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

1 files changed, 5 insertions, 0 deletions

diff --git a/packages/meshbay-node/src/meshbay_node/roots.py b/packages/meshbay-node/src/meshbay_node/roots.py
index 410fe68..7854e28 100644
--- a/packages/meshbay-node/src/meshbay_node/roots.py
+++ b/packages/meshbay-node/src/meshbay_node/roots.py
@@ -384,6 +384,11 @@ class RootSet:
         return out
 
 
+# What a transport answers when `entry_abs_path` gives None: the entry is in the
+# index but its root is ejected, unplugged, or not in the set being served.
+ROOT_NOT_SERVED = "File not available: its folder is not readable right now"
+
+
 def entry_abs_path(roots: RootSet, entry) -> Path | None:
     """
     Where an index entry actually lives, or None if its root is gone.