1nbdkit-filter(3) NBDKIT nbdkit-filter(3)
2
6 nbdkit-filter - how to write nbdkit filters
7
9 #include <nbdkit-filter.h>
10 static int
12 myfilter_config (nbdkit_next_config *next, void *nxdata,
13 const char *key, const char *value)
14 {
15 if (strcmp (key, "myparameter") == 0) {
16 // ...
17 return 0;
18 }
19 else {
20 // pass through to next filter or plugin
21 return next (nxdata, key, value);
22 }
23 }
24 static struct nbdkit_filter filter = {
26 .name = "filter",
27 .config = myfilter_config,
28 /* etc */
29 };
30 NBDKIT_REGISTER_FILTER(filter)
32 When this has been compiled to a shared library, do:
34 nbdkit [--args ...] --filter=./myfilter.so plugin [key=value ...]
36 When debugging, use the -fv options:
38 nbdkit -fv --filter=./myfilter.so plugin [key=value ...]
40
42 One or more nbdkit filters can be placed in front of an nbdkit plugin
43 to modify the behaviour of the plugin. This manual page describes how
44 to create an nbdkit filter.
45 Filters can be used for example to limit requests to an offset/limit,
47 add copy-on-write support, or inject delays or errors (for testing).
48 Different filters can be stacked:
50 NBD ┌─────────┐ ┌─────────┐ ┌────────┐
52 client ───▶│ filter1 │───▶│ filter2 │── ─ ─ ──▶│ plugin │
53 request └─────────┘ └─────────┘ └────────┘
54 Each filter intercepts plugin functions (see nbdkit-plugin(3)) and can
56 call the next filter or plugin in the chain, modifying parameters,
57 calling before the filter function, in the middle or after. Filters
58 may even short-cut the chain. As an example, to process its own
59 parameters the filter can intercept the ".config" method:
60 static int
62 myfilter_config (nbdkit_next_config *next, void *nxdata,
63 const char *key, const char *value)
64 {
65 if (strcmp (key, "myparameter") == 0) {
66 // ...
67 // here you would handle this key, value
68 // ...
69 return 0;
70 }
71 else {
72 // pass through to next filter or plugin
73 return next (nxdata, key, value);
74 }
75 }
76 static struct nbdkit_filter filter = {
78 // ...
79 .config = myfilter_config,
80 // ...
81 };
82 The call to "next (nxdata, ...)" calls the ".config" method of the next
84 filter or plugin in the chain. In the example above any instances of
85 "myparameter=..." on the command line would not be seen by the plugin.
86 To see example filters:
88 https://gitlab.com/nbdkit/nbdkit/tree/master/filters
89 Filters must be written in C.
91 Unlike plugins, where we provide a stable ABI guarantee that permits
93 operation across version differences, filters can only be run with the
94 same version of nbdkit that they were compiled with. The reason for
95 this is two-fold: the filter API includes access to struct
96 nbdkit_next_ops that is likely to change if new callbacks are added
97 (old nbdkit cannot safely run new filters that access new methods); and
98 if we added new methods then an old filter would not see them and so
99 they would be passed unmodified through the filter, and in some cases
100 that leads to data corruption (new nbdkit cannot safely run old filters
101 unaware of new methods). Therefore, unlike plugins, you should not
102 expect to distribute filters separately from nbdkit.
103
105 All filters should start by including this header file.
106
108 All filters must define and register one "struct nbdkit_filter", which
109 contains the name of the filter and pointers to plugin methods that the
110 filter wants to intercept.
111 static struct nbdkit_filter filter = {
113 .name = "filter",
114 .longname = "My Filter",
115 .description = "This is my great filter for nbdkit",
116 .config = myfilter_config,
117 /* etc */
118 };
119 NBDKIT_REGISTER_FILTER(filter)
121 The ".name" field is the name of the filter. This is the only field
123 which is required.
124
126 nbdkit-filter.h defines some function types ("nbdkit_next_config",
127 "nbdkit_next_config_complete", "nbdkit_next_preconnect",
128 "nbdkit_next_list_exports", "nbdkit_next_default_export",
129 "nbdkit_next_open") and a structure called "struct nbdkit_next_ops".
130 These abstract the next plugin or filter in the chain. There is also
131 an opaque pointer "backend", "context" or "nxdata" which must be passed
132 along when calling these functions. The value of "backend" is stable
133 between ".after_fork", ".preconnect", ".list_exports", and
134 ".default_export", and can also be obtained by using
135 "nbdkit_context_get_backend" on the "context" parameter to ".open".
136 Meanwhile, if the filter does not use "nbdkit_context_set_next", the
138 value of "next" passed to ".prepare" has a stable lifetime that lasts
139 to the corresponding ".finalize", with all intermediate functions (such
140 as ".pread") receiving the same value for convenience. Functions where
141 "nxdata" is not reused are ".config", ".config_complete", and
142 ".get_ready", which are all called during initialization outside any
143 connections. The value of "backend" passed to ".after_fork" also
144 occurs without connections, but is shared with ".preconnect",
145 ".list_exports", and ".default_export", and can also be obtained from
146 the "context" passed to ".open", and has a lifetime that lasts to
147 ".cleanup" for use by "nbdkit_next_context_open". In turn, the value
148 of "context" passed to ".open" has a lifetime that lasts until the
149 matching ".close" for use by "nbdkit_context_get_backend" and
150 "nbdkit_context_set_next".
151 Next config, open and close
153 The filter’s ".config", ".config_complete", ".get_ready",
154 ".after_fork", ".preconnect", ".list_exports", ".default_export" and
155 ".open" methods may only call the next ".config", ".config_complete",
156 ".get_ready", ".after_fork", ".preconnect", ".list_exports",
157 ".default_export" and ".open" method in the chain (optionally for
158 ".config" and ".open").
159 The filter’s ".close" method is called when an old connection closed,
161 and this has no "next" parameter because it cannot be short-circuited.
162 "nbdkit_next"
164 The filter generally needs to call into the underlying plugin, which is
165 done via a pointer to "struct nbdkit_next_ops", also available as the
166 typedef "nbdkit_next". The most common behavior is to create a next
167 context per connection by calling the "next_open" parameter during
168 ".open", at which point the next context will be automatically provided
169 to the filter’s other methods like ".prepare", ".get_size", ".pread"
170 etc. The "nbdkit_next" struct contains a comparable set of accessors
171 to plugin methods that can be called during a connection. When using
172 automatic registration, the "next" parameter is stable between
173 ".prepare" and ".finalize", and nbdkit automatically prepares,
174 finalizes, and closes the next context at the right point in the filter
175 connection lifecycle.
176 Alternatively, the filter can manage plugin contexts manually, whether
178 to multiplex multiple client connections through a single context into
179 the plugin, or to open multiple plugin contexts to perform retries or
180 otherwise service a single client connection more efficiently. In this
181 mode of operation, the filter uses "nbdkit_next_context_open" to open a
182 plugin context using the "backend" parameter passed to ".after_fork",
183 ".preconnect", ".list_exports", ".default_export", or obtained from
184 using "nbdkit_context_get_backend" on the "context" parameter to
185 ".open". The resulting next context has a lifecycle under manual
186 control, where the filter must use "next->prepare (next)" before using
187 any other function pointers within the next context, and must reclaim
188 the memory using "next->finalize (next)" and
189 "nbdkit_next_context_close" when done. A filter using manual lifecycle
190 management may use "nbdkit_context_set_next" to associate the next
191 context into the current connection, which lets nbdkit then pass that
192 context as the "next" parameter to future connection-related functions
193 like ".pread" and take over lifecycle responsibility.
194 "nbdkit_context_get_backend"
196 "nbdkit_next_context_open"
198 "nbdkit_next_context_close"
200 "nbdkit_context_set_next"
202 nbdkit_backend *nbdkit_context_get_backend (nbdkit_context *context);
204 Obtains the backend pointer from the "context" parameter to ".open",
206 matching the backend pointer available to ".after_fork", ".preconnect",
207 ".list_exports", and ".default_export". This backend pointer has a
208 stable lifetime from the time of ".after_fork" until ".cleanup".
209 nbdkit_next *nbdkit_next_context_open (nbdkit_backend *backend,
211 int readonly, const char *exportname,
212 int shared);
213 This function attempts to open a new context into the plugin in
215 relation to the filter's current "backend". The "readonly" and
216 "exportname" parameters behave the same as documented in ".open". The
217 resulting context will be under the filter's manual lifecycle control
218 unless the filter associates it into the connection with
219 "nbdkit_context_set_next". The filter should be careful to not violate
220 any threading model restrictions of the plugin if it opens more than
221 one context.
222 If "shared" is false, this function must be called while servicing an
224 existing client connection, and the new context will share the same
225 connection details (export name, tls status, and shorter interned
226 string lifetimes) as the current connection, and thus should not be
227 used after the client connection ends. Conversely, if "shared" is
228 true, this function may be called outside of a current client
229 connection (such as during ".after_fork"), and the resulting context
230 may be freely shared among multiple client connections. In shared
231 mode, it will not be possible for the plugin to differentiate content
232 based on the client export name, the result of the plugin calling
233 nbdkit_is_tls() will depend solely whether --tls=require was on the
234 command line, the lifetime of interned strings (via
235 "nbdkit_strdup_intern" and friends) lasts for the life of the filter,
236 and the filter must take care to not expose potentially-secure
237 information from the backend to an insecure client.
238 void nbdkit_next_context_close (nbdkit_next *next);
240 This function closes a context into the plugin. If the context has
242 previously been prepared, it should first be finalized before using
243 this function. This function does not need to be called for a plugin
244 context that has been associated with the filter connection via
245 "nbdkit_context_set_next" prior to the ".close" callback.
246 nbdkit_next *nbdkit_context_set_next (nbdkit_context *context,
248 nbdkit_next *next);
249 This function associates a plugin context with the filter's current
251 connection context, given by the "context" parameter to ".open". Once
252 associated, this plugin context will be given as the "next" parameter
253 to all other connection-specific callbacks. If associated during
254 ".open", nbdkit will take care of preparing the context prior to
255 ".prepare"; if still associated before ".finalize", nbdkit will take
256 care of finalizing the context, and also for closing it. A filter may
257 also pass "NULL" for "next", to remove any association; if no plugin
258 context is associated with the connection, then filter callbacks such
259 as ".pread" will receive "NULL" for their "next" parameter.
260 This function returns the previous context that had been associated
262 with the connection prior to switching the association to "next"; this
263 result will be "NULL" if there was no previous association. The filter
264 assumes manual responsibility for any remaining lifecycle functions
265 that must be called on the returned context.
266 Using "nbdkit_next"
268 Regardless of whether the plugin context is managed automatically or
269 manually, it is possible for a filter to issue (for example) extra
270 "next->pread" calls in response to a single ".pwrite" call.
271 The "next" parameter serves two purposes: it serves as the struct to
273 access the pointers to all the plugin connection functions, and it
274 serves as the opaque data that must be passed as the first parameter to
275 those functions. For example, calling the plugin's can_flush
276 functionality would be done via
277 next-E<gt>can_flush (next)
279 Note that the semantics of the functions in "struct nbdkit_next_ops"
281 are slightly different from what a plugin implements: for example, when
282 a plugin's ".pread" returns -1 on error, the error value to advertise
283 to the client is implicit (via the plugin calling "nbdkit_set_error" or
284 setting "errno"), whereas "next->pread" exposes this via an explicit
285 parameter, allowing a filter to learn or modify this error if desired.
286 Use of "next->prepare" and "next->finalize" is only needed when
288 manually managing the plugin context lifetime.
289 Other considerations
291 You can modify parameters when you call the "next" function. However
292 be careful when modifying strings because for some methods (eg.
293 ".config") the plugin may save the string pointer that you pass along.
294 So you may have to ensure that the string is not freed for the lifetime
295 of the server; you may find "nbdkit_strdup_intern" helpful for avoiding
296 a memory leak while still obeying lifecycle constraints.
297 Note that if your filter registers a callback but in that callback it
299 doesn't call the "next" function then the corresponding method in the
300 plugin will never be called. In particular, your ".open" method, if
301 you have one, must call the "next" method if you want the underlying
302 plugin to be available to all further "nbdkit_next" use.
303
305 "struct nbdkit_filter" has some static fields describing the filter and
306 optional callback functions which can be used to intercept plugin
307 methods.
308 ".name"
310 const char *name;
311 This field (a string) is required, and must contain only ASCII
313 alphanumeric characters or non-leading dashes, and be unique amongst
314 all filters.
315 ".longname"
317 const char *longname;
318 An optional free text name of the filter. This field is used in error
320 messages.
321 ".description"
323 const char *description;
324 An optional multi-line description of the filter.
326 ".load"
328 void load (void);
329 This is called once just after the filter is loaded into memory. You
331 can use this to perform any global initialization needed by the filter.
332 ".unload"
334 void unload (void);
335 This may be called once just before the filter is unloaded from memory.
337 Note that it's not guaranteed that ".unload" will always be called (eg.
338 the server might be killed or segfault), so you should try to make the
339 filter as robust as possible by not requiring cleanup. See also
340 "SHUTDOWN" in nbdkit-plugin(3).
341 ".config"
343 int (*config) (nbdkit_next_config *next, void *nxdata,
344 const char *key, const char *value);
345 This intercepts the plugin ".config" method and can be used by the
347 filter to parse its own command line parameters. You should try to
348 make sure that command line parameter keys that the filter uses do not
349 conflict with ones that could be used by a plugin.
350 If there is an error, ".config" should call "nbdkit_error" with an
352 error message and return "-1".
353 ".config_complete"
355 int (*config_complete) (nbdkit_next_config_complete *next, void *nxdata);
356 This intercepts the plugin ".config_complete" method and can be used to
358 ensure that all parameters needed by the filter were supplied on the
359 command line.
360 If there is an error, ".config_complete" should call "nbdkit_error"
362 with an error message and return "-1".
363 ".config_help"
365 const char *config_help;
366 This optional multi-line help message should summarize any "key=value"
368 parameters that it takes. It does not need to repeat what already
369 appears in ".description".
370 If the filter doesn't take any config parameters you should probably
372 omit this.
373 ".thread_model"
375 int (*thread_model) (void);
376 Filters may tighten (but not relax) the thread model of the plugin, by
378 defining this callback. Note that while plugins use a compile-time
379 definition of "THREAD_MODEL", filters do not need to declare a model at
380 compile time; instead, this callback is called after ".config_complete"
381 and before any connections are created. See "THREADS" in
382 nbdkit-plugin(3) for a discussion of thread models.
383 The final thread model used by nbdkit is the smallest (ie. most
385 serialized) out of all the filters and the plugin, and applies for all
386 connections. Requests for a model larger than permitted by the plugin
387 are silently ignored. It is acceptable for decisions made during
388 ".config" and ".config_complete" to determine which model to request.
389 This callback is optional; if it is not present, the filter must be
391 written to handle fully parallel requests, including when multiple
392 requests are issued in parallel on the same connection, similar to a
393 plugin requesting "NBDKIT_THREAD_MODEL_PARALLEL". This ensures the
394 filter doesn't slow down other filters or plugins.
395 If there is an error, ".thread_model" should call "nbdkit_error" with
397 an error message and return "-1".
398 ".get_ready"
400 int (*get_ready) (int thread_model);
401 This optional callback is reached if the plugin ".get_ready" method
403 succeeded (if the plugin failed, nbdkit has already exited), and can be
404 used by the filter to get ready to serve requests.
405 The "thread_model" parameter informs the filter about the final thread
407 model chosen by nbdkit after considering the results of ".thread_model"
408 of all filters in the chain after ".config_complete".
409 If there is an error, ".get_ready" should call "nbdkit_error" with an
411 error message and return "-1".
412 ".after_fork"
414 int (*after_fork) (nbdkit_backend *backend);
415 This optional callback is reached after the plugin ".after_fork" method
417 has succeeded (if the plugin failed, nbdkit has already exited), and
418 can be used by the filter to start background threads. The "backend"
419 parameter is valid until ".cleanup", for creating manual contexts into
420 the backend with "nbdkit_next_context_open".
421 If there is an error, ".after_fork" should call "nbdkit_error" with an
423 error message and return "-1".
424 ".cleanup"
426 int (cleanup) (nbdkit_backend *backend);
427 This optional callback is reached once after all client connections
429 have been closed, but before the underlying plugin ".cleanup" or any
430 ".unload" callbacks. It can be used by the filter to gracefully close
431 any background threads created during ".after_fork", as well as close
432 any manual contexts into "backend" previously opened with
433 "nbdkit_next_context_open".
434 Note that it's not guaranteed that ".cleanup" will always be called
436 (eg. the server might be killed or segfault), so you should try to make
437 the filter as robust as possible by not requiring cleanup. See also
438 "SHUTDOWN" in nbdkit-plugin(3).
439 ".preconnect"
441 int (*preconnect) (nbdkit_next_preconnect *next, nbdkit_backend *nxdata,
442 int readonly);
443 This intercepts the plugin ".preconnect" method and can be used to
445 filter access to the server.
446 If there is an error, ".preconnect" should call "nbdkit_error" with an
448 error message and return "-1".
449 ".list_exports"
451 int (*list_exports) (nbdkit_next_list_exports *next, nbdkit_backend *nxdata,
452 int readonly, int is_tls,
453 struct nbdkit_exports *exports);
454 This intercepts the plugin ".list_exports" method and can be used to
456 filter which exports are advertised.
457 The "readonly" parameter matches what is passed to <.preconnect> and
459 ".open", and may be changed by the filter when calling into the plugin.
460 The "is_tls" parameter informs the filter whether TLS negotiation has
461 been completed by the client, but is not passed on to "next" because it
462 cannot be altered.
463 It is possible for filters to transform the exports list received back
465 from the layer below. Without error checking it would look like this:
466 myfilter_list_exports (...)
468 {
469 size_t i;
470 struct nbdkit_exports *exports2;
471 struct nbdkit_export e;
472 char *name, *desc;
473 exports2 = nbdkit_exports_new ();
475 next_list_exports (nxdata, readonly, exports);
476 for (i = 0; i < nbdkit_exports_count (exports2); ++i) {
477 e = nbdkit_get_export (exports2, i);
478 name = adjust (e.name);
479 desc = adjust (e.desc);
480 nbdkit_add_export (exports, name, desc);
481 free (name);
482 free (desc);
483 }
484 nbdkit_exports_free (exports2);
485 }
486 If there is an error, ".list_exports" should call "nbdkit_error" with
488 an error message and return "-1".
489 Allocating and freeing nbdkit_exports list
491 Two functions are provided to filters only for allocating and freeing
493 the list:
494 struct nbdkit_exports *nbdkit_exports_new (void);
496 Allocates and returns a new, empty exports list.
498 On error this function can return "NULL". In this case it calls
500 "nbdkit_error" as required. "errno" will be set to a suitable value.
501 void nbdkit_exports_free (struct nbdkit_exports *);
503 Frees an existing exports list.
505 Iterating over nbdkit_exports list
507 Two functions are provided to filters only to iterate over the exports
509 in order:
510 size_t nbdkit_exports_count (const struct nbdkit_exports *);
512 Returns the number of exports in the list.
514 struct nbdkit_export {
516 char *name;
517 char *description;
518 };
519 const struct nbdkit_export nbdkit_get_export (const struct nbdkit_exports *,
520 size_t i);
521 Returns a copy of the "i"'th export.
523 ".default_export"
525 const char *default_export (nbdkit_next_default_export *next,
526 nbdkit_backend *nxdata,
527 int readonly, int is_tls)
528 This intercepts the plugin ".default_export" method and can be used to
530 alter the canonical export name used in place of the default "".
531 The "readonly" parameter matches what is passed to <.preconnect> and
533 ".open", and may be changed by the filter when calling into the plugin.
534 The "is_tls" parameter informs the filter whether TLS negotiation has
535 been completed by the client, but is not passed on to "next" because it
536 cannot be altered.
537 ".open"
539 void * (*open) (nbdkit_next_open *next, nbdkit_context *context,
540 int readonly, const char *exportname, int is_tls);
541 This is called when a new client connection is opened and can be used
543 to allocate any per-connection data structures needed by the filter.
544 The handle (which is not the same as the plugin handle) is passed back
545 to other filter callbacks and could be freed in the ".close" callback.
546 Note that the handle is completely opaque to nbdkit, but it must not be
548 NULL. If you don't need to use a handle, return
549 "NBDKIT_HANDLE_NOT_NEEDED" which is a static non-NULL pointer.
550 If there is an error, ".open" should call "nbdkit_error" with an error
552 message and return "NULL".
553 This callback is optional, but if provided, it should call "next",
555 passing "readonly" and "exportname" possibly modified according to how
556 the filter plans to use the plugin ("is_tls" is not passed, because a
557 filter cannot modify it). Typically, the filter passes the same values
558 as it received, or passes readonly=true to provide a writable layer on
559 top of a read-only backend. However, it is also acceptable to attempt
560 write access to the plugin even if this filter is readonly, such as
561 when a file system mounted read-only still requires write access to the
562 underlying device in case a journal needs to be replayed for
563 consistency as part of the mounting process.
564 The "exportname" string is only guaranteed to be available during the
566 call (different than the lifetime for the return of
567 "nbdkit_export_name" used by plugins). If the filter needs to use it
568 (other than immediately passing it down to the next layer) it must take
569 a copy, although "nbdkit_strdup_intern" is useful for this task. The
570 "exportname" and "is_tls" parameters are provided so that filters do
571 not need to use the plugin-only interfaces of "nbdkit_export_name" and
572 "nbdkit_is_tls".
573 The filter should generally call "next" as its first step, to allocate
575 from the plugin outwards, so that ".close" running from the outer
576 filter to the plugin will be in reverse. Skipping a call to "next" is
577 acceptable if the filter will not access "nbdkit_next" during any of
578 the remaining callbacks reached on the same connection. The "next"
579 function is provided for convenience; the same functionality can be
580 obtained manually (other than error checking) by using the following:
581 nbdkit_context_set_next (context, nbdkit_next_context_open
583 (nbdkit_context_get_backend (context), readonly, exportname, false));
584 The value of "context" in this call has a lifetime that lasts until the
586 counterpart ".close", and it is this value that may be passed to
587 "nbdkit_context_get_backend" to obtain the "backend" parameter used to
588 open a plugin context with "nbdkit_next_context_open", as well as the
589 "context" parameter used to associate a plugin context into the current
590 connection with "nbdkit_context_set_next".
591 ".close"
593 void (*close) (void *handle);
594 This is called when the client closes the connection. It should clean
596 up any per-connection resources used by the filter. It is called
597 beginning with the outermost filter and ending with the plugin (the
598 opposite order of ".open" if all filters call "next" first), although
599 this order technically does not matter since the callback cannot report
600 failures or access the underlying plugin.
601 ".prepare"
603 ".finalize"
604 int (*prepare) (nbdkit_next *next, void *handle, int readonly);
605 int (*finalize) (nbdkit_next *next, void *handle);
606 These two methods can be used to perform any necessary operations just
608 after opening the connection (".prepare") or just before closing the
609 connection (".finalize").
610 For example if you need to scan the underlying disk to check for a
612 partition table, you could do it in your ".prepare" method (calling the
613 plugin's ".get_size" and ".pread" methods via "next"). Or if you need
614 to cleanly update superblock data in the image on close you can do it
615 in your ".finalize" method (calling the plugin's ".pwrite" method).
616 Doing these things in the filter's ".open" or ".close" method is not
617 possible without using manual context lifecycle management.
618 For ".prepare", the value of "readonly" is the same as was passed to
620 ".open", declaring how this filter will be used.
621 Note that nbdkit performs sanity checking on requests made to the
623 underlying plugin; for example, "next->pread" cannot be called on a
624 given connection unless "next->get_size" has first been called at least
625 once in the same connection (to ensure the read requests are in
626 bounds), and "next->pwrite" further requires an earlier successful call
627 to "next->can_write". In many filters, these prerequisites will be
628 automatically called during the client negotiation phase, but there are
629 cases where a filter overrides query functions or makes I/O calls into
630 the plugin before handshaking is complete, where the filter needs to
631 make those prerequisite calls manually during ".prepare".
632 While there are "next->prepare" and "next->finalize" functions, these
634 are different from other filter methods, in that any plugin context
635 associated with the current connection (via the "next" parameter to
636 ".open", or via "nbdkit_context_set_next", is prepared and finalized
637 automatically by nbdkit, so they are only used during manual lifecycle
638 management. Prepare methods are called starting with the filter
639 closest to the plugin and proceeding outwards (matching the order of
640 ".open" if all filters call "next" before doing anything locally), and
641 only when an outer filter did not skip the "next" call during ".open".
642 Finalize methods are called in the reverse order of prepare methods,
643 with the outermost filter first (and matching the order of ".close"),
644 and only if the prepare method succeeded.
645 If there is an error, both callbacks should call "nbdkit_error" with an
647 error message and return "-1". An error in ".prepare" is reported to
648 the client, but leaves the connection open (a client may try again with
649 a different export name, for example); while an error in ".finalize"
650 forces the client to disconnect.
651 ".get_size"
653 int64_t (*get_size) (nbdkit_next *next, void *handle);
654 This intercepts the plugin ".get_size" method and can be used to read
656 or modify the apparent size of the block device that the NBD client
657 will see.
658 The returned size must be ≥ 0. If there is an error, ".get_size"
660 should call "nbdkit_error" with an error message and return "-1". This
661 function is only called once per connection and cached by nbdkit.
662 Similarly, repeated calls to "next->get_size" will return a cached
663 value.
664 ".export_description"
666 const char *export_description (nbdkit_next *next, void *handle);
667 This intercepts the plugin ".export_description" method and can be used
669 to read or modify the export description that the NBD client will see.
670 ".can_write"
672 ".can_flush"
673 ".is_rotational"
674 ".can_trim"
675 ".can_zero"
676 ".can_fast_zero"
677 ".can_extents"
678 ".can_fua"
679 ".can_multi_conn"
680 ".can_cache"
681 int (*can_write) (nbdkit_next *next, void *handle);
682 int (*can_flush) (nbdkit_next *next, void *handle);
683 int (*is_rotational) (nbdkit_next *next, void *handle);
684 int (*can_trim) (nbdkit_next *next, void *handle);
685 int (*can_zero) (nbdkit_next *next, void *handle);
686 int (*can_fast_zero) (nbdkit_next *next, void *handle);
687 int (*can_extents) (nbdkit_next *next, void *handle);
688 int (*can_fua) (nbdkit_next *next, void *handle);
689 int (*can_multi_conn) (nbdkit_next *next, void *handle);
690 int (*can_cache) (nbdkit_next *next, void *handle);
691 These intercept the corresponding plugin methods, and control feature
693 bits advertised to the client.
694 Of note, the semantics of ".can_zero" callback in the filter are
696 slightly different from the plugin, and must be one of three success
697 values visible only to filters:
698 "NBDKIT_ZERO_NONE"
700 Completely suppress advertisement of write zero support (this can
701 only be done from filters, not plugins).
702 "NBDKIT_ZERO_EMULATE"
704 Inform nbdkit that write zeroes should immediately fall back to
705 ".pwrite" emulation without trying ".zero" (this value is returned
706 by "next->can_zero" if the plugin returned false in its
707 ".can_zero").
708 "NBDKIT_ZERO_NATIVE"
710 Inform nbdkit that write zeroes should attempt to use ".zero",
711 although it may still fall back to ".pwrite" emulation for
712 "ENOTSUP" or "EOPNOTSUPP" failures (this value is returned by
713 "next->can_zero" if the plugin returned true in its ".can_zero").
714 Remember that most of the feature check functions return merely a
716 boolean success value, while ".can_zero", ".can_fua" and ".can_cache"
717 have three success values.
718 The difference between ".can_fua" values may affect choices made in the
720 filter: when splitting a write request that requested FUA from the
721 client, if "next->can_fua" returns "NBDKIT_FUA_NATIVE", then the filter
722 should pass the FUA flag on to each sub-request; while if it is known
723 that FUA is emulated by a flush because of a return of
724 "NBDKIT_FUA_EMULATE", it is more efficient to only flush once after all
725 sub-requests have completed (often by passing "NBDKIT_FLAG_FUA" on to
726 only the final sub-request, or by dropping the flag and ending with a
727 direct call to "next->flush").
728 If there is an error, the callback should call "nbdkit_error" with an
730 error message and return "-1". These functions are called at most once
731 per connection and cached by nbdkit. Similarly, repeated calls to any
732 of the "nbdkit_next" counterparts will return a cached value; by
733 calling into the plugin during ".prepare", you can ensure that later
734 use of the cached values during data commands like <.pwrite> will not
735 fail.
736 ".pread"
738 int (*pread) (nbdkit_next *next,
739 void *handle, void *buf, uint32_t count, uint64_t offset,
740 uint32_t flags, int *err);
741 This intercepts the plugin ".pread" method and can be used to read or
743 modify data read by the plugin.
744 The parameter "flags" exists in case of future NBD protocol extensions;
746 at this time, it will be 0 on input, and the filter should not pass any
747 flags to "next->pread".
748 If there is an error (including a short read which couldn't be
750 recovered from), ".pread" should call "nbdkit_error" with an error
751 message and return -1 with "err" set to the positive errno value to
752 return to the client.
753 ".pwrite"
755 int (*pwrite) (nbdkit_next *next,
756 void *handle,
757 const void *buf, uint32_t count, uint64_t offset,
758 uint32_t flags, int *err);
759 This intercepts the plugin ".pwrite" method and can be used to modify
761 data written by the plugin.
762 This function will not be called if ".can_write" returned false; in
764 turn, the filter should not call "next->pwrite" if "next->can_write"
765 did not return true.
766 The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on
768 the result of ".can_fua". In turn, the filter should only pass
769 "NBDKIT_FLAG_FUA" on to "next->pwrite" if "next->can_fua" returned a
770 positive value.
771 If there is an error (including a short write which couldn't be
773 recovered from), ".pwrite" should call "nbdkit_error" with an error
774 message and return -1 with "err" set to the positive errno value to
775 return to the client.
776 ".flush"
778 int (*flush) (nbdkit_next *next,
779 void *handle, uint32_t flags, int *err);
780 This intercepts the plugin ".flush" method and can be used to modify
782 flush requests.
783 This function will not be called if ".can_flush" returned false; in
785 turn, the filter should not call "next->flush" if "next->can_flush" did
786 not return true.
787 The parameter "flags" exists in case of future NBD protocol extensions;
789 at this time, it will be 0 on input, and the filter should not pass any
790 flags to "next->flush".
791 If there is an error, ".flush" should call "nbdkit_error" with an error
793 message and return -1 with "err" set to the positive errno value to
794 return to the client.
795 ".trim"
797 int (*trim) (nbdkit_next *next,
798 void *handle, uint32_t count, uint64_t offset,
799 uint32_t flags, int *err);
800 This intercepts the plugin ".trim" method and can be used to modify
802 trim requests.
803 This function will not be called if ".can_trim" returned false; in
805 turn, the filter should not call "next->trim" if "next->can_trim" did
806 not return true.
807 The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on
809 the result of ".can_fua". In turn, the filter should only pass
810 "NBDKIT_FLAG_FUA" on to "next->trim" if "next->can_fua" returned a
811 positive value.
812 If there is an error, ".trim" should call "nbdkit_error" with an error
814 message and return -1 with "err" set to the positive errno value to
815 return to the client.
816 ".zero"
818 int (*zero) (nbdkit_next *next,
819 void *handle, uint32_t count, uint64_t offset, uint32_t flags,
820 int *err);
821 This intercepts the plugin ".zero" method and can be used to modify
823 zero requests.
824 This function will not be called if ".can_zero" returned
826 "NBDKIT_ZERO_NONE"; in turn, the filter should not call "next->zero" if
827 "next->can_zero" returned "NBDKIT_ZERO_NONE".
828 On input, the parameter "flags" may include "NBDKIT_FLAG_MAY_TRIM"
830 unconditionally, "NBDKIT_FLAG_FUA" based on the result of ".can_fua",
831 and "NBDKIT_FLAG_FAST_ZERO" based on the result of ".can_fast_zero".
832 In turn, the filter may pass "NBDKIT_FLAG_MAY_TRIM" unconditionally,
833 but should only pass "NBDKIT_FLAG_FUA" or "NBDKIT_FLAG_FAST_ZERO" on to
834 "next->zero" if the corresponding "next->can_fua" or
835 "next->can_fast_zero" returned a positive value.
836 Note that unlike the plugin ".zero" which is permitted to fail with
838 "ENOTSUP" or "EOPNOTSUPP" to force a fallback to ".pwrite", the
839 function "next->zero" will not fail with "err" set to "ENOTSUP" or
840 "EOPNOTSUPP" unless "NBDKIT_FLAG_FAST_ZERO" was used, because otherwise
841 the fallback has already taken place.
842 If there is an error, ".zero" should call "nbdkit_error" with an error
844 message and return -1 with "err" set to the positive errno value to
845 return to the client. The filter should not fail with "ENOTSUP" or
846 "EOPNOTSUPP" unless "flags" includes "NBDKIT_FLAG_FAST_ZERO" (while
847 plugins have automatic fallback to ".pwrite", filters do not).
848 ".extents"
850 int (*extents) (nbdkit_next *next,
851 void *handle, uint32_t count, uint64_t offset, uint32_t flags,
852 struct nbdkit_extents *extents,
853 int *err);
854 This intercepts the plugin ".extents" method and can be used to modify
856 extent requests.
857 This function will not be called if ".can_extents" returned false; in
859 turn, the filter should not call "next->extents" if "next->can_extents"
860 did not return true.
861 It is possible for filters to transform the extents list received back
863 from the layer below. Without error checking it would look like this:
864 myfilter_extents (..., uint32_t count, uint64_t offset, ...)
866 {
867 size_t i;
868 struct nbdkit_extents *extents2;
869 struct nbdkit_extent e;
870 int64_t size;
871 size = next->get_size (next);
873 extents2 = nbdkit_extents_new (offset + shift, size);
874 next->extents (next, count, offset + shift, flags, extents2, err);
875 for (i = 0; i < nbdkit_extents_count (extents2); ++i) {
876 e = nbdkit_get_extent (extents2, i);
877 e.offset -= shift;
878 nbdkit_add_extent (extents, e.offset, e.length, e.type);
879 }
880 nbdkit_extents_free (extents2);
881 }
882 If there is an error, ".extents" should call "nbdkit_error" with an
884 error message and return -1 with "err" set to the positive errno value
885 to return to the client.
886 Allocating and freeing nbdkit_extents list
888 Two functions are provided to filters only for allocating and freeing
890 the map:
891 struct nbdkit_extents *nbdkit_extents_new (uint64_t start, uint64_t end);
893 Allocates and returns a new, empty extents list. The "start" parameter
895 is the start of the range described in the list, and the "end"
896 parameter is the offset of the byte beyond the end. Normally you would
897 pass in "offset" as the start and the size of the plugin as the end,
898 but for filters which adjust offsets, they should pass in the adjusted
899 offset.
900 On error this function can return "NULL". In this case it calls
902 "nbdkit_error" and/or "nbdkit_set_error" as required. "errno" will be
903 set to a suitable value.
904 void nbdkit_extents_free (struct nbdkit_extents *);
906 Frees an existing extents list.
908 Iterating over nbdkit_extents list
910 Two functions are provided to filters only to iterate over the extents
912 in order:
913 size_t nbdkit_extents_count (const struct nbdkit_extents *);
915 Returns the number of extents in the list.
917 struct nbdkit_extent {
919 uint64_t offset;
920 uint64_t length;
921 uint32_t type;
922 };
923 struct nbdkit_extent nbdkit_get_extent (const struct nbdkit_extents *,
924 size_t i);
925 Returns a copy of the "i"'th extent.
927 Reading the full extents from the plugin
929 A convenience function is provided to filters only which makes one or
931 more requests to the underlying plugin until we have a full set of
932 extents covering the region "[offset..offset+count-1]".
933 struct nbdkit_extents *nbdkit_extents_full (
935 nbdkit_next *next,
936 uint32_t count, uint64_t offset,
937 uint32_t flags, int *err);
938 Note this allocates a new "struct nbdkit_extents" which the caller must
940 free. "flags" is passed through to the underlying plugin, but
941 "NBDKIT_FLAG_REQ_ONE" is removed from the set of flags so that the
942 plugin returns as much information as possible (this is usually what
943 you want).
944 On error this function can return "NULL". In this case it calls
946 "nbdkit_error" and/or "nbdkit_set_error" as required. *err will be set
947 to a suitable value.
948 Enforcing alignment of an nbdkit_extents list
950 A convenience function is provided to filters only which makes it
952 easier to ensure that the client only encounters aligned extents.
953 int nbdkit_extents_aligned (nbdkit_next *next,
955 uint32_t count, uint64_t offset,
956 uint32_t flags, uint32_t align,
957 struct nbdkit_extents *extents, int *err);
958 Calls "next->extents" as needed until at least "align" bytes are
960 obtained, where "align" is a power of 2. Anywhere the underlying
961 plugin returns differing extents within "align" bytes, this function
962 treats that portion of the disk as a single extent with zero and sparse
963 status bits determined by the intersection of all underlying extents.
964 It is an error to call this function with "count" or "offset" that is
965 not already aligned.
966 ".cache"
968 int (*cache) (nbdkit_next *next,
969 void *handle, uint32_t count, uint64_t offset,
970 uint32_t flags, int *err);
971 This intercepts the plugin ".cache" method and can be used to modify
973 cache requests.
974 This function will not be called if ".can_cache" returned
976 "NBDKIT_CACHE_NONE" or "NBDKIT_CACHE_EMULATE"; in turn, the filter
977 should not call "next->cache" unless "next->can_cache" returned
978 "NBDKIT_CACHE_NATIVE".
979 The parameter "flags" exists in case of future NBD protocol extensions;
981 at this time, it will be 0 on input, and the filter should not pass any
982 flags to "next->cache".
983 If there is an error, ".cache" should call "nbdkit_error" with an error
985 message and return -1 with "err" set to the positive errno value to
986 return to the client.
987
989 If there is an error in the filter itself, the filter should call
990 "nbdkit_error" to report an error message. If the callback is involved
991 in serving data, the explicit "err" parameter determines the error code
992 that will be sent to the client; other callbacks should return the
993 appropriate error indication, eg. "NULL" or "-1".
994 "nbdkit_error" has the following prototype and works like printf(3):
996 void nbdkit_error (const char *fs, ...);
998 void nbdkit_verror (const char *fs, va_list args);
999 For convenience, "nbdkit_error" preserves the value of "errno", and
1001 also supports the glibc extension of a single %m in a format string
1002 expanding to "strerror(errno)", even on platforms that don't support
1003 that natively.
1004
1006 Run the server with -f and -v options so it doesn't fork and you can
1007 see debugging information:
1008 nbdkit -fv --filter=./myfilter.so plugin [key=value [key=value [...]]]
1010 To print debugging information from within the filter, call
1012 "nbdkit_debug", which has the following prototype and works like
1013 printf(3):
1014 void nbdkit_debug (const char *fs, ...);
1016 void nbdkit_vdebug (const char *fs, va_list args);
1017 For convenience, "nbdkit_debug" preserves the value of "errno", and
1019 also supports the glibc extension of a single %m in a format string
1020 expanding to "strerror(errno)", even on platforms that don't support
1021 that natively. Note that "nbdkit_debug" only prints things when the
1022 server is in verbose mode (-v option).
1023 Debug Flags
1025 Debug Flags in filters work exactly the same way as plugins. See
1026 "Debug Flags" in nbdkit-plugin(3).
1027
1029 The filter is a "*.so" file and possibly a manual page. You can of
1030 course install the filter "*.so" file wherever you want, and users will
1031 be able to use it by running:
1032 nbdkit --filter=/path/to/filter.so plugin [args]
1034 However if the shared library has a name of the form
1036 "nbdkit-name-filter.so" and if the library is installed in the
1037 $filterdir directory, then users can be run it by only typing:
1038 nbdkit --filter=name plugin [args]
1040 The location of the $filterdir directory is set when nbdkit is compiled
1042 and can be found by doing:
1043 nbdkit --dump-config
1045 If using the pkg-config/pkgconf system then you can also find the
1047 filter directory at compile time by doing:
1048 pkg-config nbdkit --variable=filterdir
1050
1052 nbdkit provides a pkg-config/pkgconf file called "nbdkit.pc" which
1053 should be installed on the correct path when the nbdkit development
1054 environment is installed. You can use this in autoconf configure.ac
1055 scripts to test for the development environment:
1056 PKG_CHECK_MODULES([NBDKIT], [nbdkit >= 1.2.3])
1058 The above will fail unless nbdkit ≥ 1.2.3 and the header file is
1060 installed, and will set "NBDKIT_CFLAGS" and "NBDKIT_LIBS" appropriately
1061 for compiling filters.
1062 You can also run pkg-config/pkgconf directly, for example:
1064 if ! pkg-config nbdkit --exists; then
1066 echo "you must install the nbdkit development environment"
1067 exit 1
1068 fi
1069 You can also substitute the filterdir variable by doing:
1071 PKG_CHECK_VAR([NBDKIT_FILTERDIR], [nbdkit], [filterdir])
1073 which defines "$(NBDKIT_FILTERDIR)" in automake-generated Makefiles.
1075
1077 nbdkit(1), nbdkit-plugin(3).
1078 Standard filters provided by nbdkit:
1080 nbdkit-blocksize-filter(1), nbdkit-cache-filter(1),
1082 nbdkit-cacheextents-filter(1), nbdkit-checkwrite-filter(1),
1083 nbdkit-cow-filter(1), nbdkit-ddrescue-filter(1),
1084 nbdkit-delay-filter(1), nbdkit-error-filter(1),
1085 nbdkit-exitlast-filter(1), nbdkit-exitwhen-filter(1),
1086 nbdkit-exportname-filter(1), nbdkit-ext2-filter(1),
1087 nbdkit-extentlist-filter(1), nbdkit-fua-filter(1),
1088 nbdkit-gzip-filter(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1),
1089 nbdkit-log-filter(1), nbdkit-multi-conn-filter(1),
1090 nbdkit-nocache-filter(1), nbdkit-noextents-filter(1),
1091 nbdkit-nofilter-filter(1), nbdkit-noparallel-filter(1),
1092 nbdkit-nozero-filter(1), nbdkit-offset-filter(1),
1093 nbdkit-partition-filter(1), nbdkit-pause-filter(1),
1094 nbdkit-rate-filter(1), nbdkit-readahead-filter(1),
1095 nbdkit-retry-filter(1), nbdkit-stats-filter(1), nbdkit-swab-filter(1),
1096 nbdkit-tar-filter(1), nbdkit-tls-fallback-filter(1),
1097 nbdkit-truncate-filter(1), nbdkit-xz-filter(1) .
1098
1100 Eric Blake
1101 Richard W.M. Jones
1103
1105 Copyright (C) 2013-2020 Red Hat Inc.
1106
1108 Redistribution and use in source and binary forms, with or without
1109 modification, are permitted provided that the following conditions are
1110 met:
1111 • Redistributions of source code must retain the above copyright
1113 notice, this list of conditions and the following disclaimer.
1114 • Redistributions in binary form must reproduce the above copyright
1116 notice, this list of conditions and the following disclaimer in the
1117 documentation and/or other materials provided with the
1118 distribution.
1119 • Neither the name of Red Hat nor the names of its contributors may
1121 be used to endorse or promote products derived from this software
1122 without specific prior written permission.
1123 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
1125 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
1126 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
1127 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
1128 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
1129 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
1130 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
1131 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
1132 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
1133 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
1134 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
1135nbdkit-1.28.2 2021-11-09 nbdkit-filter(3)