Re: [Qemu-devel] [PATCH V2] block: Add BlockDriver.bdrv_check_ext_snapshot.

[Jeff Cody <jcody@redhat.com>]

On Thu, Sep 26, 2013 at 04:33:49PM +0200, Benoît Canet wrote:
> This field is used by blkverify to disable external snapshots creation.
> I will also be used by block filters like quorum to disable external snapshots
> creation.
> 
> Signed-off-by: Benoit Canet <benoit@irqsave.net>
> ---
>  block.c                   | 14 ++++++++++++++
>  block/blkverify.c         |  2 ++
>  blockdev.c                |  5 +++++
>  include/block/block.h     |  7 +++++++
>  include/block/block_int.h |  8 ++++++++
>  5 files changed, 36 insertions(+)
> 
> diff --git a/block.c b/block.c
> index 4833b37..4da6fd9 100644
> --- a/block.c
> +++ b/block.c
> @@ -4632,3 +4632,17 @@ int bdrv_amend_options(BlockDriverState *bs, QEMUOptionParameter *options)
>      }
>      return bs->drv->bdrv_amend_options(bs, options);
>  }
> +
> +bool bdrv_check_ext_snapshot(BlockDriverState *bs)
> +{
> +    /* external snashots are enabled by defaults */
> +    if (!bs->drv->bdrv_check_ext_snapshot) {
> +        return true;
> +    }
> +    return bs->drv->bdrv_check_ext_snapshot(bs);
> +}
> +
> +bool bdrv_forbid_ext_snapshot(BlockDriverState *bs)
> +{
> +    return false;
> +}

The only problem I have with this now, is that
"bdrv_forbid_ext_snapshot()" returns false, to indicate that "forbid
ext snapshot" is true.  Looking at the function above, I would come to
the opposite conclusion as to what it does.

I understand why - you want the function name assigned to
.bdrv_check_ext_snapshot to reflect the action, but then that causes
the boolean return to be misleading.  Maybe returning an enum would be
more natural?

I apologize if this seems too pedantic.  :)

Thanks,
Jeff
> diff --git a/block/blkverify.c b/block/blkverify.c
> index 2077d8a..c548923 100644
> --- a/block/blkverify.c
> +++ b/block/blkverify.c
> @@ -313,6 +313,8 @@ static BlockDriver bdrv_blkverify = {
>      .bdrv_aio_readv         = blkverify_aio_readv,
>      .bdrv_aio_writev        = blkverify_aio_writev,
>      .bdrv_aio_flush         = blkverify_aio_flush,
> +
> +    .bdrv_check_ext_snapshot = bdrv_forbid_ext_snapshot,
>  };
>  
>  static void bdrv_blkverify_init(void)
> diff --git a/blockdev.c b/blockdev.c
> index 8aa66a9..5c16f1b 100644
> --- a/blockdev.c
> +++ b/blockdev.c
> @@ -1131,6 +1131,11 @@ static void external_snapshot_prepare(BlkTransactionState *common,
>          }
>      }
>  
> +    if (!bdrv_check_ext_snapshot(state->old_bs)) {
> +        error_set(errp, QERR_FEATURE_DISABLED, "snapshot");
> +        return;
> +    }
> +
>      flags = state->old_bs->open_flags;
>  
>      /* create new image w/backing file */
> diff --git a/include/block/block.h b/include/block/block.h
> index f808550..df19610 100644
> --- a/include/block/block.h
> +++ b/include/block/block.h
> @@ -244,6 +244,13 @@ int bdrv_check(BlockDriverState *bs, BdrvCheckResult *res, BdrvCheckMode fix);
>  
>  int bdrv_amend_options(BlockDriverState *bs_new, QEMUOptionParameter *options);
>  
> +/* external snapshots */
> +
> +/* return true if external snapshot is allowed, false if not */
> +bool bdrv_check_ext_snapshot(BlockDriverState *bs);
> +/* helper used to forbid external snapshots like in blkverify */
> +bool bdrv_forbid_ext_snapshot(BlockDriverState *bs);
> +
>  /* async block I/O */
>  typedef void BlockDriverDirtyHandler(BlockDriverState *bs, int64_t sector,
>                                       int sector_num);
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index 211087a..cb92355 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -67,6 +67,14 @@ typedef struct BdrvTrackedRequest {
>  struct BlockDriver {
>      const char *format_name;
>      int instance_size;
> +
> +    /* if not defined external snapshots are allowed
> +     * if return true external snapshots are allowed
> +     * if return false external snapshots are not allowed
> +     * future block filters will query their children to build the response
> +     */
> +    bool (*bdrv_check_ext_snapshot)(BlockDriverState *bs);
> +
>      int (*bdrv_probe)(const uint8_t *buf, int buf_size, const char *filename);
>      int (*bdrv_probe_device)(const char *filename);
>  
> -- 
> 1.8.1.2
> 
>