[Libguestfs] [PATCH nbdkit] server: Implement extents/can_extents calls for plugins and filters.

[Martin Kletzander]

On Tue, Mar 12, 2019 at 11:20:25AM +0000, Richard W.M. Jones wrote:
>This pair of calls allows plugins to describe which extents in the
>virtual disk are allocated, holes or zeroes.
>---
> docs/nbdkit-filter.pod  | 22 +++++++++++
> docs/nbdkit-plugin.pod  | 86 +++++++++++++++++++++++++++++++++++++++++
> include/nbdkit-common.h | 14 ++++++-
> include/nbdkit-filter.h | 12 +++++-
> include/nbdkit-plugin.h |  6 ++-
> server/internal.h       |  7 +++-
> server/filters.c        | 61 +++++++++++++++++++++++++++++
> server/plugins.c        | 72 +++++++++++++++++++++++++++++++++-
> 8 files changed, 275 insertions(+), 5 deletions(-)
>

[...]

>diff --git a/docs/nbdkit-plugin.pod b/docs/nbdkit-plugin.pod
>index 47545f3..9cc62b6 100644
>--- a/docs/nbdkit-plugin.pod
>+++ b/docs/nbdkit-plugin.pod

[...]

>@@ -717,6 +731,78 @@ If there is an error, C<.zero> should call C<nbdkit_error> with an
> error message, and C<nbdkit_set_error> to record an appropriate error
> (unless C<errno> is sufficient), then return C<-1>.
>
>+=head2 C<.extents>
>+
>+ int zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags,

s/zero/extents/

[...]

>diff --git a/server/filters.c b/server/filters.c
>index 5b7abc4..d6cc00f 100644
>--- a/server/filters.c
>+++ b/server/filters.c

[...]

>@@ -646,6 +680,31 @@ filter_zero (struct backend *b, struct connection *conn,
>                                   count, offset, flags, err);
> }
>
>+static int
>+filter_extents (struct backend *b, struct connection *conn,
>+                uint32_t count, uint64_t offset, uint32_t flags,
>+                size_t *nr_extents, struct nbdkit_extent **extents,
>+                int *err)
>+{
>+  struct backend_filter *f = container_of (b, struct backend_filter, backend);
>+  void *handle = connection_get_handle (conn, f->backend.i);
>+  struct b_conn nxdata = { .b = f->backend.next, .conn = conn };
>+
>+  assert (!(flags & ~NBDKIT_FLAG_REQ_ONE));
>+
>+  debug ("%s: extents count=%" PRIu32 " offset=%" PRIu64 " flags=0x%" PRIx32,
>+         f->name, count, offset, flags);
>+
>+  if (f->filter.extents)
>+    return f->filter.extents (&next_ops, &nxdata, handle,
>+                              count, offset, flags,
>+                              nr_extents, extents, err);
>+  else
>+    return f->backend.next->extents (f->backend.next, conn,
>+                                     count, offset, flags,
>+                                     nr_extents, extents, err);

Do I understand it correctly that if a filter does not support extents, then its
function will only be applied on the allocated blocks?  It makes sense, but I
feel like it would be nice to have that mentioned somewhere.

Other than that it looks OK to me, I'll try to cook up a test filter to test
this, but mainly as a fun exercise for myself.

Have a nice day,
Martin
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