block: add QAPI command to allow live backing file change

This allows a user to make a live change to the backing file recorded in
an open image.

The image file to modify can be specified 2 ways:

1) image filename
2) image node-name

Note: this does not cause the backing file itself to be reopened; it
merely changes the backing filename in the image file structure, and
in internal BDS structures.

It is the responsibility of the user to pass a filename string that
can be resolved when the image chain is reopened, and the filename
string is not validated.

A good analogy for this command is that it is a live version of
'qemu-img rebase -u', with respect to changing the backing file string.

[Jeff is offline so I respun this patch in his absence.  Dropped image
filename since using node-name is preferred and this is a new command.
No need to introduce the limitations of finding images by filename.
--Stefan]

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 6a697f1458..aa1252778c 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -680,6 +680,32 @@
   'data': 'BlockdevSnapshot' }
 
 ##
+# @change-backing-file
+#
+# Change the backing file in the image file metadata.  This does not
+# cause QEMU to reopen the image file to reparse the backing filename
+# (it may, however, perform a reopen to change permissions from
+# r/o -> r/w -> r/o, if needed). The new backing file string is written
+# into the image file metadata, and the QEMU internal strings are
+# updated.
+#
+# @image-node-name: The name of the block driver state node of the
+#                   image to modify.
+#
+# @device:          The name of the device that owns image-node-name.
+#
+# @backing-file:    The string to write as the backing file.  This
+#                   string is not validated, so care should be taken
+#                   when specifying the string or the image chain may
+#                   not be able to be reopened again.
+#
+# Since: 2.1
+##
+{ 'command': 'change-backing-file',
+  'data': { 'device': 'str', 'image-node-name': 'str',
+            'backing-file': 'str' } }
+
+##
 # @block-commit
 #
 # Live commit of data from overlay image nodes into backing nodes - i.e.,