block: Deprecate "backing": ""

We have a clear replacement, so let's deprecate it.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NAlberto Garcia <berto@igalia.com>
Message-Id: <20180224154033.29559-8-mreitz@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

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");
     }

qapi/block-core.json

@@ -1174,7 +1174,7 @@
 # @overlay: reference to the existing block device that will become
 #           the overlay of @node, as part of creating the snapshot.
 #           It must not have a current backing file (this can be
-#           achieved by passing "backing": "" to blockdev-add).
+#           achieved by passing "backing": null to blockdev-add).
 #
 # Since: 2.5
 ##
@@ -1347,7 +1347,7 @@
 #                     "node-name": "node1534",
 #                     "file": { "driver": "file",
 #                               "filename": "hd1.qcow2" },
-#                     "backing": "" } }
+#                     "backing": null } }
 #
 # <- { "return": {} }
 #

qemu-doc.texi

@@ -2789,6 +2789,13 @@ support page sizes < 4096 any longer.
 The ``xlnx-ep108'' machine has been replaced by the ``xlnx-zcu102'' machine.
 The ``xlnx-zcu102'' machine has the same features and capabilites in QEMU.
 
+@section Block device options
+
+@subsection "backing": "" (since 2.12.0)
+
+In order to prevent QEMU from automatically opening an image's backing
+chain, use ``"backing": null'' instead.
+
 @node License
 @appendix License
 

qemu-options.hx

@@ -743,8 +743,8 @@ Reference to or definition of the data source block driver node
 
 @item backing
 Reference to or definition of the backing file block device (default is taken
-from the image file). It is allowed to pass an empty string here in order to
-disable the default backing file.
+from the image file). It is allowed to pass @code{null} here in order to disable
+the default backing file.
 
 @item lazy-refcounts
 Whether to enable the lazy refcounts feature (on/off; default is taken from the