Initial structure

internal/transport/frame.go

@@ -0,0 +1,45 @@
+package transport
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+)
+
+// MaxFrameSize caps the size of an individual on-wire message. 16 MiB
+// is comfortably above any plausible cluster.yaml or status payload
+// and rejects malicious giants up front.
+const MaxFrameSize = 16 * 1024 * 1024
+
+// writeFrame emits a single length-prefixed message: 4-byte big-endian
+// length followed by the body.
+func writeFrame(w io.Writer, body []byte) error {
+	if len(body) > MaxFrameSize {
+		return errors.New("frame too large")
+	}
+	var hdr [4]byte
+	binary.BigEndian.PutUint32(hdr[:], uint32(len(body)))
+	if _, err := w.Write(hdr[:]); err != nil {
+		return err
+	}
+	_, err := w.Write(body)
+	return err
+}
+
+// readFrame reads the next length-prefixed message. Returns io.EOF
+// cleanly when the connection closes on a frame boundary.
+func readFrame(r io.Reader) ([]byte, error) {
+	var hdr [4]byte
+	if _, err := io.ReadFull(r, hdr[:]); err != nil {
+		return nil, err
+	}
+	n := binary.BigEndian.Uint32(hdr[:])
+	if n > MaxFrameSize {
+		return nil, errors.New("incoming frame exceeds MaxFrameSize")
+	}
+	buf := make([]byte, n)
+	if _, err := io.ReadFull(r, buf); err != nil {
+		return nil, err
+	}
+	return buf, nil
+}