fix: Cap max_depth to 50 in hierarchy endpoint

src/service/rooms/summary/mod.rs

@@ -19,6 +19,7 @@ use ruma::{
 	},
 	room::{JoinRuleSummary, RestrictedSummary, RoomSummary},
 	serde::Raw,
+	uint,
 };
 
 use crate::{Dep, rooms, sending};
@@ -188,6 +189,12 @@ impl Service {
 		// This function traverses the space hierarchy tree depth-first as required by
 		// the specification.
 
+		// Limit max depth to 50 to keep cycles in the space graph from blowing up the
+		// traversal. If you actually have over fifty nested spaces, and you're
+		// looking at this function to figure out what the issue is, I suggest
+		// you reconsider some of the choices you made which led you to this point.
+		let max_depth = max_depth.unwrap_or_else(|| uint!(50));
+
 		// Check accessibility of the root room first, because we need to error out
 		// if it isn't accessible.
 		// TODO refactor this once the Try trait is stable
@@ -259,7 +266,7 @@ impl Service {
 				reason = "queue.len() should never be large enough to cause strange behavior \
 				          here"
 			)]
-			if max_depth.is_some_and(|max_depth| (queue.len() as u64 + 1) > max_depth.into()) {
+			if (queue.len() as u64 + 1) > max_depth.into() {
 				continue;
 			}