block: Pass truncate exact=true where reasonable

This is a change in behavior, so all instances need a good
justification.  The comments added here should explain my reasoning.

qed already had a comment that suggests it always expected
bdrv_truncate()/blk_truncate() to behave as if exact=true were passed
(c743849bee7 came eight months before 55b949c8476), so it was simply
broken until now.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190918095144.955-8-mreitz@redhat.com
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
[mreitz: Changed comment in qed.c to explain why a new QED file must be
         empty, as requested and suggested by Maxim]
Signed-off-by: Max Reitz <mreitz@redhat.com>

1 files changed, 6 insertions, 1 deletions

diff --git a/qemu-img.c b/qemu-img.c
index c738297b04..b288c967b5 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -3831,7 +3831,12 @@ static int img_resize(int argc, char **argv)
         }
     }
 
-    ret = blk_truncate(blk, total_size, false, prealloc, &err);
+    /*
+     * The user expects the image to have the desired size after
+     * resizing, so pass @exact=true.  It is of no use to report
+     * success when the image has not actually been resized.
+     */
+    ret = blk_truncate(blk, total_size, true, prealloc, &err);
     if (ret < 0) {
         error_report_err(err);
         goto out;