[PATCH] Add --skip option similar to dd skip=N option

[Eric Seifert]

Hi Artem, thanks for the comments, I will look at re-implementing as you
suggest. One thing though is that since skip can be any number of bytes,
the start of where we write can be mid-block, so you may have one block
sent to be written where only part of it will be used. So I think at least
some change in the write path will be needed to split that first block up
if needed and adjust the seek offset.

-Eric

On Sat, Feb 28, 2015 at 6:05 AM, Artem Bityutskiy <dedekind1 at gmail.com>
wrote:

> Hi Eric,
>
> thanks for the patch. I know that this is what I suggested, but now,
> when I actually think about this, I am not sure if this is the right way
> to implement --skip.
>
> Let see. if my image is corrupted at the area that we are skipping, do
> we expect bmaptool to error out? I think now, we are not writing the
> corrupted part, we should proceed. Wit this implementation, the reading
> function will complain about incorrect SHA checksum and the operation
> will fail.
>
> Instead of adding the --skip implementation to the write path, we need
> to add it to the read path. That is, we do not even read and verify the
> part of the image that we are skipping.
>
> It occurs to me that the right place to add the code to is the
> _get_block_ranges() generator. Just make sure that it does not yield
> block ranges which are in the area that we are skipping.
>
> And I think if you do this, you'll patch will be a lot shorter.
>
> How does this sound to you?
>
> Also, would you please run the files you modify through 'pylint' to
> ensure the style and check against small mistakes?
>
> Thank you!
>
>
> On Fri, 2015-02-27 at 16:33 -0800, Eric Seifert wrote:
> > This will skip N bytes from the input file when writing the output file.
> > This is useful if splitting images (say by partition). As an example, if
> you
> > have a disk image sda.img and a bmap sda.img.bmap and you want to extract
> > the first partition sda1, you can run this command:
> >
> > bmaptool copy --skip 1048576 --bmap sda.img.bmap sda.img sda1.img
> >
> > Where 1048576 bytes is the offset into the disk where to first
> > partition is (2048 * 512 byte sectors).
>
> ... snip ...
>
>
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