nbdkit-python-plugin man page

nbdkit-python-plugin — nbdkit python plugin

Synopsis

 nbdkit python /path/to/plugin.py [arguments...]

Description

nbdkit-python-plugin is an embedded Python interpreter for nbdkit(1), allowing you to write nbdkit plugins in Python.

If you have been given an nbdkit Python plugin

Assuming you have a Python script which is an nbdkit plugin, you run it like this:

 nbdkit python /path/to/plugin.py

You may have to add further key=value arguments to the command line.  Read the Python script to see if it requires any.

Writing a Python Nbdkit Plugin

For an example plugin written in Python, see: https://github.com/libguestfs/nbdkit/blob/master/plugins/python/example.py

Broadly speaking, Python nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.

To write a Python nbdkit plugin, you create a Python file which contains at least the following required functions (in the top level __main__ module):

 def open(readonly):
   # see below
 def get_size(h):
   # see below
 def pread(h, count, offset):
   # see below

Note that the subroutines must have those literal names (like open), because the C part looks up and calls those functions directly.  You may want to include documentation and globals (eg. for storing global state).  Any other top level statements are run when the script is loaded, just like ordinary Python.

Python 2 and Python 3

The Python plugin has to be compiled for either Python 2 or Python 3 when building nbdkit.  You can set the PYTHON environment variable to the desired interpreter, otherwise nbdkit will use the interpreter called python on the current $PATH.  For example:

 PYTHON=/usr/bin/python3 ./configure

To find out which version the Python plugin was compiled for, use the --dump-plugin option, eg:

 $ nbdkit python --dump-plugin
 ...
 python_version=3.7.0
 python_pep_384_abi_version=3

Executable script

If you want you can make the script executable and include a “shebang” at the top:

 #!/usr/sbin/nbdkit python

See also “Shebang scripts” in nbdkit(1).

These scripts can also be installed in the $plugindir.  See “WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES” in nbdkit-plugin(3).

Methods

Your script may use import nbdkit to have access to the following methods in the nbdkit module:

 nbdkit.set_error(err)

Record err as the reason you are about to throw an exception. err should correspond to usual errno values, where it may help to import errno.

Exceptions

Python callbacks should throw exceptions to indicate errors.  Remember to use nbdkit.set_error if you need to control which error is sent back to the client; if omitted, the client will see an error of EIO.

Python callbacks

This just documents the arguments to the callbacks in Python, and any way that they differ from the C callbacks.  In all other respects they work the same way as the C callbacks, so you should go and read nbdkit-plugin(3).

dump_plugin

(Optional)

There are no arguments or return value.

config

(Optional)

 def config(key, value):
   # no return value
config_complete

(Optional)

There are no arguments or return value.

open

(Required)

 def open(readonly):
   # return handle

You can return any non-NULL Python value as the handle.  It is passed back in subsequent calls.

close

(Optional)

 def close(h):
   # no return value

After close returns, the reference count of the handle is decremented in the C part, which usually means that the handle and its contents will be garbage collected.

get_size

(Required)

 def get_size(h):
   # return the size of the disk
can_write

(Optional)

 def can_write(h):
   # return a boolean
can_flush

(Optional)

 def can_flush(h):
   # return a boolean
is_rotational

(Optional)

 def is_rotational(h):
   # return a boolean
can_trim

(Optional)

 def can_trim(h):
   # return a boolean
pread

(Required)

 def pread(h, count, offset):
   # construct a bytearray of length count bytes and return it

The body of your pread function should construct a buffer of length (at least) count bytes.  You should read count bytes from the disk starting at offset.

NBD only supports whole reads, so your function should try to read the whole region (perhaps requiring a loop).  If the read fails or is partial, your function should throw an exception, optionally using nbdkit.set_error first.

pwrite

(Optional)

 def pwrite(h, buf, offset):
   length = len (buf)
   # no return value

The body of your pwrite function should write the buf string to the disk.  You should write count bytes to the disk starting at offset.

NBD only supports whole writes, so your function should try to write the whole region (perhaps requiring a loop).  If the write fails or is partial, your function should throw an exception,
optionally using nbdkit.set_error first.

flush

(Optional)

 def flush(h):
   # no return value

The body of your flush function should do a sync(2) or fdatasync(2) or equivalent on the backing store.

If the flush fails, your function should throw an exception, optionally using nbdkit.set_error first.

trim

(Optional)

 def trim(h, count, offset):
   # no return value

The body of your trim function should “punch a hole” in the backing store.  If the trim fails, your function should throw an exception, optionally using nbdkit.set_error first.

zero

(Optional)

 def zero(h, count, offset, may_trim):
   # no return value

The body of your zero function should ensure that count bytes of the disk, starting at offset, will read back as zero.  If may_trim is true, the operation may be optimized as a trim as long as subsequent reads see zeroes.

NBD only supports whole writes, so your function should try to write the whole region (perhaps requiring a loop).  If the write fails or is partial, your function should throw an exception, optionally using nbdkit.set_error first.  In particular, if you would like to automatically fall back to pwrite (perhaps because there is nothing to optimize if may_trim is false), use nbdkit.set_error(errno.EOPNOTSUPP).

Missing callbacks

Missing: load and unload

These are not needed because you can just use ordinary Python constructs.

Missing: name, version, longname, description, config_help, can_fua, can_cache, cache

These are not yet supported.

Threads

The thread model for Python callbacks currently cannot be set from Python.  It is hard-coded in the C part to NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS.  This may change or be settable in future.

See Also

nbdkit(1), nbdkit-plugin(3), python(1).

Authors

Eric Blake

Richard W.M. Jones

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND 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.

Referenced By

nbdkit(1), nbdkit-plugin(3).

2019-08-19 nbdkit-1.13.8 NBDKIT