Re: [Qemu-devel] [PATCH v5 2/4] block.c: adding bdrv_delete_file

[Kevin Wolf <kwolf@redhat.com>]

Am 29.08.2019 um 04:07 hat John Snow geschrieben:
> 
> 
> On 8/7/19 10:21 AM, Daniel Henrique Barboza wrote:
> > Using the new 'bdrv_co_delete_file' interface, bdrv_delete_file
> > can be used in a way similar of the existing bdrv_create_file to
> > to clean up a created file.
> > 
> > The logic is also similar to what is already done in bdrv_create_file:
> > a qemu_coroutine is created if needed, a specialized function
> > bdrv_delete_co_entry is used to call the bdrv_co_delete_file
> > co-routine of the driver, if the driver implements it.
> > 
> > Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
> > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > ---
> >  block.c               | 77 +++++++++++++++++++++++++++++++++++++++++++
> >  include/block/block.h |  1 +
> >  2 files changed, 78 insertions(+)
> > 
> > diff --git a/block.c b/block.c
> > index cbd8da5f3b..1e20250627 100644
> > --- a/block.c
> > +++ b/block.c
> > @@ -547,6 +547,83 @@ int bdrv_create_file(const char *filename, QemuOpts *opts, Error **errp)
> >      return ret;
> >  }
> >  
> > +typedef struct DeleteCo {
> > +    BlockDriver *drv;
> > +    BlockDriverState *bs;
> > +    int ret;
> > +    Error *err;
> > +} DeleteCo;
> > +
> > +static void coroutine_fn bdrv_delete_co_entry(void *opaque)
> > +{
> > +    Error *local_err = NULL;
> > +    DeleteCo *dco = opaque;
> > +
> > +    assert(dco->bs);
> > +
> > +    dco->ret = dco->drv->bdrv_co_delete_file(dco->bs, &local_err);
> > +    error_propagate(&dco->err, local_err);
> > +}
> > +
> > +int bdrv_delete_file(const char *filename, Error **errp)
> > +{
> > +    BlockDriver *drv = bdrv_find_protocol(filename, true, NULL);
> > +    BlockDriverState *bs = bdrv_open(filename, NULL, NULL,
> > +                                     BDRV_O_RDWR | BDRV_O_PROTOCOL, NULL);
> > +    DeleteCo dco = {
> > +        .drv = drv,
> > +        .bs = bs,
> > +        .ret = NOT_DONE,
> > +        .err = NULL,
> > +    };
> > +    Coroutine *co;
> > +    int ret;
> > +
> > +    if (!drv) {
> > +        error_setg(errp, "File '%s' has unknown format", filename);
> > +        ret = -ENOENT;
> > +        goto out;
> > +    }
> > +
> 
> I was going to say that ENOENT is a weird error here, but I see it used
> for !drv a few other places in block.c too, alongside EINVAL and
> ENOMEDIUM. ENOMEDIUM loks like the most popular.
> 
> > +    if (!drv->bdrv_co_delete_file) {
> > +        error_setg(errp, "Driver '%s' does not support image delete",
> > +                   drv->format_name);
> > +        ret = -ENOTSUP;
> > +        goto out;
> > +    }
> > +
> > +    if (!bs) {
> > +        error_setg(errp, "Could not open image '%s' for erasing",
> > +                   filename);
> > +        ret = 1;
> 
> Please keep all errors negative (or at least consistent within a function).
> 
> 
> I'm also wondering if we want a version of delete that doesn't try to
> open a file directly -- i.e. a version that exists like this:
> 
> bdrv_co_delete_file(BlockDriverState *bs, Error **errp);
> 
> That simply dispatches based on bs->drv to the correct routine.
> 
> Then, you are free to have bdrv_delete_file handle the open (and let the
> opening figure out what driver it needs), and just hand off the bds to
> bdrv_co_delete_file.
> 
> I'm not the authority for block.c, though, so maaaybe I'm giving you bad
> advice here. Kevin's away on PTO for a bit and gave you advice most
> recently, so I might try to gently ask him for more feedback next week.

Yes, this was definitely the idea I had in mind when I suggested that
bdrv_co_delete_file() should take a BDS.

And I think the callers that want to call this function (for failures
during image creation) all already have a BDS pointer, so nobody will
actually need the additional open.

const char *filename only works for the local filesystem (and even then
I think not for all filenames) and some URLs, so this is not what we
want to have in a public interface to identify an image file.

Kevin