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

[John Snow]

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 <address@hidden>
> Signed-off-by: Daniel Henrique Barboza <address@hidden>
> ---
>  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.

> +        goto out;
> +    }
> +
> +    if (qemu_in_coroutine()) {
> +        /* Fast-path if already in coroutine context */
> +        bdrv_delete_co_entry(&dco);
> +    } else {
> +        co = qemu_coroutine_create(bdrv_delete_co_entry, &dco);
> +        qemu_coroutine_enter(co);
> +        while (dco.ret == NOT_DONE) {
> +            aio_poll(qemu_get_aio_context(), true);
> +        }
> +    }
> +
> +    ret = dco.ret;
> +    if (ret < 0) {
> +        if (dco.err) {
> +            error_propagate(errp, dco.err);
> +        } else {
> +            error_setg_errno(errp, -ret, "Could not delete image");
> +        }
> +    }
> +
> +out:
> +    bdrv_unref(bs);
> +    return ret;
> +}
> +
>  /**
>   * Try to get @bs's logical and physical block size.
>   * On success, store them in @bsz struct and return 0.
> diff --git a/include/block/block.h b/include/block/block.h
> index 50a07c1c33..5e83532364 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -369,6 +369,7 @@ bool bdrv_is_backing_chain_frozen(BlockDriverState *bs, 
> BlockDriverState *base,
>  int bdrv_freeze_backing_chain(BlockDriverState *bs, BlockDriverState *base,
>                                Error **errp);
>  void bdrv_unfreeze_backing_chain(BlockDriverState *bs, BlockDriverState 
> *base);
> +int bdrv_delete_file(const char *filename, Error **errp);
>  
>  
>  typedef struct BdrvCheckResult {
>