初版功能完成

internal/subnetgateway/udprelay/relay.go

@@ -0,0 +1,68 @@
+// Package udprelay preserves UDP datagram boundaries across a host socket flow.
+package udprelay
+
+import (
+	"context"
+	"errors"
+	"net"
+	"sync/atomic"
+	"time"
+)
+
+// Relay copies connected UDP datagrams until idle timeout or cancellation.
+func Relay(ctx context.Context, left, right net.Conn, idleTimeout time.Duration) error {
+	parent := ctx
+	ctx, cancel := context.WithCancel(parent)
+	defer cancel()
+	if idleTimeout <= 0 {
+		idleTimeout = 60 * time.Second
+	}
+	var lastActivity atomic.Int64
+	lastActivity.Store(time.Now().UnixNano())
+	results := make(chan error, 2)
+	copyDatagrams := func(destination, source net.Conn) {
+		buffer := make([]byte, 65535)
+		for {
+			deadline := time.Now().Add(min(idleTimeout/2, time.Second))
+			_ = source.SetReadDeadline(deadline)
+			count, err := source.Read(buffer)
+			if err != nil {
+				if timeout, ok := err.(net.Error); ok && timeout.Timeout() {
+					last := time.Unix(0, lastActivity.Load())
+					if time.Since(last) < idleTimeout {
+						continue
+					}
+				}
+				results <- err
+				return
+			}
+			if _, err := destination.Write(buffer[:count]); err != nil {
+				results <- err
+				return
+			}
+			lastActivity.Store(time.Now().UnixNano())
+		}
+	}
+	go copyDatagrams(left, right)
+	go copyDatagrams(right, left)
+	go func() {
+		<-ctx.Done()
+		_ = left.Close()
+		_ = right.Close()
+	}()
+	err := <-results
+	cancel()
+	_ = left.Close()
+	_ = right.Close()
+	<-results
+	if parent.Err() != nil {
+		return parent.Err()
+	}
+	if timeout, ok := err.(net.Error); ok && timeout.Timeout() {
+		return nil
+	}
+	if errors.Is(err, net.ErrClosed) {
+		return nil
+	}
+	return err
+}