[Qemu-devel] [PATCH for-1.7 1/2] block/drive-mirror: Reuse backing HD for sync=none

[Max Reitz <mreitz@redhat.com>]

For "none" sync mode in "absolute-paths" mode, the current image should
be used as the backing file for the newly created image.

The current behavior is:
a) If the image to be mirrored has a backing file, use that (which is
   wrong, since the operations recorded by "none" are applied to the
   image itself, not to its backing file).
b) If the image to be mirrored lacks a backing file, the target doesn't
   have one either (which is not really wrong, but not really right,
   either; "none" records a set of operations executed on the image
   file, therefore having no backing file to apply these operations on
   seems rather pointless).

For a, this is clearly a bugfix. For b, it is still a bugfix, although
it might break existing API - but since that case crashed qemu just
three weeks ago (before 1452686495922b81d6cf43edf025c1aef15965c0), we
can safely assume there is no such API relying on that case yet.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 blockdev.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/blockdev.c b/blockdev.c
index 330aa4a..44755e1 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -2021,6 +2021,9 @@ void qmp_drive_mirror(const char *device, const char *target,
     if (!source && sync == MIRROR_SYNC_MODE_TOP) {
         sync = MIRROR_SYNC_MODE_FULL;
     }
+    if (sync == MIRROR_SYNC_MODE_NONE) {
+        source = bs;
+    }
 
     size = bdrv_getlength(bs);
     if (size < 0) {
-- 
1.8.4.2