初版功能完成

internal/subnetgateway/tcprelay/relay.go

@@ -0,0 +1,49 @@
+// Package tcprelay performs protocol-agnostic TCP byte forwarding.
+package tcprelay
+
+import (
+	"context"
+	"errors"
+	"io"
+	"net"
+	"sync"
+)
+
+const bufferSize = 32 * 1024
+
+var bufferPool = sync.Pool{New: func() any { return make([]byte, bufferSize) }}
+
+// Relay copies both directions until EOF, cancellation, or an I/O failure.
+func Relay(ctx context.Context, left, right net.Conn) error {
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
+	results := make(chan error, 2)
+	copyDirection := func(destination, source net.Conn) {
+		buffer := bufferPool.Get().([]byte)
+		_, err := io.CopyBuffer(destination, source, buffer)
+		bufferPool.Put(buffer)
+		if closeWriter, ok := destination.(interface{ CloseWrite() error }); ok {
+			_ = closeWriter.CloseWrite()
+		}
+		results <- err
+	}
+	go copyDirection(left, right)
+	go copyDirection(right, left)
+	go func() {
+		<-ctx.Done()
+		_ = left.Close()
+		_ = right.Close()
+	}()
+	first := <-results
+	cancel()
+	_ = left.Close()
+	_ = right.Close()
+	second := <-results
+	if first != nil && !errors.Is(first, net.ErrClosed) {
+		return first
+	}
+	if second != nil && !errors.Is(second, net.ErrClosed) {
+		return second
+	}
+	return nil
+}