fix xvol false-positive;

given the following config:

* volume /a mapped to /srv/nas/
* volume /b mapped to /srv/nas/foo/
* anyone can read volume /a but not /b

accessing /a/foo/ would incorrectly fail because the xvol-check
would select /b based on its abspath being physically closer,
not considering that the same abspath is reachable from /a

copyparty/authsrv.py

@@ -881,6 +881,15 @@ class VFS(object):
                 return None
 
         if "xvol" in self.flags:
+            self_ap = self.realpath + os.sep
+            if aps.startswith(self_ap):
+                vp = aps[len(self_ap) :]
+                if ANYWIN:
+                    vp = vp.replace(os.sep, "/")
+                vn2, _ = self._find(vp)
+                if self == vn2:
+                    return self
+
             all_aps = self.shr_all_aps or self.root.all_aps
 
             for vap, vns in all_aps: