Enhance Options Buffer for >16 Message Unmarshaling

Previously, the connection was being closed due to unmarshaling failures when
more than 16 options were included in the message. This fix addresses the issue
by increasing the buffer size to accommodate a higher number of options.

Author: jkralik

message/pool/message.go

@@ -523,14 +523,28 @@ func (r *Message) MarshalWithEncoder(encoder Encoder) ([]byte, error) {
 	return r.bufferMarshal, nil
 }
 
+func (r *Message) decode(decoder Decoder) (int, error) {
+	var n int
+	var err error
+	for {
+		n, err = decoder.Decode(r.bufferUnmarshal, &r.msg)
+		if errors.Is(err, message.ErrOptionsTooSmall) {
+			// increase buffer size and try again
+			r.msg.Options = make(message.Options, 0, len(r.msg.Options)*2)
+			continue
+		}
+		return n, err
+	}
+}
+
 func (r *Message) UnmarshalWithDecoder(decoder Decoder, data []byte) (int, error) {
 	if len(r.bufferUnmarshal) < len(data) {
 		r.bufferUnmarshal = append(r.bufferUnmarshal, make([]byte, len(data)-len(r.bufferUnmarshal))...)
 	}
 	copy(r.bufferUnmarshal, data)
 	r.body = nil
 	r.bufferUnmarshal = r.bufferUnmarshal[:len(data)]
-	n, err := decoder.Decode(r.bufferUnmarshal, &r.msg)
+	n, err := r.decode(decoder)
 	if err != nil {
 		return n, err
 	}

message/pool/message_test.go

@@ -4,11 +4,13 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"strings"
 	"testing"
 
 	"github.com/plgd-dev/go-coap/v3/message"
 	"github.com/plgd-dev/go-coap/v3/message/pool"
+	"github.com/plgd-dev/go-coap/v3/tcp/coder"
 	"github.com/plgd-dev/go-coap/v3/test/net"
 	"github.com/stretchr/testify/require"
 )
@@ -360,3 +362,32 @@ func TestMessageClone(t *testing.T) {
 	require.Error(t, err)
 	require.Contains(t, err.Error(), "read error")
 }
+
+func TestUnmarshalMessageWithMultipleOptions(t *testing.T) {
+	tests := []struct {
+		numOptions int
+	}{
+		{numOptions: 0},
+		{numOptions: 8},
+		{numOptions: 16},
+		{numOptions: 32},
+		{numOptions: 64},
+		{numOptions: 1023},
+	}
+
+	for _, tt := range tests {
+		t.Run(fmt.Sprintf("num-options-%v", tt.numOptions), func(t *testing.T) {
+			req := pool.NewMessage(context.Background())
+			for i := 0; i < 64; i++ {
+				req.AddOptionUint32(message.URIQuery, uint32(i))
+			}
+			data, err := req.MarshalWithEncoder(coder.DefaultCoder)
+			require.NoError(t, err)
+			msg := pool.NewMessage(context.Background())
+			n, err := msg.UnmarshalWithDecoder(coder.DefaultCoder, data)
+			require.NoError(t, err)
+			require.Equal(t, n, len(data))
+			require.Equal(t, req.Options(), msg.Options())
+		})
+	}
+}