[Libguestfs] [PATCH nbdkit] python: Pass memoryview to pwrite()

[Nir Soffer]

Passing a memoryview we avoid unneeded copy of the original buffer. On
the python side memoryview object can be used for slicing, writing to
file, or sending to socket.

This may break plugins assuming that the they get a bytearray, but
good python code should not care about the type of the buffer, only
about the behaviour.

Testing with a plugin writing to /dev/null shows 2.7x speedup. Real
plugin will probably show much smaller improvement.

Without patch:

$ time qemu-img convert -p -f raw -O raw -n /var/tmp/disk.img nbd://localhost/
    (100.00/100%)

real	0m1.284s
user	0m0.091s
sys	0m0.576s

With patch:

$ time qemu-img convert -p -f raw -O raw -n /var/tmp/disk.img nbd://localhost/
    (100.00/100%)

real	0m0.477s
user	0m0.078s
sys	0m0.653s
---

More info on how I tested this:

# Creating test image
$ dd if=/dev/zero bs=1M count=1024 | tr "\0" "U" > /var/tmp/disk.img

$ cat zero.py
import builtins

def open(readonly):
    return builtins.open("/dev/zero", "r+b")

def get_size(h):
    return 1024**3

def pwrite(h, buf, offset):
    h.write(buf)

def pread(h, count, offset):
    raise NotImplementedError

def close(h):
    h.close()

# Running nbdkit
$ ./nbdkit -f -v python zero.py

 plugins/python/python.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/plugins/python/python.c b/plugins/python/python.c
index 214fffb..2b4361a 100644
--- a/plugins/python/python.c
+++ b/plugins/python/python.c
@@ -496,8 +496,8 @@ py_pwrite (void *handle, const void *buf,
     PyErr_Clear ();
 
     r = PyObject_CallFunction (fn, "ONL", obj,
-                               PyByteArray_FromStringAndSize (buf, count),
-                               offset, NULL);
+            PyMemoryView_FromMemory ((char *)buf, count, PyBUF_READ),
+            offset, NULL);
     Py_DECREF (fn);
     if (check_python_failure ("pwrite") == -1)
       return -1;
-- 
2.21.0