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 3.

   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 example plugins written in Python, see:
       https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/python/examples

       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):

        API_VERSION = 2
        def open(readonly):
          # see below
        def get_size(h):
          # see below
        def pread(h, buf, offset, flags):
          # 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
       Since nbdkit ≥ 1.16 only Python 3 is supported.  If you wish to continue using nbdkit
       plugins written in Python 2 then you must use nbdkit ≤ 1.14, but we advise you to update
       your plugins.

       The version of Python 3 is chosen when nbdkit is built.  This is compiled in and can't be
       changed at runtime.  "./configure" looks for (in order):

       •   the "PYTHON" variable (eg "./configure PYTHON=/usr/bin/python3.9")

       •   python3 on $PATH

       •   python on $PATH

       "./configure" will fail if the first interpreter found is a Python 2 interpreter.

       To find out which version of Python "nbdkit-python-plugin" was compiled for, use the
       --dump-plugin option:

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

   API versions
       The nbdkit API has evolved and new versions are released periodically.  To ensure
       backwards compatibility plugins have to opt in to the new version.  From Python you do
       this by declaring a constant in your module:

        API_VERSION = 2

       (where 2 is the latest version at the time this documentation was written).  All newly
       written Python modules must have this constant.

   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).

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

       "nbdkit.debug(msg)"

       Send a debug message to stderr or syslog if verbose messages are enabled.

       "nbdkit.export_name()"

       Return the export name negotiated with the client as a Unicode string.  Note this should
       not be trusted because the client can send whatever it wants.

       "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".

       "nbdkit.parse_size(str)"

       Parse a string (such as "100M") into a size in bytes. Wraps the "nbdkit_parse_size()" C
       function.

       "nbdkit.shutdown()"

       Request asynchronous server shutdown.

   Module constants
       After "import nbdkit" the following constants are available.  These are used in the
       callbacks below.

       "nbdkit.THREAD_MODEL_SERIALIZE_CONNECTIONS"
       "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
       "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS"
       "nbdkit.THREAD_MODEL_PARALLEL"
           Possible return values from "thread_model()".

       "nbdkit.FLAG_MAY_TRIM"
       "nbdkit.FLAG_FUA"
       "nbdkit.FLAG_REQ_ONE"
       "nbdkit.FLAG_FAST_ZERO"
           Flags bitmap passed to certain plugin callbacks.  Not all callbacks with a flags
           parameter use all of these flags, consult the documentation below and
           nbdkit-plugin(3).

       "nbdkit.FUA_NONE"
       "nbdkit.FUA_EMULATE"
       "nbdkit.FUA_NATIVE"
           Possible return values from "can_fua()".

       "nbdkit.CACHE_NONE"
       "nbdkit.CACHE_EMULATE"
       "nbdkit.CACHE_NATIVE"
           Possible return values from "can_cache()".

       "nbdkit.EXTENT_HOLE"
       "nbdkit.EXTENT_ZERO"
           Used in the "type" field returned by "extents()".

   Threads
       The thread model for Python callbacks defaults to
       "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS".

       Since nbdkit 1.22 it has been possible to set this by implementing a "thread_model()"
       function which returns one of the constants "nbdkit.THREAD_MODEL_*".

       The Python Global Interpreter Lock (GIL) is still used, so Python code does not run in
       parallel.  However if a plugin callback calls a library which blocks (eg. to make an HTTP
       request), then another callback might be executed in parallel.  Plugins which use
       "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS" or "nbdkit.THREAD_MODEL_PARALLEL" may need to use
       locks on shared data.

   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.

       "thread_model"
           (Optional, nbdkit ≥ 1.22)

            def thread_model():
              return nbdkit.THEAD_MODEL_SERIALIZE_ALL_REQUESTS

           See "Threads" above.

       "get_ready"
           (Optional)

           There are no arguments or return value.

       "after_fork"
           (Optional, nbdkit ≥ 1.26)

           There are no arguments or return value.

       "cleanup"
           (Optional, nbdkit ≥ 1.28)

           There are no arguments or return value.

       "list_exports"
           (Optional)

            def list_exports(readonly, is_tls):
              # return an iterable object (eg. list) of
              # (name, description) tuples or bare names:
              return [ (name1, desc1), name2, (name3, desc3), ... ]

       "default_export"
           (Optional)

            def default_export(readonly, is_tls):
              # return a string
              return "name"

       "preconnect"
           (Optional, nbdkit ≥ 1.26)

            def preconnect(readonly):
              # no 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.

       "export_description"
           (Optional)

            def export_description(h):
              # return a string
              return "description"

       "get_size"
           (Required)

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

       "block_size"
           (Option)

            def block_size(h):
              # return triple (minimum, preferred, maximum) block size

       "is_rotational"
           (Optional)

            def is_rotational(h):
              # return a boolean

       "can_multi_conn"
           (Optional)

            def can_multi_conn(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

       "can_zero"
           (Optional)

            def can_zero(h):
              # return a boolean

       "can_fast_zero"
           (Optional)

            def can_fast_zero(h):
              # return a boolean

       "can_fua"
           (Optional)

            def can_fua(h):
              # return nbdkit.FUA_NONE or nbdkit.FUA_EMULATE
              # or nbdkit.FUA_NATIVE

       "can_cache"
           (Optional)

            def can_cache(h):
              # return nbdkit.CACHE_NONE or nbdkit.CACHE_EMULATE
              # or nbdkit.CACHE_NATIVE

       "can_extents"
           (Optional)

            def can_extents(h):
              # return a boolean

       "pread"
           (Required)

            def pread(h, buf, offset, flags):
              # read into the buffer

           The body of your "pread" function should read exactly "len(buf)" bytes of data
           starting at disk "offset" and write it into the buffer "buf".  "flags" is always 0.

           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, flags):
              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".  "flags" may contain
           "nbdkit.FLAG_FUA".

           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, flags):
              # no return value

           The body of your "flush" function should do a sync(2) or fdatasync(2) or equivalent on
           the backing store.  "flags" is always 0.

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

       "trim"
           (Optional)

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

           The body of your "trim" function should "punch a hole" in the backing store.  "flags"
           may contain "nbdkit.FLAG_FUA".  If the trim fails, your function should throw an
           exception, optionally using "nbdkit.set_error" first.

       "zero"
           (Optional)

            def zero(h, count, offset, flags):
              # 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.  "flags" is a bitmask which may include
           "nbdkit.FLAG_MAY_TRIM", "nbdkit.FLAG_FUA", "nbdkit.FLAG_FAST_ZERO".

           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
           "flags & nbdkit.FLAG_MAY_TRIM" is false), use "nbdkit.set_error(errno.EOPNOTSUPP)".

       "cache"
           (Optional)

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

           The body of your "cache" function should prefetch data in the indicated range.

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

       "extents"
           (Optional)

            def extents(h, count, offset, flags):
              # return an iterable object (eg. list) of
              # (offset, length, type) tuples:
              return [ (off1, len1, type1), (off2, len2, type2), ... ]

   Missing callbacks
       Missing: "load"
           This is not needed since you can use regular Python mechanisms like top level
           statements to run code when the module is loaded.

       Missing: "unload"
           This is missing, but in nbdkit ≥ 1.28 you can put code in the "cleanup()" function to
           have it run when nbdkit exits.  In earlier versions of nbdkit, using a Python atexit
           handler is recommended.

       Missing: "name", "version", "longname", "description", "config_help", "magic_config_key".
           These are not yet supported.

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) 2013-2021 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
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