commit 269d39afe5c59ecb3d3d64dba52f8cfa8d63d197
Author: Guido Günther <agx@sigxcpu.org>
Date:   Sun Aug 23 22:03:54 2015 +0200

    storage: only run safezero if allocation is > 0
    
    While a zero allocation in safezero should be fine it isn't when we use
    posix_fallocate which returns EINVAL on a zero allocation.
    
    While we could skip the zero allocation in safezero_posix_fallocate it's
    an optimization to do it for all allocations.
    
    This fixes vm installation via virtinst for me which otherwise aborts
    like:
    
       Starting install...
       Retrieving file linux...               | 5.9 MB     00:01 ...
       Retrieving file initrd.gz...           |  29 MB     00:07 ...
       ERROR    Couldn't create storage volume 'virtinst-linux.sBgds4': 'cannot fill file '/var/lib/libvirt/boot/virtinst-linux.sBgds4': Invalid argument'
    
    The error was introduced by e30297b0 as spotted by Chunyan Liu

Index: libvirt-1.2.18/src/storage/storage_backend.c
===================================================================
--- libvirt-1.2.18.orig/src/storage/storage_backend.c
+++ libvirt-1.2.18/src/storage/storage_backend.c
@@ -455,7 +455,7 @@ createRawFile(int fd, virStorageVolDefPt
         pos = inputvol->target.capacity - remain;
     }
 
-    if (need_alloc) {
+    if (need_alloc && (vol->target.allocation - pos > 0)) {
         if (safezero(fd, pos, vol->target.allocation - pos) < 0) {
             ret = -errno;
             virReportSystemError(errno, _("cannot fill file '%s'"),