nbdkit-python-plugin(3)

NAME

   nbdkit-python-plugin - nbdkit python plugin

SYNOPSIS

    nbdkit python script=/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.

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

   USING A PYTHON NBDKIT PLUGIN
   Assuming you have a Python script which is an nbdkit plugin, you run it
   like this:

    nbdkit python script=/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.  "script=..."  must
   come first on the command line.

WRITING A PYTHON NBDKIT PLUGIN

   There is an example Python nbdkit plugin called "example.py" which
   ships with the nbdkit source.

   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.

   The file does not need to include a "#!" (hash-bang) at the top, and
   does not need to be executable.  In fact it's a good idea not to do
   that, because running the plugin directly as a Python script won't
   work.

   EXCEPTIONS
   Python callbacks should throw exceptions to indicate errors.

   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).

   "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.

   "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.

   "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.

   "trim"
       (Optional)

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

       The body of your "trim" function should "punch a hole" in the
       backing store.

   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"
       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

   Richard W.M. Jones

COPYRIGHT

   Copyright (C) 2013-2014 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.

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