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nbdkit-filter(3) NBDKIT nbdkit-filter(3)

NAME

nbdkit-filter - how to write nbdkit filters

SYNOPSIS

 #include <nbdkit-filter.h>
 
 #define THREAD_MODEL NBDKIT_THREAD_MODEL_PARALLEL
 
 static int
 myfilter_config (nbdkit_next_config *next, void *nxdata,
                  const char *key, const char *value)
 {
   if (strcmp (key, "myparameter") == 0) {
     // ...
     return 0;
   }
   else {
     // pass through to next filter or plugin
     return next (nxdata, key, value);
   }
 }
 
 static struct nbdkit_filter filter = {
   .name              = "filter",
   .config            = myfilter_config,
   /* etc */
 };
 
 NBDKIT_REGISTER_FILTER(filter)

When this has been compiled to a shared library, do:

 nbdkit [--args ...] --filter=./myfilter.so plugin [key=value ...]

When debugging, use the -fv options:

 nbdkit -fv --filter=./myfilter.so plugin [key=value ...]

DESCRIPTION

One or more nbdkit filters can be placed in front of an nbdkit plugin to modify the behaviour of the plugin. This manual page describes how to create an nbdkit filter.

Filters can be used for example to limit requests to an offset/limit, add copy-on-write support, or inject delays or errors (for testing).

Different filters can be stacked:

     NBD     ┌─────────┐    ┌─────────┐          ┌────────┐
  client ───▶│ filter1 │───▶│ filter2 │── ─ ─ ──▶│ plugin │
 request     └─────────┘    └─────────┘          └────────┘

Each filter intercepts plugin functions (see nbdkit-plugin(3)) and can call the next filter or plugin in the chain, modifying parameters, calling before the filter function, in the middle or after. Filters may even short-cut the chain. As an example, to process its own parameters the filter can intercept the ".config" method:

 static int
 myfilter_config (nbdkit_next_config *next, void *nxdata,
                  const char *key, const char *value)
 {
   if (strcmp (key, "myparameter") == 0) {
     // ...
     // here you would handle this key, value
     // ...
     return 0;
   }
   else {
     // pass through to next filter or plugin
     return next (nxdata, key, value);
   }
 }
 
 static struct nbdkit_filter filter = {
   // ...
   .config            = myfilter_config,
   // ...
 };

The call to "next (nxdata, ...)" calls the ".config" method of the next filter or plugin in the chain. In the example above any instances of "myparameter=..." on the command line would not be seen by the plugin.

To see example filters: https://github.com/libguestfs/nbdkit/tree/master/filters

Filters must be written in C.

Unlike plugins, where we provide a stable ABI guarantee that permits operation across version differences, filters can only be run with the same version of nbdkit that they were compiled with. The reason for this is two-fold: the filter API includes access to struct nbdkit_next_ops that is likely to change if new callbacks are added (old nbdkit cannot safely run new filters that access new methods); and if we added new methods then an old filter would not see them and so they would be passed unmodified through the filter, and in some cases that leads to data corruption (new nbdkit cannot safely run old filters unaware of new methods). Therefore, unlike plugins, you should not expect to distribute filters separately from nbdkit.

"#include <nbdkit-filter.h>"

All filters should start by including this header file.

"#define THREAD_MODEL"

All filters must define a thread model. See "THREADS" in nbdkit-plugin(3) for a discussion of thread models.

The final thread model is the smallest (ie. most serialized) out of all the filters and the plugin. Filters cannot alter the thread model to make it larger (more parallel).

If possible filters should be be written to handle fully parallel requests ("NBDKIT_THREAD_MODEL_PARALLEL", even multiple requests issued in parallel on the same connection). This ensures that they don't slow down other filters or plugins.

"struct nbdkit_filter"

All filters must define and register one "struct nbdkit_filter", which contains the name of the filter and pointers to plugin methods that the filter wants to intercept. 

 static struct nbdkit_filter filter = {
   .name              = "filter",
   .longname          = "My Filter",
   .description       = "This is my great filter for nbdkit",
   .config            = myfilter_config,
   /* etc */
 };
 
 NBDKIT_REGISTER_FILTER(filter)

The ".name" field is the name of the filter. This is the only field which is required.

NEXT PLUGIN

nbdkit-filter.h defines three function types ("nbdkit_next_config", "nbdkit_next_config_complete", "nbdkit_next_open") and a structure called "struct nbdkit_next_ops". These abstract the next plugin or filter in the chain. There is also an opaque pointer "nxdata" which must be passed along when calling these functions.

The filter’s ".config", ".config_complete" and ".open" methods may only call the next ".config", ".config_complete" and ".open" method in the chain (optionally for ".config").

The filter’s ".close" method is called when an old connection closed, and this has no "next" parameter because it cannot be short-circuited.

The filter’s other methods like ".prepare", ".get_size", ".pread" etc ― always called in the context of a connection ― are passed a pointer to "struct nbdkit_next_ops" which contains a comparable set of accessors to plugin methods that can be called during a connection. It is possible for a filter to issue (for example) extra read calls in response to a single ".pwrite" call. Note that the semantics of the functions in "struct nbdkit_next_ops" are slightly different from what a plugin implements: for example, when a plugin's ".pread" returns -1 on error, the error value to advertise to the client is implicit (via the plugin calling "nbdkit_set_error" or setting "errno"), whereas "next_ops->pread" exposes this via an explicit parameter, allowing a filter to learn or modify this error if desired.

You can modify parameters when you call the "next" function. However be careful when modifying strings because for some methods (eg. ".config") the plugin may save the string pointer that you pass along. So you may have to ensure that the string is not freed for the lifetime of the server.

Note that if your filter registers a callback but in that callback it doesn't call the "next" function then the corresponding method in the plugin will never be called. In particular, your ".open" method, if you have one, must call the ".next" method.

CALLBACKS

"struct nbdkit_filter" has some static fields describing the filter and optional callback functions which can be used to intercept plugin methods.

".name"

 const char *name;

This field (a string) is required, and must contain only ASCII alphanumeric characters and be unique amongst all filters.

".version"

 const char *version;

Filters may optionally set a version string which is displayed in help and debugging output.

".longname"

 const char *longname;

An optional free text name of the filter. This field is used in error messages.

".description"

 const char *description;

An optional multi-line description of the filter.

".load"

 void load (void);

This is called once just after the filter is loaded into memory. You can use this to perform any global initialization needed by the filter.

".unload"

 void unload (void);

This may be called once just before the filter is unloaded from memory. Note that it's not guaranteed that ".unload" will always be called (eg. the server might be killed or segfault), so you should try to make the filter as robust as possible by not requiring cleanup. See also "SHUTDOWN" in nbdkit-plugin(3).

".config"

 int (*config) (nbdkit_next_config *next, void *nxdata,
                const char *key, const char *value);

This intercepts the plugin ".config" method and can be used by the filter to parse its own command line parameters. You should try to make sure that command line parameter keys that the filter uses do not conflict with ones that could be used by a plugin.

If there is an error, ".config" should call "nbdkit_error" with an error message and return "-1".

".config_complete"

 int (*config_complete) (nbdkit_next_config_complete *next, void *nxdata);

This intercepts the plugin ".config_complete" method and can be used to ensure that all parameters needed by the filter were supplied on the command line.

If there is an error, ".config_complete" should call "nbdkit_error" with an error message and return "-1".

".config_help"

 const char *config_help;

This optional multi-line help message should summarize any "key=value" parameters that it takes. It does not need to repeat what already appears in ".description".

If the filter doesn't take any config parameters you should probably omit this.

".open"

 void * (*open) (nbdkit_next_open *next, void *nxdata,
                 int readonly);

This is called when a new client connection is opened and can be used to allocate any per-connection data structures needed by the filter. The handle (which is not the same as the plugin handle) is passed back to other filter callbacks and could be freed in the ".close" callback.

Note that the handle is completely opaque to nbdkit, but it must not be NULL. If you don't need to use a handle, return "NBDKIT_HANDLE_NOT_NEEDED" which is a static non-NULL pointer.

If there is an error, ".open" should call "nbdkit_error" with an error message and return "NULL".

".close"

 void (*close) (void *handle);

This is called when the client closes the connection. It should clean up any per-connection resources used by the filter.

".prepare"

".finalize"

  int (*prepare) (struct nbdkit_next_ops *next_ops, void *nxdata,
                  void *handle);
  int (*finalize) (struct nbdkit_next_ops *next_ops, void *nxdata,
                   void *handle);

These two methods can be used to perform any necessary operations just after opening the connection (".prepare") or just before closing the connection (".finalize").

For example if you need to scan the underlying disk to check for a partition table, you could do it in your ".prepare" method (calling the plugin's ".pread" method via "next_ops"). Or if you need to cleanly update superblock data in the image on close you can do it in your ".finalize" method (calling the plugin's ".pwrite" method). Doing these things in the filter's ".open" or ".close" method is not possible.

There is no "next_ops->prepare" or "next_ops->finalize". Unlike other filter methods, prepare and finalize are not chained through the "next_ops" structure. Instead the core nbdkit server calls the prepare and finalize methods of all filters. Prepare methods are called starting with the filter closest to the plugin and proceeding outwards. Finalize methods are called in the reverse order of prepare methods.

If there is an error, both callbacks should call "nbdkit_error" with an error message and return "-1".

".get_size"

 int64_t (*get_size) (struct nbdkit_next_ops *next_ops, void *nxdata,
                      void *handle);

This intercepts the plugin ".get_size" method and can be used to read or modify the apparent size of the block device that the NBD client will see.

The returned size must be ≥ 0. If there is an error, ".get_size" should call "nbdkit_error" with an error message and return "-1". If this function is called more than once for the same connection, it should return the same value; similarly, the filter may cache "next_ops->get_size" for a given connection rather than repeating calls.

".can_write"

".can_flush"

".is_rotational"

".can_trim"

".can_zero"

".can_fua"

".can_multi_conn"

 int (*can_write) (struct nbdkit_next_ops *next_ops, void *nxdata,
                   void *handle);
 int (*can_flush) (struct nbdkit_next_ops *next_ops, void *nxdata,
                   void *handle);
 int (*is_rotational) (struct nbdkit_next_ops *next_ops,
                       void *nxdata,
                       void *handle);
 int (*can_trim) (struct nbdkit_next_ops *next_ops, void *nxdata,
                  void *handle);
 int (*can_zero) (struct nbdkit_next_ops *next_ops, void *nxdata,
                  void *handle);
 int (*can_fua) (struct nbdkit_next_ops *next_ops, void *nxdata,
                 void *handle);
 int (*can_multi_conn) (struct nbdkit_next_ops *next_ops, void *nxdata,
                        void *handle);

These intercept the corresponding plugin methods, and control feature bits advertised to the client.

Of note, the ".can_zero" callback in the filter controls whether the server advertises zero support to the client, which is slightly different semantics than the plugin; that is, "next_ops->can_zero" always returns true for a plugin, even when the plugin's own ".can_zero" callback returned false, because nbdkit implements a fallback to ".pwrite" at the plugin layer.

Remember that most of the feature check functions return merely a boolean success value, while ".can_fua" has three success values. The difference between values may affect choices made in the filter: when splitting a write request that requested FUA from the client, if "next_ops->can_fua" returns "NBDKIT_FUA_NATIVE", then the filter should pass the FUA flag on to each sub-request; while if it is known that FUA is emulated by a flush because of a return of "NBDKIT_FUA_EMULATE", it is more efficient to only flush once after all sub-requests have completed (often by passing "NBDKIT_FLAG_FUA" on to only the final sub-request, or by dropping the flag and ending with a direct call to "next_ops->flush").

If there is an error, the callback should call "nbdkit_error" with an error message and return "-1". If these functions are called more than once for the same connection, they should return the same value; similarly, the filter may cache the results of each counterpart in "next_ops" for a given connection rather than repeating calls.

".pread"

 int (*pread) (struct nbdkit_next_ops *next_ops, void *nxdata,
               void *handle, void *buf, uint32_t count, uint64_t offset,
               uint32_t flags, int *err);

This intercepts the plugin ".pread" method and can be used to read or modify data read by the plugin.

The parameter "flags" exists in case of future NBD protocol extensions; at this time, it will be 0 on input, and the filter should not pass any flags to "next_ops->pread".

If there is an error (including a short read which couldn't be recovered from), ".pread" should call "nbdkit_error" with an error message and return -1 with "err" set to the positive errno value to return to the client.

".pwrite"

 int (*pwrite) (struct nbdkit_next_ops *next_ops, void *nxdata,
                void *handle,
                const void *buf, uint32_t count, uint64_t offset,
                uint32_t flags, int *err);

This intercepts the plugin ".pwrite" method and can be used to modify data written by the plugin.

This function will not be called if ".can_write" returned false; in turn, the filter should not call "next_ops->pwrite" if "next_ops->can_write" did not return true.

The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on the result of ".can_fua". In turn, the filter should only pass "NBDKIT_FLAG_FUA" on to "next_ops->pwrite" if "next_ops->can_fua" returned a positive value.

If there is an error (including a short write which couldn't be recovered from), ".pwrite" should call "nbdkit_error" with an error message and return -1 with "err" set to the positive errno value to return to the client.

".flush"

 int (*flush) (struct nbdkit_next_ops *next_ops, void *nxdata,
               void *handle, uint32_t flags, int *err);

This intercepts the plugin ".flush" method and can be used to modify flush requests.

This function will not be called if ".can_flush" returned false; in turn, the filter should not call "next_ops->flush" if "next_ops->can_flush" did not return true.

The parameter "flags" exists in case of future NBD protocol extensions; at this time, it will be 0 on input, and the filter should not pass any flags to "next_ops->flush".

If there is an error, ".flush" should call "nbdkit_error" with an error message and return -1 with "err" set to the positive errno value to return to the client.

".trim"

 int (*trim) (struct nbdkit_next_ops *next_ops, void *nxdata,
              void *handle, uint32_t count, uint64_t offset,
              uint32_t flags, int *err);

This intercepts the plugin ".trim" method and can be used to modify trim requests.

This function will not be called if ".can_trim" returned false; in turn, the filter should not call "next_ops->trim" if "next_ops->can_trim" did not return true.

The parameter "flags" may include "NBDKIT_FLAG_FUA" on input based on the result of ".can_fua". In turn, the filter should only pass "NBDKIT_FLAG_FUA" on to "next_ops->trim" if "next_ops->can_fua" returned a positive value.

If there is an error, ".trim" should call "nbdkit_error" with an error message and return -1 with "err" set to the positive errno value to return to the client.

".zero"

 int (*zero) (struct nbdkit_next_ops *next_ops, void *nxdata,
              void *handle, uint32_t count, uint64_t offset, uint32_t flags,
              int *err);

This intercepts the plugin ".zero" method and can be used to modify zero requests.

This function will not be called if ".can_zero" returned false; in turn, the filter should not call "next_ops->zero" if "next_ops->can_zero" did not return true.

On input, the parameter "flags" may include "NBDKIT_FLAG_MAY_TRIM" unconditionally, and "NBDKIT_FLAG_FUA" based on the result of ".can_fua". In turn, the filter may pass "NBDKIT_FLAG_MAY_TRIM" unconditionally, but should only pass "NBDKIT_FLAG_FUA" on to "next_ops->zero" if "next_ops->can_fua" returned a positive value.

Note that unlike the plugin ".zero" which is permitted to fail with "EOPNOTSUPP" to force a fallback to ".pwrite", the function "next_ops->zero" will never fail with "err" set to "EOPNOTSUPP" because the fallback has already taken place.

If there is an error, ".zero" should call "nbdkit_error" with an error message and return -1 with "err" set to the positive errno value to return to the client. The filter should never fail with "EOPNOTSUPP" (while plugins have automatic fallback to ".pwrite", filters do not).

ERROR HANDLING

If there is an error in the filter itself, the filter should call "nbdkit_error" to report an error message. If the callback is involved in serving data, the explicit "err" parameter determines the error code that will be sent to the client; other callbacks should return the appropriate error indication, eg. "NULL" or "-1".

"nbdkit_error" has the following prototype and works like printf(3):

 void nbdkit_error (const char *fs, ...);
 void nbdkit_verror (const char *fs, va_list args);

For convenience, "nbdkit_error" preserves the value of "errno", and also supports the glibc extension of a single %m in a format string expanding to "strerror(errno)", even on platforms that don't support that natively.

DEBUGGING

Run the server with -f and -v options so it doesn't fork and you can see debugging information: 

 nbdkit -fv --filter=./myfilter.so plugin [key=value [key=value [...]]]

To print debugging information from within the filter, call "nbdkit_debug", which has the following prototype and works like printf(3):

 void nbdkit_debug (const char *fs, ...);
 void nbdkit_vdebug (const char *fs, va_list args);

For convenience, "nbdkit_debug" preserves the value of "errno", and also supports the glibc extension of a single %m in a format string expanding to "strerror(errno)", even on platforms that don't support that natively. Note that "nbdkit_debug" only prints things when the server is in verbose mode (-v option).

Debug Flags

Debug Flags in filters work exactly the same way as plugins. See "Debug Flags" in nbdkit-plugin(3).

INSTALLING THE FILTER

The filter is a "*.so" file and possibly a manual page. You can of course install the filter "*.so" file wherever you want, and users will be able to use it by running: 

 nbdkit --filter=/path/to/filter.so plugin [args]

However if the shared library has a name of the form "nbdkit-name-filter.so" and if the library is installed in the $filterdir directory, then users can be run it by only typing:

 nbdkit --filter=name plugin [args]

The location of the $filterdir directory is set when nbdkit is compiled and can be found by doing:

 nbdkit --dump-config

If using the pkg-config/pkgconf system then you can also find the filter directory at compile time by doing:

 pkgconf nbdkit --variable=filterdir

PKG-CONFIG/PKGCONF

nbdkit provides a pkg-config/pkgconf file called "nbdkit.pc" which should be installed on the correct path when the nbdkit development environment is installed. You can use this in autoconf configure.ac scripts to test for the development environment: 

 PKG_CHECK_MODULES([NBDKIT], [nbdkit >= 1.2.3])

The above will fail unless nbdkit ≥ 1.2.3 and the header file is installed, and will set "NBDKIT_CFLAGS" and "NBDKIT_LIBS" appropriately for compiling filters.

You can also run pkg-config/pkgconf directly, for example:

 if ! pkgconf nbdkit --exists; then
   echo "you must install the nbdkit development environment"
   exit 1
 fi

You can also substitute the filterdir variable by doing:

 PKG_CHECK_VAR([NBDKIT_FILTERDIR], [nbdkit], [filterdir])

which defines "$(NBDKIT_FILTERDIR)" in automake-generated Makefiles.

SEE ALSO

nbdkit(1), nbdkit-plugin(3).

Standard filters provided by nbdkit:

nbdkit-blocksize-filter(1), nbdkit-cache-filter(1), nbdkit-cow-filter(1), nbdkit-delay-filter(1), nbdkit-error-filter(1), nbdkit-fua-filter(1), nbdkit-log-filter(1), nbdkit-nozero-filter(1), nbdkit-offset-filter(1), nbdkit-partition-filter(1), nbdkit-truncate-filter(1), nbdkit-xz-filter(1) .

AUTHORS

Eric Blake

Richard W.M. Jones

COPYRIGHT

Copyright (C) 2013-2018 Red Hat Inc.

LICENSE

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
  • Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

2019-01-26 nbdkit-1.10.3