block: extend block-commit to accept a string for the backing file

On some image chains, QEMU may not always be able to resolve the
filenames properly, when updating the backing file of an image
after a block commit.

For instance, certain relative pathnames may fail, or drives may
have been specified originally by file descriptor (e.g. /dev/fd/???),
or a relative protocol pathname may have been used.

In these instances, QEMU may lack the information to be able to make
the correct choice, but the user or management layer most likely does
have that knowledge.

With this extension to the block-commit api, the user is able to change
the backing file of the overlay image as part of the block-commit
operation.

This allows the change to be 'safe', in the sense that if the attempt
to write the overlay image metadata fails, then the block-commit
operation returns failure, without disrupting the guest.

If the commit top is the active layer, then specifying the backing
file string will be treated as an error (there is no overlay image
to modify in that case).

If a backing file string is not specified in the command, the backing
file string to use is determined in the same manner as it was
previously.

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

1 files changed, 18 insertions, 1 deletions

diff --git a/qmp-commands.hx b/qmp-commands.hx
index 8534948..a3f932c 100644
--- a/qmp-commands.hx
+++ b/qmp-commands.hx
@@ -985,7 +985,7 @@ EQMP
 
     {
         .name       = "block-commit",
-        .args_type  = "device:B,base:s?,top:s?,speed:o?",
+        .args_type  = "device:B,base:s?,top:s?,backing-file:s?,speed:o?",
         .mhandler.cmd_new = qmp_marshal_input_block_commit,
     },
 
@@ -1006,6 +1006,23 @@ Arguments:
           which contains the topmost data to be committed down. If
           not specified, this is the active layer. (json-string, optional)
 
+- backing-file:     The backing file string to write into the overlay
+                    image of 'top'.  If 'top' is the active layer,
+                    specifying a backing file string is an error. This
+                    filename is not validated.
+
+                    If a pathname string is such that it cannot be
+                    resolved by QEMU, that means that subsequent QMP or
+                    HMP commands must use node-names for the image in
+                    question, as filename lookup methods will fail.
+
+                    If not specified, QEMU will automatically determine
+                    the backing file string to use, or error out if
+                    there is no obvious choice. Care should be taken
+                    when specifying the string, to specify a valid
+                    filename or protocol.
+                    (json-string, optional) (Since 2.1)
+
           If top == base, that is an error.
           If top == active, the job will not be completed by itself,
           user needs to complete the job with the block-job-complete