Libguestfs - Nov 2019 - [PATCH] python: Support buffer protocol for pread() result

pread() can return now any object supporting the buffer protocol[1] such
as bytearray, bytes, or memoryview.

[1] https://docs.python.org/3/c-api/buffer.html
---

When the new python tests will be in master we can add some automated tests.

For now I tested this manually using the example plugin as is and with
these changes:

 def pread(h, count, offset):
     global disk
     return bytes(disk[offset:offset+count])
 
 def pread(h, count, offset):
     global disk
     return memoryview(disk)[offset:offset+count]

 plugins/python/python.c | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/plugins/python/python.c b/plugins/python/python.c
index 148097f..ed299ff 100644
--- a/plugins/python/python.c
+++ b/plugins/python/python.c
@@ -445,10 +445,12 @@ py_pread (void *handle, void *buf,
   PyObject *obj = handle;
   PyObject *fn;
   PyObject *r;
+  Py_buffer view = {0};
+  int ret = -1;
 
   if (!callback_defined ("pread", &fn)) {
     nbdkit_error ("%s: missing callback: %s", script,
"pread");
-    return -1;
+    return ret;
   }
 
   PyErr_Clear ();
@@ -456,25 +458,30 @@ py_pread (void *handle, void *buf,
   r = PyObject_CallFunction (fn, "OiL", obj, count, offset, NULL);
   Py_DECREF (fn);
   if (check_python_failure ("pread") == -1)
-    return -1;
+    return ret;
 
-  if (!PyByteArray_Check (r)) {
-    nbdkit_error ("%s: value returned from pread method is not a byte
array",
+  if (PyObject_GetBuffer (r, &view, PyBUF_SIMPLE) == -1) {
+    nbdkit_error ("%s: value returned from pread does not support the
"
+                  "buffer protocol",
                   script);
-    Py_DECREF (r);
-    return -1;
+    goto out;
   }
 
-  if (PyByteArray_Size (r) < count) {
-    nbdkit_error ("%s: byte array returned from pread is too small",
script);
-    Py_DECREF (r);
-    return -1;
+  if (view.len < count) {
+    nbdkit_error ("%s: buffer returned from pread is too small",
script);
+    goto out;
   }
 
-  memcpy (buf, PyByteArray_AsString (r), count);
+  memcpy (buf, view.buf, count);
+  ret = 0;
+
+out:
+  if (view.obj)
+    PyBuffer_Release (&view);
+
   Py_DECREF (r);
 
-  return 0;
+  return ret;
 }
 
 static int
-- 
2.21.0

On Sat, Nov 23, 2019 at 03:27:54AM +0200, Nir Soffer
wrote:
> pread() can return now any object supporting the buffer protocol[1] such
> as bytearray, bytes, or memoryview.
> 
> [1] https://docs.python.org/3/c-api/buffer.html
ACK, I have pushed this.
> ---
> 
> When the new python tests will be in master we can add some automated
tests.
Don't worry about updating the tests, I've already done it
and will post a renewed series testing this feature shortly.

Thanks,

Rich.

-- 
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