            for p in protocols.keys()
+            if protocols[p]
+        }
 
     async def _get_services_for_event(
         self, event: EventBase