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.disconnect(force)"
107 Disconnect from the client. If "force" is "True" then nbdkit will
109 disconnect the client immediately.
110 "nbdkit.export_name()"
112 Return the export name negotiated with the client as a Unicode string.
114 Note this should not be trusted because the client can send whatever it
115 wants.
116 "nbdkit.parse_size(str)"
118 Parse a string (such as "100M") into a size in bytes. Wraps the
120 nbdkit_parse_size() C function.
121 "nbdkit.parse_probability(what, str)"
123 Parse a string (such as "100%") into a probability, returning a
125 floating point number. Wraps the nbdkit_parse_probability() C function.
126 "nbdkit.set_error(err)"
128 Record "err" as the reason you are about to throw an exception. "err"
130 should correspond to usual errno values, where it may help to "import
131 errno".
132 "nbdkit.shutdown()"
134 Request asynchronous server shutdown.
136 Module constants
138 After "import nbdkit" the following constants are available. These are
139 used in the callbacks below.
140 "nbdkit.THREAD_MODEL_SERIALIZE_CONNECTIONS"
142 "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS"
143 "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS"
144 "nbdkit.THREAD_MODEL_PARALLEL"
145 Possible return values from thread_model().
146 "nbdkit.FLAG_MAY_TRIM"
148 "nbdkit.FLAG_FUA"
149 "nbdkit.FLAG_REQ_ONE"
150 "nbdkit.FLAG_FAST_ZERO"
151 Flags bitmap passed to certain plugin callbacks. Not all callbacks
152 with a flags parameter use all of these flags, consult the
153 documentation below and nbdkit-plugin(3).
154 "nbdkit.FUA_NONE"
156 "nbdkit.FUA_EMULATE"
157 "nbdkit.FUA_NATIVE"
158 Possible return values from can_fua().
159 "nbdkit.CACHE_NONE"
161 "nbdkit.CACHE_EMULATE"
162 "nbdkit.CACHE_NATIVE"
163 Possible return values from can_cache().
164 "nbdkit.EXTENT_HOLE"
166 "nbdkit.EXTENT_ZERO"
167 Used in the "type" field returned by extents().
168 Threads
170 The thread model for Python callbacks defaults to
171 "nbdkit.THREAD_MODEL_SERIALIZE_ALL_REQUESTS".
172 Since nbdkit 1.22 it has been possible to set this by implementing a
174 thread_model() function which returns one of the constants
175 "nbdkit.THREAD_MODEL_*".
176 The Python Global Interpreter Lock (GIL) is still used, so Python code
178 does not run in parallel. However if a plugin callback calls a library
179 which blocks (eg. to make an HTTP request), then another callback might
180 be executed in parallel. Plugins which use
181 "nbdkit.THREAD_MODEL_SERIALIZE_REQUESTS" or
182 "nbdkit.THREAD_MODEL_PARALLEL" may need to use locks on shared data.
183 Exceptions
185 Python callbacks should throw exceptions to indicate errors. Remember
186 to use "nbdkit.set_error" if you need to control which error is sent
187 back to the client; if omitted, the client will see an error of "EIO".
188 Python callbacks
190 This just documents the arguments to the callbacks in Python, and any
191 way that they differ from the C callbacks. In all other respects they
192 work the same way as the C callbacks, so you should go and read
193 nbdkit-plugin(3).
194 "dump_plugin"
196 (Optional)
197 There are no arguments or return value.
199 "config"
201 (Optional)
202 def config(key, value):
204 # no return value
205 "config_complete"
207 (Optional)
208 There are no arguments or return value.
210 "thread_model"
212 (Optional, nbdkit ≥ 1.22)
213 def thread_model():
215 return nbdkit.THEAD_MODEL_SERIALIZE_ALL_REQUESTS
216 See "Threads" above.
218 "get_ready"
220 (Optional)
221 There are no arguments or return value.
223 "after_fork"
225 (Optional, nbdkit ≥ 1.26)
226 There are no arguments or return value.
228 "cleanup"
230 (Optional, nbdkit ≥ 1.28)
231 There are no arguments or return value.
233 "list_exports"
235 (Optional)
236 def list_exports(readonly, is_tls):
238 # return an iterable object (eg. list) of
239 # (name, description) tuples or bare names:
240 return [ (name1, desc1), name2, (name3, desc3), ... ]
241 "default_export"
243 (Optional)
244 def default_export(readonly, is_tls):
246 # return a string
247 return "name"
248 "preconnect"
250 (Optional, nbdkit ≥ 1.26)
251 def preconnect(readonly):
253 # no return value
254 "open"
256 (Required)
257 def open(readonly):
259 # return handle
260 You can return any Python value (even "None") as the handle. It is
262 passed back as the first arg 'h' in subsequent calls. To return an
263 error from this method you must throw an exception.
264 "close"
266 (Optional)
267 def close(h):
269 # no return value
270 After "close" returns, the reference count of the handle is
272 decremented in the C part, which usually means that the handle and
273 its contents will be garbage collected.
274 "export_description"
276 (Optional)
277 def export_description(h):
279 # return a string
280 return "description"
281 "get_size"
283 (Required)
284 def get_size(h):
286 # return the size of the disk
287 "block_size"
289 (Option)
290 def block_size(h):
292 # return triple (minimum, preferred, maximum) block size
293 "is_rotational"
295 (Optional)
296 def is_rotational(h):
298 # return a boolean
299 "can_multi_conn"
301 (Optional)
302 def can_multi_conn(h):
304 # return a boolean
305 "can_write"
307 (Optional)
308 def can_write(h):
310 # return a boolean
311 "can_flush"
313 (Optional)
314 def can_flush(h):
316 # return a boolean
317 "can_trim"
319 (Optional)
320 def can_trim(h):
322 # return a boolean
323 "can_zero"
325 (Optional)
326 def can_zero(h):
328 # return a boolean
329 "can_fast_zero"
331 (Optional)
332 def can_fast_zero(h):
334 # return a boolean
335 "can_fua"
337 (Optional)
338 def can_fua(h):
340 # return nbdkit.FUA_NONE or nbdkit.FUA_EMULATE
341 # or nbdkit.FUA_NATIVE
342 "can_cache"
344 (Optional)
345 def can_cache(h):
347 # return nbdkit.CACHE_NONE or nbdkit.CACHE_EMULATE
348 # or nbdkit.CACHE_NATIVE
349 "can_extents"
351 (Optional)
352 def can_extents(h):
354 # return a boolean
355 "pread"
357 (Required)
358 def pread(h, buf, offset, flags):
360 # read into the buffer
361 The body of your "pread" function should read exactly len(buf)
363 bytes of data starting at disk "offset" and write it into the
364 buffer "buf". "flags" is always 0.
365 NBD only supports whole reads, so your function should try to read
367 the whole region (perhaps requiring a loop). If the read fails or
368 is partial, your function should throw an exception, optionally
369 using "nbdkit.set_error" first.
370 "pwrite"
372 (Optional)
373 def pwrite(h, buf, offset, flags):
375 length = len(buf)
376 # no return value
377 The body of your "pwrite" function should write the buffer "buf" to
379 the disk. You should write "count" bytes to the disk starting at
380 "offset". "flags" may contain "nbdkit.FLAG_FUA".
381 NBD only supports whole writes, so your function should try to
383 write the whole region (perhaps requiring a loop). If the write
384 fails or is partial, your function should throw an exception,
385 optionally using "nbdkit.set_error" first.
386 "flush"
388 (Optional)
389 def flush(h, flags):
391 # no return value
392 The body of your "flush" function should do a sync(2) or
394 fdatasync(2) or equivalent on the backing store. "flags" is always
395 0.
396 If the flush fails, your function should throw an exception,
398 optionally using "nbdkit.set_error" first.
399 "trim"
401 (Optional)
402 def trim(h, count, offset, flags):
404 # no return value
405 The body of your "trim" function should "punch a hole" in the
407 backing store. "flags" may contain "nbdkit.FLAG_FUA". If the trim
408 fails, your function should throw an exception, optionally using
409 "nbdkit.set_error" first.
410 "zero"
412 (Optional)
413 def zero(h, count, offset, flags):
415 # no return value
416 The body of your "zero" function should ensure that "count" bytes
418 of the disk, starting at "offset", will read back as zero. "flags"
419 is a bitmask which may include "nbdkit.FLAG_MAY_TRIM",
420 "nbdkit.FLAG_FUA", "nbdkit.FLAG_FAST_ZERO".
421 NBD only supports whole writes, so your function should try to
423 write the whole region (perhaps requiring a loop).
424 If the write fails or is partial, your function should throw an
426 exception, optionally using "nbdkit.set_error" first. In
427 particular, if you would like to automatically fall back to
428 "pwrite" (perhaps because there is nothing to optimize if
429 "flags & nbdkit.FLAG_MAY_TRIM" is false), use
430 "nbdkit.set_error(errno.EOPNOTSUPP)".
431 "cache"
433 (Optional)
434 def cache(h, count, offset, flags):
436 # no return value
437 The body of your "cache" function should prefetch data in the
439 indicated range.
440 If the cache operation fails, your function should throw an
442 exception, optionally using "nbdkit.set_error" first.
443 "extents"
445 (Optional)
446 def extents(h, count, offset, flags):
448 # return an iterable object (eg. list) of
449 # (offset, length, type) tuples:
450 return [ (off1, len1, type1), (off2, len2, type2), ... ]
451 Missing callbacks
453 Missing: "load"
454 This is not needed since you can use regular Python mechanisms like
455 top level statements to run code when the module is loaded.
456 Missing: "unload"
458 This is missing, but in nbdkit ≥ 1.28 you can put code in the
459 cleanup() function to have it run when nbdkit exits. In earlier
460 versions of nbdkit, using a Python atexit handler is recommended.
461 Missing: "name", "version", "longname", "description", "config_help",
463 "magic_config_key".
464 These are not yet supported.
465
467 $plugindir/nbdkit-python-plugin.so
468 The plugin.
469 Use "nbdkit --dump-config" to find the location of $plugindir.
471
473 "nbdkit-python-plugin" first appeared in nbdkit 1.2.
474
476 nbdkit(1), nbdkit-plugin(3), python(1).
477
479 Eric Blake
480 Richard W.M. Jones
482 Nir Soffer
484
486 Copyright Red Hat
487
489 Redistribution and use in source and binary forms, with or without
490 modification, are permitted provided that the following conditions are
491 met:
492 • Redistributions of source code must retain the above copyright
494 notice, this list of conditions and the following disclaimer.
495 • Redistributions in binary form must reproduce the above copyright
497 notice, this list of conditions and the following disclaimer in the
498 documentation and/or other materials provided with the
499 distribution.
500 • Neither the name of Red Hat nor the names of its contributors may
502 be used to endorse or promote products derived from this software
503 without specific prior written permission.
504 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
506 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
507 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
508 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
509 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
510 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
511 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
512 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
513 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
514 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
515 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
516nbdkit-1.36.2 2023-11-26 nbdkit-python-plugin(3)