commit cc954f01e3c004aad081aa36736a17e842b80211
author Fam Zheng <famz@redhat.com> Fri Dec 15 16:04:45 2017 +0800
committer Kevin Wolf <kwolf@redhat.com> Fri Dec 22 15:03:41 2017 +0100
tree eef52c34f276f7c4f3593a6b23f44a97bc101741
parent 1ee24514aed34760fb2863d98bea3a1b705d9c9f

block: Open backing image in force share mode for size probe

Management tools create overlays of running guests with qemu-img:

  $ qemu-img create -b /image/in/use.qcow2 -f qcow2 /overlay/image.qcow2

but this doesn't work anymore due to image locking:

    qemu-img: /overlay/image.qcow2: Failed to get shared "write" lock
    Is another process using the image?
    Could not open backing image to determine size.
Use the force share option to allow this use case again.

Cc: qemu-stable@nongnu.org
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

diff --git a/block.c b/block.c
index db0be7e..dd90dca 100644
--- a/block.c
+++ b/block.c
@@ -4605,10 +4605,11 @@
         back_flags = flags;
         back_flags &= ~(BDRV_O_RDWR | BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING);
 
+        backing_options = qdict_new();
         if (backing_fmt) {
-            backing_options = qdict_new();
             qdict_put_str(backing_options, "driver", backing_fmt);
         }
+        qdict_put_bool(backing_options, BDRV_OPT_FORCE_SHARE, true);
 
         bs = bdrv_open(full_backing, NULL, backing_options, back_flags,
                        &local_err);