Re: [Qemu-devel] [PATCH 01/15] block: Fail gracefully when using a format driver on protocol level

[Kevin Wolf <kwolf@redhat.com>]

Am 13.04.2013 um 00:50 hat Eric Blake geschrieben:
> On 04/12/2013 02:47 PM, Kevin Wolf wrote:
> > Specifying the wrong driver could fail an assertion:
> > 
> > $ qemu-system-x86_64 -drive file.driver=qcow2,file=x
> > qemu-system-x86_64: block.c:721: bdrv_open_common: Assertion `file !=
> > ((void *)0)' failed.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  block.c                    |  7 +++++++
> >  tests/qemu-iotests/051     |  7 +++++++
> >  tests/qemu-iotests/051.out | 10 ++++++++++
> >  3 files changed, 24 insertions(+)
> > 
> > diff --git a/block.c b/block.c
> > index 602d8a4..f23bdcc 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -718,6 +718,13 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
> >          assert(drv->bdrv_parse_filename || filename != NULL);
> >          ret = drv->bdrv_file_open(bs, filename, options, open_flags);
> >      } else {
> > +        if (file == NULL) {
> > +            qerror_report(ERROR_CLASS_GENERIC_ERROR, "The '%s' block driver is "
> > +                          "not suitable for the bottom level",
> > +                          drv->format_name);
> > +            ret = -EINVAL;
> > +            goto free_and_fail;
> > +        }
> >          assert(file != NULL);
> 
> Is it really necessary to leave the assert in place, now that you have a
> check for NULL followed by unconditional goto?

Not really. I'll send a cleanup.

> Just reading that error message, I'm not quite sure what you meant by
> "not suitable for the bottom level".  I guess the intent is that
> file.driver specifies the protocol, and that both raw and qcow2 are
> formats possible on the file protocol, rather than qcow2 being a file
> protocol itself.
> 
> > +Testing: -drive file=TEST_DIR/t.qcow2,file.driver=qcow2
> > +qemu: -drive file=TEST_DIR/t.qcow2,file.driver=qcow2: The 'qcow2' block driver is not suitable for the bottom level
> > +qemu: -drive file=TEST_DIR/t.qcow2,file.driver=qcow2: could not open disk image TEST_DIR/t.qcow2: Invalid argument
> 
> Maybe a better error message would be this?
> 
> Attempt to use format driver 'qcow2' where a protocol driver was expected

I was trying to avoid the format/protocol discussion this time by
choosing a different phrasing in the first place. Seems this approach
doesn't work better either...

Problems with this terminology start when you have more than two drivers
involved. Currently, this is only with the more exotic cases like when
you have qcow2 -> blkdebug -> file, but if we follow through with our
-blockdev plans, arbitrary stacking of BlockDriverStates will become
more common.

Markus likes to describe it as a graph of nodes of the same kind, where
the leaves (i.e. the protocols) just happen to not have a .file option.

Kevin