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

On Tue, Apr 21, 2020 at 11:44:59AM +0100, Richard W.M. Jones wrote: 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) } 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 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|