[PATCH v2] storage: only run safezero if allocation is > 0

[Guido Günther <agx@xxxxxxxxxxx>]

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
---
Only the commit message got adjusted.

 src/storage/storage_backend.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/storage/storage_backend.c b/src/storage/storage_backend.c
index db49739..0418473 100644
--- a/src/storage/storage_backend.c
+++ b/src/storage/storage_backend.c
@@ -455,7 +455,7 @@ createRawFile(int fd, virStorageVolDefPtr vol,
         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'"),
-- 
2.1.4

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