[Libguestfs] [PATCH nbdkit v2] Add the ability to write plugins in golang.

[Daniel P. Berrangé]

On Tue, Apr 21, 2020 at 11:44:59AM +0100, Richard W.M. Jones wrote:
> Thanks: Dan Berrangé
> 
> XXX UNFINISHED:
> 
>  - Is using uintptr for the handle a good idea?  Plugins must return
>    something != 0.  In other languages we would allow plugins to
>    return an arbitrary object here, but this is not possible in golang
>    because of lack of GC roots.

Yeah, this is the bit that looks wierd to me when thinking about this
from a Go dev POV.  You asked me on IRC whether we should have a separate
interface for a connection object, and this makes me think that we should
indeed do that.

Of course we still have the problem that C code is forbidden to keep
a pointer to a Go object across method calls.

IIUC, the C layuer in nbdkit treats the "void *" as a completely
opaque value, so we don't actually have to store any valid pointer
as the handle at all. It can be literally any value that fits in
a 32-bit pointer. Thus we can keep a global variable mapping the
GO objects to fake C "pointers"

So it would look like this:

 // The plugin interface.
 type PluginInterface interface {
 	// Open, GetSize and PRead are required for all plugins.
 	// Other methods are optional.
 	Config(key string, value string) error
 	ConfigComplete() error
 	Open(readonly bool) (PluginConnectionInterface, error)
 }

 type PluginConnectionInterface interface {
 	// Open, GetSize and PRead are required for all plugins.
 	// Other methods are optional.
 	Close()
 	GetSize() (uint64, error)
 	PRead(buf []byte, offset uint64, flags uint32) error
 }


Now, we need todo some mapping

  var connectionID uint
  var connectionMap map[uint]PluginConnectionInterface

 //export implOpen
 func implOpen(c_readonly C.int) unsafe.Pointer {
 	readonly := false
 	if c_readonly != 0 {
 		readonly = true
 	}
 	h, err := pluginImpl.Open(readonly)
 	if err != nil {
 		set_error(err)
 		return nil
 	}
 	if h == 0 {
 		panic("Open method: handle must be != 0")
 	}
	id := connectionID++
	connectionMap[id] = h
 	return unsafe.Pointer(uintptr(id))
 }

 func getConn(handle unsafe.Pointer) PluginConnectionInterface {
        id := uint(uintptr(handle))
	conn, ok = connectionMap[id]
	if !ok {
	    panic("Connection %d was not open", id)
	}
 }

 //export implClose
 func implClose(handle unsafe.Pointer) {
        conn := getConn(handle)
 	conn.Close()

        delete(connectionMap, id)
 }

 //export implGetSize
 func implGetSize(handle unsafe.Pointer) C.int64_t {
        conn := getConn(handle)
 	size, err := conn.GetSize()
 	if err != nil {
 		set_error(err)
 		return -1
 	}
 	return C.int64_t(size)
 }

> diff --git a/plugins/golang/test/test.go b/plugins/golang/test/test.go
> new file mode 100644
> index 00000000..f5b1a33b
> --- /dev/null
> +++ b/plugins/golang/test/test.go

 type TestPlugin struct {
 	nbdkit.Plugin
 }

 type TestPluginConnection struct {
        nbdkit.PluginConnection
	...other per-connection fields...
 }

 var pluginName = "test"
 var size uint64
 var size_set = false
 
 func (p TestPlugin) Config(key string, value string) error {
 	if key == "size" {
 		var err error
 		size, err = strconv.ParseUint(value, 0, 64)
 		if err != nil {
 			return err
 		}
 		size_set = true
 		return nil
 	} else {
 		return nbdkit.PluginError{Errmsg: "unknown parameter"}
 	}
 }
 
 func (p TestPlugin) ConfigComplete() error {
 	if !size_set {
>+		return nbdkit.PluginError{Errmsg: "size parameter is required"}
 	}
 	return nil
 }
 
 func (p TestPlugin) Open(readonly bool) (nbdkit.PluginConnectionInterface, error) {
 	nbdkit.Debug("golang code running in the .open callback")
 	return &TestPluginConnection{}, nil
 }
 
 func (p TestPluginConnection) GetSize() (uint64, error) {
 	nbdkit.Debug("golang code running in the .get_size callback")
 	return size, nil
 }
 
 func (p TestPluginConnection) PRead(buf []byte, offset uint64, flags uint32) error {
 	nbdkit.Debug("golang code running in the .pread callback")
 	for i := 0; i < len(buf); i++ {
 		buf[i] = 0
 	}
 	return nil
 }
 

Regards,
Daniel
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