[Libguestfs] [PATCH nbdkit] blocksize: Export block size constraints

[Richard W.M. Jones]

This filter is a little unusual because it allows clients to send a
wider range of request sizes than the underlying plugin allows.
Therefore we advertise the widest possible minimum and maximum block
size to clients.

We still need to pick a suitable preferred block size assuming the
plugin itself doesn't advertise one.
---
 filters/blocksize/blocksize.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/filters/blocksize/blocksize.c b/filters/blocksize/blocksize.c
index a6fa00cb..46e759b0 100644
--- a/filters/blocksize/blocksize.c
+++ b/filters/blocksize/blocksize.c
@@ -156,6 +156,29 @@ blocksize_get_size (nbdkit_next *next,
   return ROUND_DOWN (size, minblock);
 }
 
+/* Block size constraints.
+ *
+ * Note that the purpose of this filter is to allow clients to send a
+ * wider range of request sizes than the underlying plugin permits,
+ * and therefore this callback advertises the full availability of the
+ * filter, likely widening the constraints from the plugin.
+ */
+static int
+blocksize_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 (*preferred == 0)
+    *preferred = MAX (4096, minblock);
+
+  *minimum = 1;
+  *maximum = 0xffffffff;
+
+  return 0;
+}
+
 static int
 blocksize_pread (nbdkit_next *next,
                  void *handle, void *b, uint32_t count, uint64_t offs,
@@ -432,6 +455,7 @@ static struct nbdkit_filter filter = {
   .config_complete   = blocksize_config_complete,
   .config_help       = blocksize_config_help,
   .get_size          = blocksize_get_size,
+  .block_size        = blocksize_block_size,
   .pread             = blocksize_pread,
   .pwrite            = blocksize_pwrite,
   .trim              = blocksize_trim,
-- 
2.35.1