If bdrv_truncate() is called, but the requested size is the same as
before, don't call posix_fallocate(), which returns -EINVAL for length
zero and would therefore make bdrv_truncate() fail.

The problem can be triggered by creating a zero-sized raw image with
'falloc' preallocation mode.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
---
 block/file-posix.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index ba14ed9459..6aed5bca0b 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1686,11 +1686,15 @@ static int raw_regular_truncate(int fd, int64_t offset, PreallocMode prealloc,
          * file systems that do not support fallocate(), trying to check if a
          * block is allocated before allocating it, so don't do that here.
          */
-        result = -posix_fallocate(fd, current_length, offset - current_length);
-        if (result != 0) {
-            /* posix_fallocate() doesn't set errno. */
-            error_setg_errno(errp, -result,
-                             "Could not preallocate new data");
+        if (offset != current_length) {
+            result = -posix_fallocate(fd, current_length, offset - current_length);
+            if (result != 0) {
+                /* posix_fallocate() doesn't set errno. */
+                error_setg_errno(errp, -result,
+                                 "Could not preallocate new data");
+            }
+        } else {
+            result = 0;
         }
         goto out;
 #endif
-- 
2.13.6