perf(node): list a group's directories off the event loop

Every full index walked all roots on the loop, and a node with several
large roots stopped answering for seconds. Walk directories only, on the
roots' disk thread; index_sync is spawned and still answers on failure.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

2 files changed, 17 insertions, 5 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/dispatch.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/dispatch.py
index 4bf9dbb..a8b5767 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/dispatch.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/dispatch.py
@@ -18,7 +18,7 @@ MAX_PRE_PROOF_FETCHES = 4
 # records it for every type.
 SPAWNED, INLINE = True, False
 _HANDLERS = {
-    MNP.INDEX_SYNC: ("_do_index_sync", INLINE),
+    MNP.INDEX_SYNC: ("_do_index_sync", SPAWNED),
     # Spawned rather than answered inline: the reply waits for room
     # on the channel, and blocking the message loop for that would
     # stop everything else this peer is doing — including the
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc/files.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc/files.py
index f629563..0434554 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc/files.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc/files.py
@@ -19,7 +19,7 @@ from meshbay_node.transport.webrtc.disk import (
     _rmdir_if_empty,
 )
 from meshbay_node.transport.webrtc.limits import CHUNK_SIZE, LEASE_GRANTED, LEASE_QUEUED
-from meshbay_node.transport.wire import index_sync_message
+from meshbay_node.transport.wire import index_sync_message, list_dirs_off_loop
 
 log = logging.getLogger("meshbay_node.transport.webrtc_server")
 
@@ -177,9 +177,21 @@ class FilesMixin:
         self._audit("dir_delete", rel)
         self._send({"type": MNP.DIR_DELETE_ACK, "v": MNP_VERSION, "dir": rel})
 
-    def _do_index_sync(self) -> None:
-        ctx = self._group_ctx()
-        self._send(index_sync_message(ctx["index"], ctx.get("roots"), self._hidden_ids()))
+    async def _do_index_sync(self) -> None:
+        try:
+            dirs = await list_dirs_off_loop(self._group_ctx().get("roots"))
+            # Read again after the walk: the entries go out as they are when the
+            # reply is sent, so no delta pushed meanwhile is older than it.
+            ctx = self._group_ctx()
+            msg = index_sync_message(ctx["index"], ctx.get("roots"), self._hidden_ids(),
+                                     dirs)
+        except Exception as e:
+            # What the dispatcher answered when this ran inline. A spawned task
+            # that fails only logs, and the client would wait out its timeout.
+            log.error("Error building index_sync: %s", e, exc_info=True)
+            self._send({"type": "error", "detail": "Request failed"})
+            return
+        self._send(msg)
 
     async def _try_serve_thumbnail(
         self, thumb_hash: str, chunk_index: int, gek: bytes | None,