From c5b1fac4c67078f0164bd23eab6d4d2b8c9830b0 Mon Sep 17 00:00:00 2001
From: "Richard W.M. Jones" <rjones@redhat.com>
Date: Thu, 21 Nov 2019 16:42:02 +0000
Subject: [PATCH] python: Implement cache.

However this does not implement can_cache, since that is not a simple
boolean.

(cherry picked from commit e61ffb73c7a0af0c383184fdb8f08d30784a195e)
---
 plugins/python/nbdkit-python-plugin.pod | 14 ++++++++++-
 plugins/python/python.c                 | 31 +++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/plugins/python/nbdkit-python-plugin.pod b/plugins/python/nbdkit-python-plugin.pod
index 4923d9d..0ea8dee 100644
--- a/plugins/python/nbdkit-python-plugin.pod
+++ b/plugins/python/nbdkit-python-plugin.pod
@@ -289,6 +289,19 @@ because there is nothing to optimize if
 S<C<flags & nbdkit.FLAG_MAY_TRIM>> is false), use
 S<C<nbdkit.set_error (errno.EOPNOTSUPP)>>.
 
+=item C<cache>
+
+(Optional)
+
+ def cache(h, count, offset, flags):
+   # no return value
+
+The body of your C<cache> function should prefetch data in the
+indicated range.
+
+If the cache operation fails, your function should throw an exception,
+optionally using C<nbdkit.set_error> first.
+
 =back
 
 =head2 Missing callbacks
@@ -317,7 +330,6 @@ C<can_zero>,
 C<can_fast_zero>,
 C<can_extents>,
 C<can_multi_conn>,
-C<cache>,
 C<extents>.
 
 These are not yet supported.
diff --git a/plugins/python/python.c b/plugins/python/python.c
index 0f28595..c5cf38e 100644
--- a/plugins/python/python.c
+++ b/plugins/python/python.c
@@ -714,6 +714,36 @@ py_zero (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
   return -1;
 }
 
+static int
+py_cache (void *handle, uint32_t count, uint64_t offset, uint32_t flags)
+{
+  PyObject *obj = handle;
+  PyObject *fn;
+  PyObject *r;
+
+  if (callback_defined ("cache", &fn)) {
+    PyErr_Clear ();
+
+    switch (py_api_version) {
+    case 1:
+    case 2:
+      r = PyObject_CallFunction (fn, "OiLI", obj, count, offset, flags, NULL);
+      break;
+    default: abort ();
+    }
+    Py_DECREF (fn);
+    if (check_python_failure ("cache") == -1)
+      return -1;
+    Py_DECREF (r);
+  }
+  else {
+    nbdkit_error ("%s not implemented", "cache");
+    return -1;
+  }
+
+  return 0;
+}
+
 static int
 boolean_callback (void *handle, const char *can_fn, const char *plain_fn)
 {
@@ -799,6 +829,7 @@ static struct nbdkit_plugin plugin = {
   .flush             = py_flush,
   .trim              = py_trim,
   .zero              = py_zero,
+  .cache             = py_cache,
 };
 
 NBDKIT_REGISTER_PLUGIN (plugin)
-- 
2.18.2