[Libguestfs] [PATCH nbdkit v2 10/10] tests: Test the Python plugin thoroughly.

[Nir Soffer]

On Sat, Nov 23, 2019 at 3:10 PM Richard W.M. Jones <rjones at redhat.com> wrote:
>
> On Sat, Nov 23, 2019 at 01:42:15AM +0200, Nir Soffer wrote:
> > On Fri, Nov 22, 2019 at 9:55 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> > > +def pread(h, count, offset, flags):
> > > +    assert flags == 0
> > > +    return h['disk'][offset:offset+count]
> >
> > Very nice and simple test plugin!
> >
> > But this returns always a bytearray, which is also what nbdkit python plugin
> > expects. But real code using HTTPConnection return bytes:
> >
> > >>> c = http.client.HTTPSConnection("www.python.org")
> > >>> c.request("GET", "/")
> > >>> r = c.getresponse()
> > >>> r.read()[:10]
> > b'<!doctype '
> >
> > I think the plugin should support both bytearray, memoryview, or
> > bytes. Supporting objects
> > implementing the buffer protocol would be best.
>
> I thought bytearray & bytes were the same thing :-/
>
> Would adding this break existing nbdkit Python scripts?  Should this
> be considered for v2 of the API?  Are there performance implications /
> improvements from doing this?

There is performance implication for v1 plugins like rhv-upload-plugin that
need to copy the bytes from imageio server on python side:

    r = http.getresponse()
    ...
    return bytearray(r.read())

This is sad because on the C side we mempcpy the data again. So with this patch
we avoid one copy of the two.

To avoid all unneeded copies, we need to change pread() to:

    def pread(h, buf, offset):

So the python side we can do:

    f.readinto(buf)

Or:

    sock.recv_info(buf)

It does not work for HTTPResponse, so in this case we have to do:

    buf[:] = r.read()

Since we work on v2 now, I think we should consider this change.

An uglier alternative is:

    def preadinto(h, buf, offset):

Matching python read() and readinto() interface.

> > test-python-plugin.py
> >
> > This text can use python docstring:
> >
> > """
> > This tests ...
> > """
>
> Good point, I'll fix this.  We could probably also have nbdkit do
> something with the docstring, such as printing it in --help output,
> although that's something for another patch series.
>
> > > +    h = nbd.NBD ()
> > > +    cfg = codecs.encode (pickle.dumps (cfg), "base64").decode()
> >
> > base64.b64encode() is better, avoiding unwanted newlines.
>
> Ah OK, I originally added strip(), but this is better.
>
> > > +    cmd = ["nbdkit", "-v", "-s", "--exit-with-parent",
> > > +           "python", srcdir + "/test-python-plugin.py",
> > > +           "cfg=" + cfg]
> > > +    h.connect_command (cmd)
> > > +    return h
> > > +
> > > +# Test we can send an empty pickled test configuration and do nothing
> > > +# else.  This is just to ensure the machinery of the test works.
> > > +h = test ({})
> >
> > So we have now running nbdkit that will exit the python collects when h
> > is implicitly closed when creating a new handle?
> >
> > This is fragile, but can be solved with the help of a testing framework.
> [...]
> > pytest test-python.py
>
> I'll probably use unittest though because it's built into Python and
> because it's what we use in libguestfs, hivex etc but yes good idea.
>
> Thanks,
>
> Rich.
>
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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