fix(node): an added root is served before it is scanned

Adding a large directory to a running group made the reload await the
scan of the new root before putting the new RootSet in the group's
context, holding _reload_lock the whole time. For the hours a large drive
takes to hash, the node served the old set:

- a file request under the new root got None from entry_abs_path and the
  handler died on None.exists() without replying;
- a writable/removable toggle answered with the live table, still the
  old one, so the directory vanished from the operator's settings;
- reconcile saw every file the scan had not reached as a missed event and
  hashed it again on the same executor, rewriting progress under the scan.

retarget now applies the set, the roots table and the watcher first, and
with wait=False scans the added roots in the background; the daemon swaps
ctx["roots"] before calling it. A scan lock shared by the initial scan,
added-root scans and reconcile makes the reconcile loop sit out a running
scan without backing off. Every transport site that resolves an entry
answers ROOT_NOT_SERVED instead of crashing, and a delete keeps the entry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T6jPTeocXA1BePekdsgPya

2 files changed, 19 insertions, 2 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
index 7b9d094..2a2b07a 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/quic_server.py
@@ -34,7 +34,7 @@ from aioquic.quic.events import QuicEvent, StreamDataReceived, StreamReset
 from cryptography.hazmat.primitives.asymmetric.ed25519 import Ed25519PrivateKey
 
 from meshbay_common import MNP_VERSION
-from meshbay_node.roots import RootSet, entry_abs_path
+from meshbay_node.roots import ROOT_NOT_SERVED, RootSet, entry_abs_path
 from meshbay_common.handshake import (
     MNP_MIN_SUPPORTED,
     NONCE_LEN,
@@ -418,6 +418,9 @@ class _MNPServerProtocol(QuicConnectionProtocol):
             return
 
         file_path = entry_abs_path(ctx["roots"], entry)
+        if file_path is None:
+            self._send(stream_id, {"type": "error", "detail": ROOT_NOT_SERVED})
+            return
         if not file_path.exists():
             self._send(stream_id, {"type": "error", "detail": "File not on disk"})
             return
diff --git a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
index bc78644..eff40f1 100644
--- a/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
+++ b/packages/meshbay-node/src/meshbay_node/transport/webrtc_server.py
@@ -137,7 +137,7 @@ from meshbay_node.media_probe import (
     probe_video as _probe_video,
 )
 from meshbay_node.roots import (
-    RootSet, entry_abs_path, SAFE_UPLOAD_NAME, safe_subdir, _free_name,
+    ROOT_NOT_SERVED, RootSet, entry_abs_path, SAFE_UPLOAD_NAME, safe_subdir, _free_name,
 )
 
 log = logging.getLogger(__name__)
@@ -3643,6 +3643,9 @@ class WebRTCPeerSession:
             return
 
         file_path = entry_abs_path(ctx["roots"], entry)
+        if file_path is None:
+            self._send({"type": "error", "detail": ROOT_NOT_SERVED})
+            return
         if not file_path.exists():
             self._send({"type": "error", "detail": "File not on disk"})
             return
@@ -3770,6 +3773,9 @@ class WebRTCPeerSession:
             self._send({"type": "error", "detail": "File not found"})
             return
         file_path = entry_abs_path(ctx["roots"], entry)
+        if file_path is None:
+            self._send({"type": "error", "detail": ROOT_NOT_SERVED})
+            return
         if not file_path.exists():
             self._send({"type": "error", "detail": "File not on disk"})
             return
@@ -5524,6 +5530,11 @@ class WebRTCPeerSession:
 
     def _exec_file_delete(self, ctx: dict, file_id: str, entry) -> None:
         file_path = entry_abs_path(ctx["roots"], entry)
+        if file_path is None:
+            # Frozen, not gone: removing the entry would lose a file that is
+            # still on a drive the node cannot read right now.
+            self._send({"type": "error", "detail": ROOT_NOT_SERVED})
+            return
         if file_path.exists():
             file_path.unlink()
             log.info("File deleted: %s", entry.name)
@@ -5723,6 +5734,9 @@ class WebRTCPeerSession:
             return
 
         file_path = entry_abs_path(ctx["roots"], entry)
+        if file_path is None:
+            self._send({"type": "error", "detail": ROOT_NOT_SERVED})
+            return
         if not file_path.exists():
             self._send({"type": "error", "detail": "File not on disk"})
             return