The manager webapp doPut() processing (which is what the Ant manager tasks use),
extracts a war file from the HTTP request, saves it to appBase, and then tries
to copy the war on to itself, resulting in a zero-length war file.  The code
that does this is in org.apache.catalina.manager.ManagerServlet line 642 in
method deploy(PrintWriter, String, String, boolean, HttpServletRequest):

            if (!isServiced(path)) {
                addServiced(path);
--->            copy(localWar, new File(getAppBase(), basename + ".war"));
                check(path);
                removeServiced(path);
            }

In the call to copy(), the source and destination files are the same.