7:17 a.m.
On Mon, Jan 31, 2022 at 3:10 PM Richard W.M. Jones <rjones(a)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(a)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.
Should I make this explicit?
def thread_model():
# Serialize requests so we can seek safely in pread and pwrite.
return nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS
Reviewed-by: Richard W.M. Jones <rjones(a)redhat.com>
Rich.
> > + assert n == len(buf)
> > +
> > +
> > +def pwrite(h, buf, offset, flags):
> > + global calls
> > + calls += 1
> > + if calls % 2:
> > + raise RuntimeError(f"pwrite error offset={offset}
count={len(buf)}")
> > +
> > + os.lseek(h["fd"], offset, os.SEEK_SET)
> > + n = os.writev(h['fd'], [buf])
> > + assert n == len(buf)
> > --
> > 2.34.1
> >
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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