block: Deprecate "backing": ""

We have a clear replacement, so let's deprecate it.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-Id: <20180224154033.29559-8-mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/block.c b/block.c
index ab77ba0233..a2caadf0a0 100644
--- a/block.c
+++ b/block.c
@@ -2649,6 +2649,10 @@ static BlockDriverState *bdrv_open_inherit(const char *filename,
     if (qobject_to(QNull, qdict_get(options, "backing")) != NULL ||
         (backing && *backing == '\0'))
     {
+        if (backing) {
+            warn_report("Use of \"backing\": \"\" is deprecated; "
+                        "use \"backing\": null instead");
+        }
         flags |= BDRV_O_NO_BACKING;
         qdict_del(options, "backing");
     }