fix another race (unpost):

unposting could collide with most other database-related activities,
causing one or the other to fail.
luckily the unprotected query performed by the unpost API happens to be
very cheap, so also the most likely to fail, and would succeed upon a
manual reattempt from the UI.
even in the worst case scenario, there would be no unrecoverable damage
as the next rescan would auto-repair any resulting inconsistencies.

copyparty/up2k.py

@@ -3000,7 +3000,8 @@ class Up2k(object):
             permsets = [[False, True]]
             vn, rem = self.asrv.vfs.get(vpath, uname, *permsets[0])
             vn, rem = vn.get_dbv(rem)
-            _, _, _, _, dip, dat = self._find_from_vpath(vn.realpath, rem)
+            with self.mutex:
+                _, _, _, _, dip, dat = self._find_from_vpath(vn.realpath, rem)
 
             t = "you cannot delete this: "
             if not dip: