[Libguestfs] [PATCH nbdkit 6/8] python: Implement cache, can_cache.

[Richard W.M. Jones]

---
 plugins/python/nbdkit-python-plugin.pod | 22 +++++++++++--
 plugins/python/python.c                 | 41 +++++++++++++++++++++++++
 2 files changed, 61 insertions(+), 2 deletions(-)

diff --git a/plugins/python/nbdkit-python-plugin.pod b/plugins/python/nbdkit-python-plugin.pod
index 0fd4dcb..2bc4722 100644
--- a/plugins/python/nbdkit-python-plugin.pod
+++ b/plugins/python/nbdkit-python-plugin.pod
@@ -212,6 +212,13 @@ contents will be garbage collected.
  def can_trim(h):
    # return a boolean
 
+=item C<can_cache>
+
+(Optional)
+
+ def can_cache(h):
+   # return a boolean
+
 =item C<pread>
 
 (Required)
@@ -293,6 +300,19 @@ 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
@@ -316,12 +336,10 @@ C<description>,
 C<config_help>,
 C<magic_config_key>,
 C<can_fua>,
-C<can_cache>,
 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 9445343..33fb6e4 100644
--- a/plugins/python/python.c
+++ b/plugins/python/python.c
@@ -683,6 +683,39 @@ 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:
+      nbdkit_error ("%s can only be called when using api_version >= 2",
+                    "cache");
+      return -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)
 {
@@ -736,6 +769,12 @@ py_can_trim (void *handle)
   return boolean_callback (handle, "can_trim", "trim");
 }
 
+static int
+py_can_cache (void *handle)
+{
+  return boolean_callback (handle, "can_cache", "cache");
+}
+
 #define py_config_help \
   "script=<FILENAME>     (required) The Python plugin to run.\n" \
   "[other arguments may be used by the plugin that you load]"
@@ -762,12 +801,14 @@ static struct nbdkit_plugin plugin = {
   .can_write         = py_can_write,
   .can_flush         = py_can_flush,
   .can_trim          = py_can_trim,
+  .can_cache         = py_can_cache,
 
   .pread             = py_pread,
   .pwrite            = py_pwrite,
   .flush             = py_flush,
   .trim              = py_trim,
   .zero              = py_zero,
+  .cache             = py_cache,
 };
 
 NBDKIT_REGISTER_PLUGIN (plugin)
-- 
2.23.0