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Date: Thu, 21 Nov 2013 15:33:09 +0100
From: Stefan Hajnoczi <stefanha@gmail.com>
Message-ID: <20131121143309.GA17336@stefanha-thinkpad.redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] block: Enable BDRV_O_SNAPSHOT with
 driver-specific options
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To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com

On Tue, Nov 19, 2013 at 04:37:27PM +0100, Kevin Wolf wrote:
> @@ -1114,6 +1093,24 @@ int bdrv_open(BlockDriverState *bs, const char *filename, QDict *options,
>              goto fail;
>          }
>  
> +        /* Prepare a new options QDict for the temporary file, where user
> +         * options refer to the backing file */
> +        if (!options) {
> +            options = qdict_new();
> +        }

You can drop this because options is never NULL:

  options = qdict_clone_shallow(options);

  /* For snapshot=on, create a temporary qcow2 overlay */
  if (flags & BDRV_O_SNAPSHOT) {
      ...

> +        if (filename) {
> +            qdict_put(options, "file.filename", qstring_from_str(filename));
> +        }
> +        if (drv) {
> +            qdict_put(options, "driver", qstring_from_str(drv->format_name));
> +        }
> +
> +        snapshot_options = qdict_new();
> +        qdict_put(snapshot_options, "backing", options);
> +        qdict_flatten(snapshot_options);
> +
> +        options = snapshot_options;

One thing I'm not sure about after these operations have been performed:

  bs->options = options;
  ...
  if (flags & BDRV_O_SNAPSHOT) {
      ...

So bs->options does not reflect what we ended up with in the
BDRV_O_SNAPSHOT case.

But git grep -- '->options' shows no users of this field.  Therefore it
won't cause a problem yet.  But can you explain what's going on here?
Either we should keep bs->options up-to-date or we should drop the
field.

Stefan