TUN-5494: Send a RPC with terminate reason to edge if the session is closed locally

datagramsession/event.go

@@ -1,6 +1,7 @@
 package datagramsession
 
 import (
+	"fmt"
 	"io"
 
 	"github.com/google/uuid"
@@ -24,6 +25,22 @@ func newRegisterSessionEvent(sessionID uuid.UUID, originProxy io.ReadWriteCloser
 // unregisterSessionEvent is an event to stop tracking and terminate the session.
 type unregisterSessionEvent struct {
 	sessionID uuid.UUID
+	err       *errClosedSession
+}
+
+// ClosedSessionError represent a condition that closes the session other than I/O
+// I/O error is not included, because the side that closes the session is ambiguous.
+type errClosedSession struct {
+	message  string
+	byRemote bool
+}
+
+func (sc *errClosedSession) Error() string {
+	if sc.byRemote {
+		return fmt.Sprintf("session closed by remote due to %s", sc.message)
+	} else {
+		return fmt.Sprintf("session closed by local due to %s", sc.message)
+	}
 }
 
 // newDatagram is an event when transport receives new datagram