1nbdkit-python-plugin(3) NBDKIT nbdkit-python-plugin(3)
2
6 nbdkit-python-plugin - nbdkit python plugin
7
9 nbdkit python /path/to/plugin.py [arguments...]
10
12 "nbdkit-python-plugin" is an embedded Python interpreter for nbdkit(1),
13 allowing you to write nbdkit plugins in Python 3.
14 If you have been given an nbdkit Python plugin
16 Assuming you have a Python script which is an nbdkit plugin, you run it
17 like this:
18 nbdkit python /path/to/plugin.py
20 You may have to add further "key=value" arguments to the command line.
22 Read the Python script to see if it requires any.
23
25 For example plugins written in Python, see:
26 https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/python/examples
27 Broadly speaking, Python nbdkit plugins work like C ones, so you should
29 read nbdkit-plugin(3) first.
30 To write a Python nbdkit plugin, you create a Python file which
32 contains at least the following required functions (in the top level
33 "__main__" module):
34 API_VERSION = 2
36 def open(readonly):
37 # see below
38 def get_size(h):
39 # see below
40 def pread(h, buf, offset, flags):
41 # see below
42 Note that the subroutines must have those literal names (like "open"),
44 because the C part looks up and calls those functions directly. You
45 may want to include documentation and globals (eg. for storing global
46 state). Any other top level statements are run when the script is
47 loaded, just like ordinary Python.
48 Python versions
50 Since nbdkit ≥ 1.16 only Python 3 is supported. If you wish to
51 continue using nbdkit plugins written in Python 2 then you must use
52 nbdkit ≤ 1.14, but we advise you to update your plugins.
53 The version of Python 3 is chosen when nbdkit is built. This is
55 compiled in and can't be changed at runtime. "./configure" looks for
56 (in order):
57 • the "PYTHON" variable (eg "./configure PYTHON=/usr/bin/python3.9")
59 • python3 on $PATH
61 • python on $PATH
63 "./configure" will fail if the first interpreter found is a Python 2
65 interpreter.
66 To find out which version of Python "nbdkit-python-plugin" was compiled
68 for, use the --dump-plugin option:
69 $ nbdkit python --dump-plugin
71 ...
72 python_version=3.7.0
73 python_pep_384_abi_version=3
74 API versions
76 The nbdkit API has evolved and new versions are released periodically.
77 To ensure backwards compatibility plugins have to opt in to the new
78 version. From Python you do this by declaring a constant in your
79 module:
80 API_VERSION = 2
82 (where 2 is the latest version at the time this documentation was
84 written). All newly written Python modules must have this constant.
85 Executable script
87 If you want you can make the script executable and include a "shebang"
88 at the top:
89 #!/usr/sbin/nbdkit python
91 See also "Shebang scripts" in nbdkit(1).
93 These scripts can also be installed in the $plugindir. See "WRITING
95 PLUGINS IN OTHER PROGRAMMING LANGUAGES" in nbdkit-plugin(3).
96 Module functions
98 Your script may use "import nbdkit" to have access to the following
99 methods in the "nbdkit" module:
100 "nbdkit.debug(msg)"
102 Send a debug message to stderr or syslog if verbose messages are
104 enabled.
105 "nbdkit.export_name()"
107 Return the export name negotiated with the client as a Unicode string.
109 Note this should not be trusted because the client can send whatever it
110 wants.
111 "nbdkit.set_error(err)"
113 Record "err" as the reason you are about to throw an exception. "err"
115 should correspond to usual errno values, where it may help to "import
116 errno".
117 "nbdkit.shutdown()"
119 Request asynchronous server shutdown.
121 Module constants
123 After "import nbdkit" the following constants are available. These are
124 used in the callbacks below.
125 "nbdkit.THREAD_MODEL_SERIALIZE_CONNECTIONS"
127 "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
128 "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS"
129 "nbdkit.THREAD_MODEL_PARALLEL"
130 Possible return values from "thread_model()".
131 "nbdkit.FLAG_MAY_TRIM"
133 "nbdkit.FLAG_FUA"
134 "nbdkit.FLAG_REQ_ONE"
135 "nbdkit.FLAG_FAST_ZERO"
136 Flags bitmap passed to certain plugin callbacks. Not all callbacks
137 with a flags parameter use all of these flags, consult the
138 documentation below and nbdkit-plugin(3).
139 "nbdkit.FUA_NONE"
141 "nbdkit.FUA_EMULATE"
142 "nbdkit.FUA_NATIVE"
143 Possible return values from "can_fua()".
144 "nbdkit.CACHE_NONE"
146 "nbdkit.CACHE_EMULATE"
147 "nbdkit.CACHE_NATIVE"
148 Possible return values from "can_cache()".
149 "nbdkit.EXTENT_HOLE"
151 "nbdkit.EXTENT_ZERO"
152 Used in the "type" field returned by "extents()".
153 Threads
155 The thread model for Python callbacks defaults to
156 "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS".
157 Since nbdkit 1.22 it has been possible to set this by implementing a
159 "thread_model()" function which returns one of the constants
160 "nbdkit.THREAD_MODEL_*".
161 The Python Global Interpreter Lock (GIL) is still used, so Python code
163 does not run in parallel. However if a plugin callback calls a library
164 which blocks (eg. to make an HTTP request), then another callback might
165 be executed in parallel. Plugins which use
166 "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS" or
167 "nbdkit.THREAD_MODEL_PARALLEL" may need to use locks on shared data.
168 Exceptions
170 Python callbacks should throw exceptions to indicate errors. Remember
171 to use "nbdkit.set_error" if you need to control which error is sent
172 back to the client; if omitted, the client will see an error of "EIO".
173 Python callbacks
175 This just documents the arguments to the callbacks in Python, and any
176 way that they differ from the C callbacks. In all other respects they
177 work the same way as the C callbacks, so you should go and read
178 nbdkit-plugin(3).
179 "dump_plugin"
181 (Optional)
182 There are no arguments or return value.
184 "config"
186 (Optional)
187 def config(key, value):
189 # no return value
190 "config_complete"
192 (Optional)
193 There are no arguments or return value.
195 "thread_model"
197 (Optional, nbdkit ≥ 1.22)
198 def thread_model():
200 return nbdkit.THEAD_MODEL_SERIALIZE_ALL_REQUESTS
201 See "Threads" above.
203 "get_ready"
205 (Optional)
206 There are no arguments or return value.
208 "after_fork"
210 (Optional, nbdkit ≥ 1.26)
211 There are no arguments or return value.
213 "cleanup"
215 (Optional, nbdkit ≥ 1.28)
216 There are no arguments or return value.
218 "list_exports"
220 (Optional)
221 def list_exports(readonly, is_tls):
223 # return an iterable object (eg. list) of
224 # (name, description) tuples or bare names:
225 return [ (name1, desc1), name2, (name3, desc3), ... ]
226 "default_export"
228 (Optional)
229 def default_export(readonly, is_tls):
231 # return a string
232 return "name"
233 "preconnect"
235 (Optional, nbdkit ≥ 1.26)
236 def preconnect(readonly):
238 # no return value
239 "open"
241 (Required)
242 def open(readonly):
244 # return handle
245 You can return any non-NULL Python value as the handle. It is
247 passed back in subsequent calls.
248 "close"
250 (Optional)
251 def close(h):
253 # no return value
254 After "close" returns, the reference count of the handle is
256 decremented in the C part, which usually means that the handle and
257 its contents will be garbage collected.
258 "export_description"
260 (Optional)
261 def export_description(h):
263 # return a string
264 return "description"
265 "get_size"
267 (Required)
268 def get_size(h):
270 # return the size of the disk
271 "is_rotational"
273 (Optional)
274 def is_rotational(h):
276 # return a boolean
277 "can_multi_conn"
279 (Optional)
280 def can_multi_conn(h):
282 # return a boolean
283 "can_write"
285 (Optional)
286 def can_write(h):
288 # return a boolean
289 "can_flush"
291 (Optional)
292 def can_flush(h):
294 # return a boolean
295 "can_trim"
297 (Optional)
298 def can_trim(h):
300 # return a boolean
301 "can_zero"
303 (Optional)
304 def can_zero(h):
306 # return a boolean
307 "can_fast_zero"
309 (Optional)
310 def can_fast_zero(h):
312 # return a boolean
313 "can_fua"
315 (Optional)
316 def can_fua(h):
318 # return nbdkit.FUA_NONE or nbdkit.FUA_EMULATE
319 # or nbdkit.FUA_NATIVE
320 "can_cache"
322 (Optional)
323 def can_cache(h):
325 # return nbdkit.CACHE_NONE or nbdkit.CACHE_EMULATE
326 # or nbdkit.CACHE_NATIVE
327 "can_extents"
329 (Optional)
330 def can_extents(h):
332 # return a boolean
333 "pread"
335 (Required)
336 def pread(h, buf, offset, flags):
338 # read into the buffer
339 The body of your "pread" function should read exactly "len(buf)"
341 bytes of data starting at disk "offset" and write it into the
342 buffer "buf". "flags" is always 0.
343 NBD only supports whole reads, so your function should try to read
345 the whole region (perhaps requiring a loop). If the read fails or
346 is partial, your function should throw an exception, optionally
347 using "nbdkit.set_error" first.
348 "pwrite"
350 (Optional)
351 def pwrite(h, buf, offset, flags):
353 length = len(buf)
354 # no return value
355 The body of your "pwrite" function should write the buffer "buf" to
357 the disk. You should write "count" bytes to the disk starting at
358 "offset". "flags" may contain "nbdkit.FLAG_FUA".
359 NBD only supports whole writes, so your function should try to
361 write the whole region (perhaps requiring a loop). If the write
362 fails or is partial, your function should throw an exception,
363 optionally using "nbdkit.set_error" first.
364 "flush"
366 (Optional)
367 def flush(h, flags):
369 # no return value
370 The body of your "flush" function should do a sync(2) or
372 fdatasync(2) or equivalent on the backing store. "flags" is always
373 0.
374 If the flush fails, your function should throw an exception,
376 optionally using "nbdkit.set_error" first.
377 "trim"
379 (Optional)
380 def trim(h, count, offset, flags):
382 # no return value
383 The body of your "trim" function should "punch a hole" in the
385 backing store. "flags" may contain "nbdkit.FLAG_FUA". If the trim
386 fails, your function should throw an exception, optionally using
387 "nbdkit.set_error" first.
388 "zero"
390 (Optional)
391 def zero(h, count, offset, flags):
393 # no return value
394 The body of your "zero" function should ensure that "count" bytes
396 of the disk, starting at "offset", will read back as zero. "flags"
397 is a bitmask which may include "nbdkit.FLAG_MAY_TRIM",
398 "nbdkit.FLAG_FUA", "nbdkit.FLAG_FAST_ZERO".
399 NBD only supports whole writes, so your function should try to
401 write the whole region (perhaps requiring a loop).
402 If the write fails or is partial, your function should throw an
404 exception, optionally using "nbdkit.set_error" first. In
405 particular, if you would like to automatically fall back to
406 "pwrite" (perhaps because there is nothing to optimize if
407 "flags & nbdkit.FLAG_MAY_TRIM" is false), use
408 "nbdkit.set_error(errno.EOPNOTSUPP)".
409 "cache"
411 (Optional)
412 def cache(h, count, offset, flags):
414 # no return value
415 The body of your "cache" function should prefetch data in the
417 indicated range.
418 If the cache operation fails, your function should throw an
420 exception, optionally using "nbdkit.set_error" first.
421 "extents"
423 (Optional)
424 def extents(h, count, offset, flags):
426 # return an iterable object (eg. list) of
427 # (offset, length, type) tuples:
428 return [ (off1, len1, type1), (off2, len2, type2), ... ]
429 Missing callbacks
431 Missing: "load"
432 This is not needed since you can use regular Python mechanisms like
433 top level statements to run code when the module is loaded.
434 Missing: "unload"
436 This is missing, but in nbdkit ≥ 1.28 you can put code in the
437 "cleanup()" function to have it run when nbdkit exits. In earlier
438 versions of nbdkit, using a Python atexit handler is recommended.
439 Missing: "name", "version", "longname", "description", "config_help",
441 "magic_config_key".
442 These are not yet supported.
443
445 $plugindir/nbdkit-python-plugin.so
446 The plugin.
447 Use "nbdkit --dump-config" to find the location of $plugindir.
449
451 "nbdkit-python-plugin" first appeared in nbdkit 1.2.
452
454 nbdkit(1), nbdkit-plugin(3), python(1).
455
457 Eric Blake
458 Richard W.M. Jones
460 Nir Soffer
462
464 Copyright (C) 2013-2021 Red Hat Inc.
465
467 Redistribution and use in source and binary forms, with or without
468 modification, are permitted provided that the following conditions are
469 met:
470 • Redistributions of source code must retain the above copyright
472 notice, this list of conditions and the following disclaimer.
473 • Redistributions in binary form must reproduce the above copyright
475 notice, this list of conditions and the following disclaimer in the
476 documentation and/or other materials provided with the
477 distribution.
478 • Neither the name of Red Hat nor the names of its contributors may
480 be used to endorse or promote products derived from this software
481 without specific prior written permission.
482 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
484 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
485 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
486 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
487 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
488 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
489 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
490 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
491 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
492 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
493 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
494nbdkit-1.28.2 2021-11-09 nbdkit-python-plugin(3)