[Libguestfs] [PATCH nbdkit] plugins: python: Add error example

[Nir Soffer]

On Mon, Jan 31, 2022 at 3:10 PM Richard W.M. Jones <rjones at redhat.com> wrote:
>
> On Mon, Jan 31, 2022 at 12:17:36AM +0200, Nir Soffer wrote:
> > On Mon, Jan 31, 2022 at 12:14 AM Nir Soffer <nsoffer at redhat.com> wrote:
> > >
> > > This plugin simulates errors in pread, pwrite, and extents. This is
> > > useful for testing error handling in NBD clients, and understanding how
> > > plugin exceptions are reported to the NBD client.
> > > ---
> > >  plugins/python/Makefile.am       |  1 +
> > >  plugins/python/examples/error.py | 77 ++++++++++++++++++++++++++++++++
> > >  2 files changed, 78 insertions(+)
> > >  create mode 100644 plugins/python/examples/error.py
> > >
> > > diff --git a/plugins/python/Makefile.am b/plugins/python/Makefile.am
> > > index eecd7e89..e6e6c9e6 100644
> > > --- a/plugins/python/Makefile.am
> > > +++ b/plugins/python/Makefile.am
> > > @@ -27,20 +27,21 @@
> > >  # ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
> > >  # OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
> > >  # OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
> > >  # SUCH DAMAGE.
> > >
> > >  include $(top_srcdir)/common-rules.mk
> > >
> > >  EXTRA_DIST = \
> > >         nbdkit-python-plugin.pod \
> > >         examples/file.py \
> > > +       examples/error.py \
> > >         examples/imageio.py \
> > >         examples/ramdisk.py \
> > >         examples/url.py \
> > >         $(NULL)
> > >
> > >  if HAVE_PYTHON
> > >
> > >  plugin_LTLIBRARIES = nbdkit-python-plugin.la
> > >
> > >  nbdkit_python_plugin_la_SOURCES = \
> > > diff --git a/plugins/python/examples/error.py b/plugins/python/examples/error.py
> > > new file mode 100644
> > > index 00000000..0331a83e
> > > --- /dev/null
> > > +++ b/plugins/python/examples/error.py
> > > @@ -0,0 +1,77 @@
> > > +# Example Python plugin.
> > > +#
> > > +# This plugin simulates errors for testing NBD client error hanlding.
> > > +# Every odd call will fail, and every even call will succeed, unless
> > > +# there a real error accesing the specified file.
> > > +#
> > > +# This example can be freely used for any purpose.
> > > +
> > > +# Run it from the build directory like this:
> > > +#
> > > +#   ./nbdkit -f -v python ./plugins/python/examples/error.py file=test.img
> > > +#
> > > +# Or run it after installing nbdkit like this:
> > > +#
> > > +#   nbdkit -f -v python ./plugins/python/examples/error.py file=test.img
> > > +#
> > > +# The -f -v arguments are optional.  They cause the server to stay in
> > > +# the foreground and print debugging, which is useful when testing.
> > > +
> > > +import os
> > > +
> > > +API_VERSION = 2
> > > +
> > > +filename = None
> > > +calls = 0
> > > +
> > > +
> > > +def config(key, value):
> > > +    global filename
> > > +    assert key == "file"
> > > +    filename = value
> > > +
> > > +
> > > +def open(readonly):
> > > +    flags = os.O_RDONLY if readonly else os.O_RDWR
> > > +    return {"fd": os.open(filename, flags)}
> > > +
> > > +
> > > +def can_extents(h):
> > > +    return True
> > > +
> > > +
> > > +def get_size(h):
> > > +    return os.stat(h["fd"]).st_size
> > > +
> > > +
> > > +def extents(h, count, offset, flags):
> > > +    global calls
> > > +    calls += 1
> > > +    if calls % 2:
> > > +        raise RuntimeError(f"extents error offset={offset} count={count}")
> > > +
> > > +    # We don't really support extents, so we report the entire file as
> > > +    # data.
> > > +    return [(offset, count, 0)]
> > > +
> > > +
> > > +def pread(h, buf, offset, flags):
> > > +    global calls
> > > +    calls += 1
> > > +    if calls % 2:
> > > +        raise RuntimeError(f"pread error offset={offset} count={len(buf)}")
> > > +
> > > +    os.lseek(h["fd"], offset, os.SEEK_SET)
> > > +    n = os.readv(h['fd'], [buf])
> >
> > We can use os.preadv in current python, but I'm trying to keep this plugin
> > useful on rhel 8 systems (python 3.6). os.preadv added in python 3.7.
>
> The default thread model for Python plugins is
> nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS, ie. you cannot have two
> pread() functions racing to do lseek + readon the same file
> descriptor, so this should be safe.
>
> Reviewed-by: Richard W.M. Jones <rjones at redhat.com>

Pushed as 096a5dd07c560439ca7d2544d9df33062df2801e.