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

[Nir Soffer]

On Sun, Nov 24, 2019, 12:42 Richard W.M. Jones <rjones at redhat.com> wrote:

> On Sat, Nov 23, 2019 at 06:11:47PM +0200, Nir Soffer wrote:
> > 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.
>
> Right, we can consider this for v2, while leaving v1 callers alone.
>
> > An uglier alternative is:
> >
> >     def preadinto(h, buf, offset):
> >
> > Matching python read() and readinto() interface.
>
> Is this different somehow from def pread(h, buf, offset) defined above?
>

We keep nicer pread for compatibility and add preadinto for people that
care about performence. This is the current approach in the standard
library.


> Rich.
>
> > > > 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
> > > Read my programming and virtualization blog: http://rwmj.wordpress.com
> > > Fedora Windows cross-compiler. Compile Windows programs, test, and
> > > build Windows installers. Over 100 libraries supported.
> > > http://fedoraproject.org/wiki/MinGW
> > >
>
> --
> Richard Jones, Virtualization Group, Red Hat
> http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> libguestfs lets you edit virtual machines.  Supports shell scripting,
> bindings from many languages.  http://libguestfs.org
>
>
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