qemu-img rebase: use empty string to rebase without backing file

This patch allows an empty filename to be passed as the new base image name
for qemu-img rebase to mean base the image on no backing file (i.e.
independent of any backing file). According to Eric Blake, qemu-img rebase
already supports this when '-u' is used; this adds support when -u is not
used.

Signed-off-by: Alex Bligh <alex@alex.org.uk>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/qemu-img.texi b/qemu-img.texi
index 8b05f2c..42ec392 100644
--- a/qemu-img.texi
+++ b/qemu-img.texi
@@ -148,7 +148,9 @@ Changes the backing file of an image. Only the formats @code{qcow2} and
 
 The backing file is changed to @var{backing_file} and (if the image format of
 @var{filename} supports this) the backing file format is changed to
-@var{backing_fmt}.
+@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
+string), then the image is rebased onto no backing file (i.e. it will exist
+independently of any backing file).
 
 There are two different modes in which @code{rebase} can operate:
 @table @option