[Libguestfs] [nbdkit PATCH] Update filters to support FUA flags.

[Eric Blake]

On 01/19/2018 10:23 AM, Eric Blake wrote:
> From: "Richard W.M. Jones" <rjones at redhat.com>

Whoops, that's supposed to be attributed to me as author if we keep this
patch separate from your work (I was still attributing it to you when I
had squashed it into your earlier patches, so this was a rebase artifact).

> 
> This patch may be worth squashing?
> ---
>  docs/nbdkit-filter.pod  | 65 +++++++++++++++++++++++++++++++++++++++++++------
>  include/nbdkit-filter.h | 29 ++++++++++++----------
>  src/filters.c           | 55 ++++++++++++++++++++---------------------
>  3 files changed, 101 insertions(+), 48 deletions(-)
> 

> @@ -333,11 +342,21 @@ message B<and> set C<errno>, then return C<-1>.
> 
>   int (*pwrite) (struct nbdkit_next_ops *next_ops, void *nxdata,
>                  void *handle,
> -                const void *buf, uint32_t count, uint64_t offset);
> +                const void *buf, uint32_t count, uint64_t offset,
> +                uint32_t flags);
> 
>  This intercepts the plugin C<.pwrite> method and can be used to modify
>  data written by the plugin.
> 
> +At this time, flags may include C<NBDKIT_FLAG_FUA> on input based on
> +the result of C<.can_flush>.  In turn, the filter may only pass
> +C<NBDKIT_FLAG_FUA> on to C<next_ops->pwrite> if C<next_ops->can_flush>
> +returned true.
> +
> +This function will not be called if C<.can_write> returned false; in
> +turn, the filter should not call C<next_ops->pwrite> if C<next_ops->can_write>
> +did not return true.

I'm wondering if we're missing documentation here (and/or in the plugins
document) that if .can_write returns true, the plugin must supply a
.pwrite; likewise for .can_trim implying a .trim, and .can_flush
implying a .flush.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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