focal (3) nbdkit-python-plugin.3.gz

NAME

       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 versions
       In nbdkit ≤ 1.14, either Python 2 or 3 could be used.  It was selected at compile time by either:

        ./configure

       which selected the version of Python by looking at the "python" interpreter found on the $PATH.  Or:

        ./configure PYTHON=/usr/bin/python3

       which allowed you to select a different interpreter and hence a different version of Python.

       nbdkit ≥ 1.16 drops all support for Python 2, since Python 2 has reached its end of life.

       The new behaviour is that "./configure" looks for "python3" or "python" (in that order) on the $PATH.  It
       will fail if the first interpreter it finds is a Python 2 interpreter.  You may also still choose a
       Python interpreter by setting the "PYTHON" variable at configure time as above.

       If you wish to continue using nbdkit plugins written in Python 2 then you must use nbdkit ≤ 1.14, but we
       would advise you to update your plugins.

       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

       "is_rotational"
           (Optional)

            def is_rotational(h):
              # return a boolean

       "can_write"
           (Optional)

            def can_write(h):
              # return a boolean

       "can_flush"
           (Optional)

            def can_flush(h):
              # return a boolean

       "can_trim"
           (Optional)

            def can_trim(h):
              # return a boolean

       "pread"
           (Required)

            def pread(h, count, offset):
              # construct a buffer 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".

           The returned buffer can be any type compatible with the Python 3 buffer protocol, such as bytearray,
           bytes or memoryview (https://docs.python.org/3/c-api/buffer.html)

           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 buffer "buf" 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: "thread_model"
           See "Threads" below.

       Missing: "name", "version", "longname", "description", "config_help", "magic_config_key", "can_fua",
       "can_cache", "can_zero", "can_fast_zero", "can_extents", "can_multi_conn", "cache", "extents".
           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.

FILES

       $plugindir/nbdkit-python-plugin.so
           The plugin.

           Use "nbdkit --dump-config" to find the location of $plugindir.

VERSION

       "nbdkit-python-plugin" first appeared in nbdkit 1.2.

SEE ALSO

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

AUTHORS

       Eric Blake

       Richard W.M. Jones

       Nir Soffer

COPYRIGHT

       Copyright (C) 2013-2019 Red Hat Inc.

LICENSE

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

       •   Redistributions of source code must retain the above copyright notice, this list of conditions and
           the following disclaimer.

       •   Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
           the following disclaimer in the documentation and/or other materials provided with the distribution.

       •   Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote
           products derived from this software without specific prior written permission.

       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.