nbdkit-golang-plugin - writing nbdkit plugins in Go
nbdkit /path/to/plugin.so [arguments...]This manual page describes how to write nbdkit plugins in compiled Golang code. Go plugins are compiled to *.so files (the same as plugins written in C) and are used in the same way.
For examples of plugins written in Go, see: https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/golang/examples
The minimal/minimal.go example is the smallest possible plugin, implementing only the required callbacks (Open, GetSize and PRead). Other examples show other nbdkit features.
Broadly speaking, Golang nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.
To write a Golang nbdkit plugin, you must:
Set the package to main
Plugins should be main packages, because that's how Golang shared libraries work.
Import "C", "unsafe" and "libguestfs.org/nbdkit"
Create plugin and connection structs
Conventionally these are named after the plugin, but you can call them whatever you want. They must derive from the nbdkit.Plugin and nbdkit.Connection structs respectively:
type MyPlugin struct {
nbdkit.Plugin
}
type MyConnection struct {
nbdkit.Connection
}The plugin struct is used to define the global callbacks, like Load, Config and GetReady. The connection struct represents a client connection and is used to define connection callbacks like GetSize and PRead. What connects the two is the Open callback which is called when the client has connected and where you should return a new instance of your connection struct. For example:
func (p *MyPlugin) Load() {
// global callback used for initializing the plugin
}
func (p *MyPlugin) Open(readonly bool) (nbdkit.ConnectionInterface, error) {
// new client has connected
return &MyConnection{}, nil
}
func (c *MyConnection) GetSize() (uint64, error) {
// called per-client
return virtual_size, nil
}You can store per-client data in the connection struct. Global data could be stored as global variables or kept in the plugin struct as you wish.
Include the plugin_init and main boilerplate
See the example plugins for the correct boilerplate to include.
Compile the plugin to a shared library
go build -o nbdkit-mygolang-plugin.so -buildmode=c-sharedThis builds the shared object (*.so) which is the plugin, and also a header file (*.h) which is irrelevant and you can delete.
Can* callbacksImportant: If you implement PWrite it will not be called unless you also implement a CanWrite callback that returns true. (This is different from plugins written in C or other languages).
The same applies to Flush (CanFlush), Trim (CanTrim) and Zero (CanZero).
func (c *MyConnection) CanWrite() (bool, error) {
return true, nil
}
func (c *MyConnection) PWrite(buf []byte, offset uint64,
flags uint32) error {
// ...
}The following callbacks are not yet implemented:
versionlongnamedescriptionmagic_config_keyconfig_helpthread_modelAt the moment the thread model is always NBDKIT_THREAD_MODEL_PARALLEL.
can_fast_zerocan_extentscan_fuacan_cacheextentscacheGolang plugins first appeared in nbdkit 1.20.
Richard W.M. Jones
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