[Libguestfs] [nbdkit PATCH 3/4] blocksize-policy: Round extents to match minimum alignment

[Eric Blake via Libguestfs]

Since a client is very likely to make further I/O calls at offsets
returned in a block status call (for example, 'nbdinfo --map
--totals'), and the blocksize-policy filter can set a minimum
alignment larger than what the plugin does by default, it makes sense
to force extents to that alignment.

Signed-off-by: Eric Blake <ebl...@redhat.com>
---
 tests/Makefile.am                 |  2 ++
 filters/blocksize-policy/policy.c | 15 ++++++++++++++-
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/tests/Makefile.am b/tests/Makefile.am
index 63730821..242a03e4 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1656,11 +1656,13 @@ TESTS += \
        test-blocksize-policy.sh \
        test-blocksize-error-policy.sh \
        test-blocksize-write-disconnect.sh \
+       test-blocksize-policy-extents.sh \
        $(NULL)
 EXTRA_DIST += \
        test-blocksize-policy.sh \
        test-blocksize-error-policy.sh \
        test-blocksize-write-disconnect.sh \
+       test-blocksize-policy-extents.sh \
        $(NULL)

 if HAVE_BZLIB
diff --git a/filters/blocksize-policy/policy.c 
b/filters/blocksize-policy/policy.c
index 51bea91e..d656c956 100644
--- a/filters/blocksize-policy/policy.c
+++ b/filters/blocksize-policy/policy.c
@@ -38,6 +38,7 @@
 #include <inttypes.h>
 #include <string.h>
 #include <errno.h>
+#include <assert.h>

 #include <nbdkit-filter.h>

@@ -373,10 +374,22 @@ policy_extents (nbdkit_next *next,
                 void *handle, uint32_t count, uint64_t offset, uint32_t flags,
                 struct nbdkit_extents *extents, int *err)
 {
+  uint32_t minimum, preferred, maximum;
+  int res;
+
   if (check_policy (next, handle, "extents", false, count, offset, err) == -1)
     return -1;

-  return next->extents (next, count, offset, flags, extents, err);
+  /* Munge the plugin's response to values that match the alignment we
+   * are enforcing; that way, a client that performs later operations
+   * at boundaries determined by the block status result will still be
+   * aligned.  Since check_policy succeeded, policy_block_size will too.
+   */
+  res = policy_block_size (next, handle,
+                           &minimum, &preferred, &maximum);
+  assert (res == 0);
+  return nbdkit_extents_aligned (next, count, offset, flags,
+                                 minimum ?: 1, extents, err);
 }

 static struct nbdkit_filter filter = {
-- 
2.49.0
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