Add syncPair

mtglib/internal/relay/sync_pair.go

@@ -0,0 +1,54 @@
+package relay
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"time"
+)
+
+type syncPair struct {
+	writer  *bufio.Writer
+	copyBuf []byte
+
+	reader net.Conn
+}
+
+func (s *syncPair) Sync() (int64, error) {
+	return io.CopyBuffer(s, s, s.copyBuf) // nolint: wrapcheck
+}
+
+func (s *syncPair) Read(p []byte) (int, error) {
+	n, err := s.readBlocking(p, false)
+
+	if errors.Is(err, os.ErrDeadlineExceeded) {
+		if err := s.writer.Flush(); err != nil {
+			return 0, fmt.Errorf("cannot flush writer hand-side: %w", err)
+		}
+
+		return s.readBlocking(p, true)
+	}
+
+	return n, err
+}
+
+func (s *syncPair) Write(p []byte) (int, error) {
+	return s.writer.Write(p) // nolint: wrapcheck
+}
+
+func (s *syncPair) readBlocking(p []byte, blocking bool) (int, error) {
+	var deadline time.Time
+
+	if !blocking {
+		deadline = time.Now().Add(readTimeout)
+	}
+
+	if err := s.reader.SetReadDeadline(deadline); err != nil {
+		return 0, fmt.Errorf("cannot set read deadline: %w", err)
+	}
+
+	return s.reader.Read(p) // nolint: wrapcheck
+}