nbdkit-sh-plugin - Man Page

Assuming you have a shell script which is an nbdkit plugin, you run it like this:

 nbdkit sh /path/to/script

You may have to add further key=value arguments to the command line.  The script must be executable (chmod +x).

It is also possible to write a shell script plugin "inline" using - as the name of the script, like this:

 nbdkit sh - <<'EOF'
   case "$1" in
     get_size) echo 1M ;;
     pread) dd if=/dev/zero count=$3 iflag=count_bytes ;;
     *) exit 2 ;;
   esac
 EOF

By default the inline script runs under /bin/sh.  You can add a shebang (#!) to use other scripting languages.  Of course, reading an inline script from stdin is incompatible with the -s (--single) mode of nbdkit that connects a client on stdin.

This plugin has a simple programming model: For every plugin method that needs to be called, the external script is invoked with parameters describing the method and its arguments.  The first parameter is always the method name.  For example:

 /path/to/script config file disk.img
                   │      │   │
                   │      │   └─ value ($3)
                   │      └── key ($2)
               method ($1)

 /path/to/script pread <handle> <count> <offset>
                   │       │       │       │
                   │       │       │       └─ offset in bytes ($4)
                   │       │       └── request size in bytes ($3)
               method ($1) └── handle ($2) ─ see "Handles" below

Scripts should ignore extra parameters that they don't understand since we may add new parameters in future.

The script should exit with specific exit codes:

0

The method was executed successfully.

1 and 16-255

There was an error.  The script may print on stderr an errno name, optionally followed by whitespace and a message, for example:

 echo 'ENOSPC Out of space' >&2
 exit 1

or if you don't need the log message:

 echo ENOSPC >&2
 exit 1

If the script doesn't print anything or the output cannot be parsed then nbdkit assumes error EIO.  Note that output to stderr is ignored if the command succeeds, so it is acceptable to output a potential error message prefix prior to attempting a command which will add further details if a failure occurs.

2

The requested method is not supported by the script.

3

For methods which return booleans, this code indicates false.

4 and 5

Triggers a call to the C function nbdkit_shutdown(3), which requests an asynchronous exit of the nbdkit server (disconnecting all clients). The client will usually get a response before shutdown is complete (although this is racy); so once the shutdown is requested, code 4 then behaves like code 0 (stderr is ignored, and the server tries to return success), and code 5 behaves like code 1 (the server tries to return an error to the client parsed from stderr, although a missing error defaults to ESHUTDOWN instead of EIO).

6

Triggers a call to the C function nbdkit_disconnect(3) with force set to true, which requests an abrupt disconnect of the current client.  The contents of stderr are irrelevant with this status, since the client will not get a response.

7 and 8

Triggers a call to the C function nbdkit_disconnect(3) with force set to false, which requests a soft disconnect of the current client (future client requests are rejected with ESHUTDOWN without calling into the plugin, but current requests may complete).  Since the client will likely get the response to this command, code 7 then behaves like code 0 (stderr is ignored, and the server tries to return success), and code 8 behaves like code 1 (the server tries to return an error to the client parsed from stderr, although a missing error defaults to ESHUTDOWN instead of EIO).

9-15

These exit codes are reserved for future use.  Note that versions of nbdkit < 1.34 documented that codes 8 through 15 behaved like code 1; although it is unlikely that many scripts relied on this similarity in practice.

In nbdkit > 1.34, it is possible to probe whether additional exit codes have been assigned meaning, by looking for the line max_known_status= in the output of nbdkit --dump-plugin sh.  If this line is not present, exit codes 4 and above behave like status 1.

A fresh script is invoked for each method call (ie. scripts are stateless), so if the script needs to store state it has to store it somewhere in the filesystem in a format and location which is left up to the author of the script.

However nbdkit helps by creating a randomly named, empty directory for the script.  This directory persists for the lifetime of nbdkit and is deleted when nbdkit exits.  The name of the directory is passed to each script invocation in the $tmpdir environment variable.

Handles are arbitrary strings, but it is best to limit them to short alphanumeric strings.

Per-connection state

The temporary directory described above can be used for state for the lifetime of the nbdkit instance (across multiple connections).  If you want to store state per connection then one way to do it is to create a randomly named subdirectory under the temporary directory:

 case "$1" in
   ...
   open)
     mktemp -d $tmpdir/handle-XXXXXX ;;

The handle will be the subdirectory name, returned to the script as $2 in all connected calls (eg. pread, get_size).  You can delete the subdirectory explicitly in close:

 case "$1" in
   ...
   close)
     rm -rf "$2" ;;

or rely on nbdkit deleting the whole temporary directory including all per-handle subdirectories when it exits.

This plugin has to fork on every request, so performance will never be great.  For best performance, consider using the nbdkit-plugin(3) API directly.  Having said that, if you have a sh plugin and want to improve performance then the following tips may help:

Relax the thread model.

The default thread_model is serialize_all_requests meaning that two instances of the script can never be running at the same time. This is safe but slow.  If your script is safe to be called in parallel, set this to parallel.

Implement the zero method.

If the zero method is not implemented then nbdkit will fall back to using pwrite which is considerably slower because nbdkit has to send blocks of zeroes to the script.

You don't have to write shell scripts.

This plugin can run any external binary, not only shell scripts.  You should get more performance by rewriting the shell script as a program in a compiled language.

This just documents the arguments to the script corresponding to each plugin method, and any way that they differ from the C callbacks.  In all other respects they work the same way as the C callbacks, so you should go and read nbdkit-plugin(3).

load

 /path/to/script load

unload

 /path/to/script unload

This is called just before nbdkit exits.  Errors from this method are ignored.

dump_plugin

 /path/to/script dump_plugin

config

 /path/to/script config <key> <value>

config_complete

 /path/to/script config_complete

magic_config_key

 /path/to/script magic_config_key

If a magic config key is needed, this should echo it to stdout. See "Magic parameters" in nbdkit(1).

thread_model

 /path/to/script thread_model

On success this should print the desired thread model of the script, one of "serialize_connections", "serialize_all_requests", "serialize_requests", or "parallel".

This method is not required; if omitted, then the plugin will be executed under the safe "serialize_all_requests" model.  However, this means that this method must be provided if you want to use the "parallel" or "serialize_requests" model.  Even then your request may be restricted for other reasons; look for thread_model in the output of nbdkit --dump-plugin sh script to see what actually gets selected.

If an error occurs, the script should output an error message and exit with status 1; unrecognized output is ignored.

get_ready

 /path/to/script get_ready

after_fork

 /path/to/script after_fork

preconnect

 /path/to/script preconnect <readonly>

list_exports

 /path/to/script list_exports <readonly> <tls>

The readonly and tls parameters will be true or false.

The first line of output informs nbdkit how to parse the rest of the output, the remaining lines then supply the inputs of the C nbdkit_add_export function (see nbdkit-plugin(3)), as follows:

NAMES

The remaining output provides one export name per line, and no export will be given a description.  For convenience, this form is also assumed if the first output line does not match one of the recognized parse modes.

INTERLEAVED

The remaining output provides pairs of lines, the first line being an export name, and the second the corresponding description.

NAMES+DESCRIPTIONS

The number of remaining lines is counted, with the first half being used as export names, and the second half providing descriptions to pair with names from the first half.

An example of using this form to list files in the current directory, followed by their ls(1) long description, would be:

 echo NAMES+DESCRIPTIONS
 ls
 ls -l

Note that other output modes might be introduced in the future; in particular, none of the existing modes allow a literal newline in an export name or description, although this could be possible under a new mode supporting escape sequences.

This method is not required; if it is absent, the list of exports advertised by nbdkit will be the single name result of default_export and no description.

default_export

 /path/to/script default_export <readonly> <tls>

The readonly and tls parameters will be true or false.

On success this should print a name on stdout to use in place of the default export "", then exit with code 0.  For convenience, the output can be any of the list forms recognized by list_exports, in which case the first listed export name is used, and where an empty list uses "".  Given the current set of recognized export lists, it is not possible for the resulting name to include a newline.

This method is not required; if it is absent, the default export name will be the empty string, "".

open

 /path/to/script open <readonly> <exportname> <tls>

The readonly parameter will be true or false.  The exportname parameter, if present, is the export name passed to the server from the client.  The tls parameter, if present, will be true or false depending on whether the client is using TLS.

On success this should print the handle (any string) on stdout and exit with code 0.  If the handle ends with a newline character then the newline is removed.

Unlike C plugins, this method is not required.  If omitted then the handle will be "" (empty string).

close

 /path/to/script close <handle>

export_description

 /path/to/script export_description <handle>

The script should print a human-readable description of the disk image on stdout.  If the description ends with a newline character then the newline is removed.

This method is not required; if it is absent, no export description will be provided to the client.

get_size

 /path/to/script get_size <handle>

The script should print the size of the disk image on stdout.  You can print the size in bytes, or use any format understood by nbdkit_parse_size(3) such as 1M.

This method is required.

block_size

 /path/to/script block_size <handle>

This script should print three numbers on stdout, separated by whitespace.  These are (in order) the minimum block size, the preferred block size, and the maximum block size.  You can print the sizes in bytes or use any format understood by nbdkit_parse_size(3) such as 1M.

can_write

can_flush

can_trim

can_zero

can_extents

Unlike in other languages, you must provide the can_* methods otherwise they are assumed to all return false and your pwrite, flush, trim, zero and extents methods will never be called.  The reason for this is obscure: In other languages we can detect if (eg) a pwrite method is defined and synthesize an appropriate response if no actual can_write method is defined. However detecting if methods are present without running them is not possible with this plugin.

 /path/to/script can_write <handle>
 /path/to/script can_flush <handle>
 /path/to/script can_trim <handle>
 /path/to/script can_zero <handle>
 /path/to/script can_extents <handle>

The script should exit with code 0 for true or code 3 for false.

is_rotational

can_fast_zero

 /path/to/script is_rotational <handle>
 /path/to/script can_fast_zero <handle>

The script should exit with code 0 for true or code 3 for false.

can_fua

can_cache

 /path/to/script can_fua <handle>
 /path/to/script can_cache <handle>

These control Forced Unit Access (FUA) and caching behaviour of the core server.

Unlike the other can_* callbacks, these two are not a boolean. They must print either "none", "emulate" or "native" to stdout.  The meaning of these is described in nbdkit-plugin(3).  Furthermore, you must provide a can_cache method if you desire the cache callback to be utilized, similar to the reasoning behind requiring can_write to utilize pwrite.

can_multi_conn

 /path/to/script can_multi_conn <handle>

The script should exit with code 0 for true or code 3 for false.

pread

 /path/to/script pread <handle> <count> <offset>

The script should print the requested binary data on stdout.  Exactly count bytes must be printed.

This method is required.

pwrite

 /path/to/script pwrite <handle> <count> <offset> <flags>

The script should read the binary data to be written from stdin.

The flags parameter can be an empty string or "fua".  In the future, a comma-separated list of flags may be present.

Unlike in other languages, if you provide a pwrite method you must also provide a can_write method which exits with code 0 (true).

flush

 /path/to/script flush <handle>

Unlike in other languages, if you provide a flush method you must also provide a can_flush method which exits with code 0 (true).

trim

 /path/to/script trim <handle> <count> <offset> <flags>

The flags parameter can be an empty string or "fua".  In the future, a comma-separated list of flags may be present.

Unlike in other languages, if you provide a trim method you must also provide a can_trim method which exits with code 0 (true).

zero

 /path/to/script zero <handle> <count> <offset> <flags>

The flags parameter can be an empty string or a comma-separated list of the flags: "fua", "may_trim", and "fast" (eg. "", "fua", "fua,may_trim,fast" are some of the 8 possible values).

Unlike in other languages, if you provide a zero method you must also provide a can_zero method which exits with code 0 (true).

To trigger a fallback to pwrite on a normal zero request, or to respond quickly to the "fast" flag that a specific zero request is no faster than a corresponding write, the script must output ENOTSUP or EOPNOTSUPP to stderr (possibly followed by a description of the problem) before exiting with code 1 (failure).

extents

 /path/to/script extents <handle> <count> <offset> <flags>

The flags parameter can be an empty string or "req_one".

This must print, one per line on stdout, a list of one or more extents in the format:

 offset length type

which correspond to the inputs of the C nbdkit_add_extent function (see nbdkit-plugin(3)).  The offset and length fields may use any format understood by nbdkit_parse_size(3).  The optional type field may be an integer, missing (same as 0), or a comma-separated list of the words hole and zero.  An example of a valid set of extents covering a 10M disk where the first megabyte only is allocated data:

 0  1M
 1M 9M  hole,zero

Unlike in other languages, if you provide an extents method you must also provide a can_extents method which exits with code 0 (true).

cache

 /path/to/script cache <handle> <count> <offset>

Unlike in other languages, if you provide a cache method you must also provide a can_cache method which prints "native" and exits with code 0 (true).

Missing: name, version, longname, description, config_help

These are not yet supported.