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

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 8/28/19 11:07 PM, John Snow wrote:
>
> 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.

Didn't spend too much time thinking about it. I copied the same behavior 
from
bdrv_create_file:

---------

int bdrv_create_file(const char *filename, QemuOpts *opts, Error **errp)
{
     (...)

     drv = bdrv_find_protocol(filename, true, errp);
     if (drv == NULL) {
         return -ENOENT;
     }
-----

I can change to ENOMEDIUM if it's indeed more informative than ENOENT.


>
>> +    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).

Got it. I'll fix it in the re-spin.


>
>
> 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.

I appreciate. I'm not acquainted with the block code at all - I'm playing
by ear since the first version. Any tip is appreciated :)


Thanks,


DHB

>
>> +        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 {
>>