[PATCH 3/4] xen-block: Fix BlockBackend leak on unplug of empty CD-ROM

[Mitsuru Kariya <Mitsuru.Kariya@xxxxxxxxxxxxxxx>]

When a Xen CD-ROM is created without a backing drive (an empty CD-ROM),
xen_cdrom_realize() allocates an anonymous BlockBackend for the empty
drive with blk_new() and attaches it to the device using
blk_attach_dev().

blk_new() returns the backend with refcnt == 1 (the creation reference
owned by the caller) and blk_attach_dev() takes an additional reference
for the device.  The creation reference was never released, so on
unplug blk_detach_dev() only drops the device's reference and the
BlockBackend is leaked.

Release the creation reference with blk_unref() right after
blk_attach_dev(), matching the idiom already used in set_drive_helper()
in hw/core/qdev-properties-system.c ("If we need to keep a reference,
blk_attach_dev() took it").

Signed-off-by: Mitsuru Kariya <Mitsuru.Kariya@xxxxxxxxxxxxxxx>
---
 hw/block/xen-block.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
index abf9df2eee..e5e14e47ee 100644
--- a/hw/block/xen-block.c
+++ b/hw/block/xen-block.c
@@ -763,6 +763,7 @@ static void xen_cdrom_realize(XenBlockDevice *blockdev, 
Error **errp)
 
         rc = blk_attach_dev(conf->blk, DEVICE(blockdev));
         assert(rc == 0);
+        blk_unref(conf->blk);
     }
 
     blockdev->info = VDISK_READONLY | VDISK_CDROM;
-- 
2.43.0