codec: Enforce NumSegments limit

Previously this was not enforced by the Encode function. While this
limit is unlikely to be hit in practice, it's important to assert it due
to the type allowing it and a count of segments greater than 2^32
causing an invalid marshaling.

codec.go

@@ -195,6 +195,10 @@ func MustUnmarshalRoot(data []byte) Ptr {
 	return p
 }
 
+var (
+	errTooManySegments = errors.New("message has too many segments")
+)
+
 // An Encoder represents a framer for serializing a particular Cap'n
 // Proto stream.
 type Encoder struct {
@@ -220,6 +224,9 @@ func (e *Encoder) Encode(m *Message) error {
 	if nsegs == 0 {
 		return errors.New("encode: message has no segments")
 	}
+	if nsegs > 1<<32 {
+		return exc.WrapError("encode", errTooManySegments)
+	}
 	e.bufs = append(e.bufs[:0], nil) // first element is placeholder for header
 	maxSeg := SegmentID(nsegs - 1)
 	hdrSize := streamHeaderSize(maxSeg)

codec_test.go

@@ -2,8 +2,11 @@ package capnp
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestEncoder(t *testing.T) {
@@ -72,6 +75,40 @@ func TestDecoder(t *testing.T) {
 	}
 }
 
+type tooManySegsArena struct {
+	data []byte
+}
+
+func (t *tooManySegsArena) NumSegments() int64 { return 1<<32 + 1 }
+
+func (t *tooManySegsArena) Data(id SegmentID) ([]byte, error) {
+	return nil, errors.New("no data")
+}
+
+func (t *tooManySegsArena) Allocate(minsz Size, segs map[SegmentID]*Segment) (SegmentID, []byte, error) {
+	return 0, nil, errors.New("cannot allocate")
+}
+
+func (t *tooManySegsArena) Release() {}
+
+// TestEncoderTooManySegments verifies attempting to encode an arena that has
+// more segments than possible.
+func TestEncoderTooManySegments(t *testing.T) {
+	t.Parallel()
+	zeroWord := []byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
+	arena := &tooManySegsArena{data: zeroWord}
+
+	// Setup via field because NewMessage checks arena has > 1 segments.
+	var msg Message
+	msg.Arena = arena
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	err := enc.Encode(&msg)
+
+	// Encoding should error with a specific error.
+	require.ErrorIs(t, err, errTooManySegments)
+}
+
 func TestDecoder_MaxMessageSize(t *testing.T) {
 	t.Parallel()