[Libguestfs] [nbdkit PATCH] swab: Implement .block_size callback

[Eric Blake]

We were previously enforcing minimum block size with EINVAL for
too-small requests.  Advertise this to the client.
---
 filters/swab/nbdkit-swab-filter.pod |  6 ++++++
 filters/swab/swab.c                 | 24 +++++++++++++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/filters/swab/nbdkit-swab-filter.pod b/filters/swab/nbdkit-swab-filter.pod
index f8500150..030a0852 100644
--- a/filters/swab/nbdkit-swab-filter.pod
+++ b/filters/swab/nbdkit-swab-filter.pod
@@ -34,6 +34,11 @@ the last few bytes, combine this filter with
 L<nbdkit-truncate-filter(1)>; fortunately, sector-based disk images
 are already suitably sized.

+Note that this filter fails operations that are not aligned to the
+swab-bits boundaries; if you need byte-level access, apply the
+L<nbdkit-blocksize-filter(1)> before this one, to get
+read-modify-write access to individual bytes.
+
 =head1 PARAMETERS

 =over 4
@@ -90,6 +95,7 @@ L<nbdkit(1)>,
 L<nbdkit-file-plugin(1)>,
 L<nbdkit-pattern-plugin(1)>,
 L<nbdkit-filter(3)>,
+L<nbdkit-blocksize-filter(1)>.
 L<nbdkit-truncate-filter(1)>.

 =head1 AUTHORS
diff --git a/filters/swab/swab.c b/filters/swab/swab.c
index 68776eee..6e8dc981 100644
--- a/filters/swab/swab.c
+++ b/filters/swab/swab.c
@@ -44,6 +44,7 @@
 #include "byte-swapping.h"
 #include "isaligned.h"
 #include "cleanup.h"
+#include "minmax.h"
 #include "rounding.h"

 /* Can only be 8 (filter disabled), 16, 32 or 64. */
@@ -85,8 +86,28 @@ swab_get_size (nbdkit_next *next,
   return ROUND_DOWN (size, bits/8);
 }

+/* Block size constraints. */
+static int
+swab_block_size (nbdkit_next *next, void *handle,
+                 uint32_t *minimum, uint32_t *preferred, uint32_t *maximum)
+{
+  if (next->block_size (next, minimum, preferred, maximum) == -1)
+    return -1;
+
+  if (*minimum == 0) {         /* No constraints set by the plugin. */
+    *minimum = bits/8;
+    *preferred = 512;
+    *maximum = 0xffffffff;
+  }
+  else {
+    *minimum = MAX (*minimum, bits/8);
+  }
+
+  return 0;
+}
+
 /* The request must be aligned.
- * XXX We could lift this restriction with more work.
+ * If you want finer alignment, use the blocksize filter.
  */
 static bool
 is_aligned (uint32_t count, uint64_t offset, int *err)
@@ -220,6 +241,7 @@ static struct nbdkit_filter filter = {
   .config            = swab_config,
   .config_help       = swab_config_help,
   .get_size          = swab_get_size,
+  .block_size        = swab_block_size,
   .pread             = swab_pread,
   .pwrite            = swab_pwrite,
   .trim              = swab_trim,
-- 
2.35.1