taprpc+tapfreighter: add next_send_state field to SendEvent

Adds a new field, next_send_state, to the SendEvent message. This
enables integration tests to determine the upcoming state in the send
state machine.

Author: ffranr

rpcserver.go

@@ -4252,6 +4252,7 @@ func marshalSendEvent(event fn.Event) (*taprpc.SendEvent, error) {
 		VirtualPackets:        make([][]byte, len(e.VirtualPackets)),
 		PassiveVirtualPackets: make([][]byte, len(e.PassivePackets)),
 		TransferLabel:         e.TransferLabel,
+		NextSendState:         e.NextSendState.String(),
 	}
 
 	if e.Error != nil {

tapfreighter/chain_porter.go

@@ -1789,6 +1789,9 @@ type AssetSendEvent struct {
 	// Error below is set, then it means executing this state failed.
 	SendState SendState
 
+	// NextSendState is the next state that will be executed.
+	NextSendState SendState
+
 	// Error is an optional error, indicating that something went wrong
 	// during the execution of the SendState above.
 	Error error
@@ -1827,8 +1830,9 @@ func newAssetSendEvent(executedState SendState,
 	pkg sendPackage) *AssetSendEvent {
 
 	newSendEvent := &AssetSendEvent{
-		timestamp: time.Now().UTC(),
-		SendState: executedState,
+		timestamp:     time.Now().UTC(),
+		SendState:     executedState,
+		NextSendState: pkg.SendState,
 		// The parcel remains static throughout the state machine, so we
 		// don't need to copy it, there can be no data race.
 		Parcel:         pkg.Parcel,
@@ -1855,6 +1859,7 @@ func newAssetSendErrorEvent(err error, executedState SendState,
 	return &AssetSendEvent{
 		timestamp:      time.Now().UTC(),
 		SendState:      executedState,
+		NextSendState:  pkg.SendState,
 		Error:          err,
 		Parcel:         pkg.Parcel,
 		TransferLabel:  pkg.Label,