[v10,03/10] qcow2_format.py: change Qcow2BitmapExt initialization method

There are two ways to initialize a class derived from Qcow2Struct:
1. Pass a block of binary data to the constructor.
2. Pass the file descriptor to allow reading the file from constructor.
Let's change the Qcow2BitmapExt initialization method from 1 to 2 to
support a scattered reading in the initialization chain.
The implementation comes with the patch that follows.

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
---
 tests/qemu-iotests/qcow2_format.py | 33 +++++++++++++++++++--------------
 1 file changed, 19 insertions(+), 14 deletions(-)

14.07.2020 00:36, Andrey Shinkevich wrote:
> There are two ways to initialize a class derived from Qcow2Struct:
> 1. Pass a block of binary data to the constructor.
> 2. Pass the file descriptor to allow reading the file from constructor.
> Let's change the Qcow2BitmapExt initialization method from 1 to 2 to
> support a scattered reading in the initialization chain.
> The implementation comes with the patch that follows.
> 
> Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> Signed-off-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
> ---
>   tests/qemu-iotests/qcow2_format.py | 33 +++++++++++++++++++--------------
>   1 file changed, 19 insertions(+), 14 deletions(-)
> 
> diff --git a/tests/qemu-iotests/qcow2_format.py b/tests/qemu-iotests/qcow2_format.py
> index 2f3681b..cbaffc4 100644
> --- a/tests/qemu-iotests/qcow2_format.py
> +++ b/tests/qemu-iotests/qcow2_format.py
> @@ -113,6 +113,11 @@ class Qcow2BitmapExt(Qcow2Struct):
>           ('u64', '{:#x}', 'bitmap_directory_offset')
>       )
>   
> +    def __init__(self, fd):
> +        super().__init__(fd=fd)
> +        pad = (struct.calcsize(self.fmt) + 7) & ~7

It's a size of structure rounded up to 8-bytes boundary. But after super init, we should alread be at the end of the structure and need to add only padding, not the whole structure rounded up. I think, correct code should be:

    tail = struct.calcsize(self.fmt) % 8
    if tail:
        fd.seek(8 - tail, 1)


> +        if pad:
> +            fd.seek(pad, 1)
>   
>   QCOW2_EXT_MAGIC_BITMAPS = 0x23852875
>   
> @@ -161,21 +166,21 @@ class QcowHeaderExtension(Qcow2Struct):
>           else:

Hmm, you parse data now only for this "else:" branch. Prior to this patch, it was parsed for "if fd is None:" branch as well, after the whole "if" statement. This is wrong.

>               assert all(v is None for v in (magic, length, data))
>               super().__init__(fd=fd)
> -            padded = (self.length + 7) & ~7
> -            self.data = fd.read(padded)
> -            assert self.data is not None
> -
> -        data_str = self.data[:self.length]
> -        if all(c in string.printable.encode('ascii') for c in data_str):
> -            data_str = f"'{ data_str.decode('ascii') }'"
> -        else:
> -            data_str = '<binary>'
> -        self.data_str = data_str
> +            if self.magic == QCOW2_EXT_MAGIC_BITMAPS:
> +                self.obj = Qcow2BitmapExt(fd=fd)
> +            else:
> +                padded = (self.length + 7) & ~7
> +                self.data = fd.read(padded)
> +                assert self.data is not None
> +                self.obj = None
> +                data_str = self.data[:self.length]
> +                if all(c in string.printable.encode(
> +                    'ascii') for c in data_str):
> +                    data_str = f"'{ data_str.decode('ascii') }'"
> +                else:
> +                    data_str = '<binary>'
> +                self.data_str = data_str
>   
> -        if self.magic == QCOW2_EXT_MAGIC_BITMAPS:
> -            self.obj = Qcow2BitmapExt(data=self.data)
> -        else:
> -            self.obj = None
>   
>       def dump(self):
>           super().dump()
>