Re: [Qemu-devel] [PATCH v2 10/11] block: add backing-file option to block-stream

[Eric Blake]

On 05/27/2014 08:28 AM, Jeff Cody wrote:
> 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 job.
> 
> 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-stream api, the user is able to change
> the backing file of the active layer as part of the block-stream
> operation.
> 
> This allows the change to be 'safe', in the sense that if the attempt
> to write the active image metadata fails, then the block-stream
> operation returns failure, without disrupting the guest.
> 
> 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.
> 
> Signed-off-by: Jeff Cody <address@hidden>
> ---
>  block/stream.c            | 13 ++++++++-----
>  blockdev.c                | 12 +++++++++++-
>  hmp-commands.hx           |  2 +-
>  hmp.c                     |  2 ++
>  include/block/block_int.h |  2 +-
>  qapi-schema.json          | 18 +++++++++++++++++-
>  qmp-commands.hx           |  2 +-
>  7 files changed, 41 insertions(+), 10 deletions(-)
> 

> @@ -220,7 +221,7 @@ void stream_start(BlockDriverState *bs, BlockDriverState 
> *base,
>                    const char *base_id, int64_t speed,
>                    BlockdevOnError on_error,
>                    BlockDriverCompletionFunc *cb,
> -                  void *opaque, Error **errp)
> +                  void *opaque, const char *backing_file_str, Error **errp)
>  {

Umm, aren't the 'base_id' and 'backing_file_str' arguments redundant
now?  You only need one, not both, because there is only one backing
string to be written (or cleared) into the active file. [1]

> @@ -237,8 +238,10 @@ void stream_start(BlockDriverState *bs, BlockDriverState 
> *base,
>      }
>  
>      s->base = base;
> -    if (base_id) {
> -        pstrcpy(s->backing_file_id, sizeof(s->backing_file_id), base_id);
> +    if (backing_file_str) {
> +        s->backing_file_str = g_strdup(backing_file_str);
> +    } else {
> +        s->backing_file_str = g_strdup(base_id);

g_strdup(NULL) is safely NULL, correct?  (You hit this case when base_id
is NULL, which happens when base is NULL).

> @@ -1941,8 +1942,17 @@ void qmp_block_stream(bool has_device, const char 
> *device,
>          return;
>      }
>  
> +    /* if we are streaming from the bottommost base image, then specifying
> +     * a backing file does not make sense, and is an error */

Misleading (back to the complaint in 9/11) - omitting base is different
from using the bottommost base image as base.  I'd word that more like:

If we are streaming the entire chain, then the result will have no
backing file and specifying a backing name is an error

> +    if (base_bs == NULL && has_backing_file) {
> +        error_setg(errp, "backing file specified, but streaming from "
> +                         "bottommost base image");
> +        return;
> +    }

The 'if' condition is correct and necessary, but the error message could
use better wording; maybe:

backing file specified but no base image supplied

>      stream_start(bs, base_bs, base_name, has_speed ? speed : 0,
> -                 on_error, block_job_cb, bs, &local_err);
> +                 on_error, block_job_cb, bs,
> +                 has_backing_file ? backing_file : NULL, &local_err);

[1] Again, how do 'base_name' and 'backing_file' differ at this point in
the game? Isn't it just simpler to do:
 has_backing_file ? backing_file : base_name
and use a single parameter?

> +++ b/qapi-schema.json
> @@ -2611,6 +2611,21 @@
>  # @base-node-name: #optional the block driver state node name of the
>  #                            common backing file.  (Since 2.1)
>  #
> +# @backing-file: #optional The backing file string to write into the active
> +#                          layer. This filename is not validated.
> +#
> +#                          If a pathname string is such that it cannot be
> +#                          resolved be QEMU, that means that subsequent QMP 
> or

Copy-and-paste strikes again :)
s/be/by/

> +++ b/qmp-commands.hx
> @@ -979,7 +979,7 @@ EQMP
>  
>      {
>          .name       = "block-stream",
> -        .args_type  = 
> "device:B?,base:s?,base-node-name:s?,speed:o?,on-error:s?",
> +        .args_type  = 
> "device:B?,base:s?,base-node-name:s?,speed:o?,backing-file:s?,on-error:s?",
>          .mhandler.cmd_new = qmp_marshal_input_block_stream,
>      },

Missing documentation of backing-file?  Oh, the entire block-stream call
is missing documentation, when compared to the block-commit call.  Oh
well, I can't fault you for that, although it might be nice to rectify
while we are improving it.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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