[Libguestfs] [nbdkit PATCH v2 22/24] truncate: Implement .cache

[Eric Blake]

In the truncate filter, all we need to do is copy-and-paste existing
per-command count rewriting (well, technically, this isn't enabled
until a later patch flips the default for .can_cache to pass-through).

Signed-off-by: Eric Blake <eblake at redhat.com>
---
 filters/truncate/truncate.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/filters/truncate/truncate.c b/filters/truncate/truncate.c
index 6408c35..38b1cd9 100644
--- a/filters/truncate/truncate.c
+++ b/filters/truncate/truncate.c
@@ -354,6 +354,29 @@ truncate_extents (struct nbdkit_next_ops *next_ops, void *nxdata,
   return 0;
 }

+/* Cache. */
+static int
+truncate_cache (struct nbdkit_next_ops *next_ops, void *nxdata,
+                void *handle, uint32_t count, uint64_t offset,
+                uint32_t flags, int *err)
+{
+  int r;
+  uint32_t n;
+  struct handle *h = handle;
+
+  if (offset < h->real_size) {
+    if (offset + count <= h->real_size)
+      n = count;
+    else
+      n = h->real_size - offset;
+    r = next_ops->cache (nxdata, n, offset, flags, err);
+    if (r == -1)
+      return -1;
+  }
+
+  return 0;
+}
+
 static struct nbdkit_filter filter = {
   .name              = "truncate",
   .longname          = "nbdkit truncate filter",
@@ -369,6 +392,7 @@ static struct nbdkit_filter filter = {
   .trim              = truncate_trim,
   .zero              = truncate_zero,
   .extents           = truncate_extents,
+  .cache             = truncate_cache,
 };

 NBDKIT_REGISTER_FILTER(filter)
-- 
2.20.1