internal/pipewire: specify opcode and file count with message

This adds checking of FileCount while writing a message. Message encoding is relocated to an exported method to be used externally, probably for test stubbing.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pipewire/pod.go

@@ -5,6 +5,7 @@ import (
 	"io"
 	"math"
 	"reflect"
+	"slices"
 	"strconv"
 )
 
@@ -573,3 +574,48 @@ func (d *SPADict) UnmarshalPOD(data []byte) (Word, error) {
 	}
 	return wireSize, nil
 }
+
+// A Message is a value that can be transmitted as a message over PipeWire protocol native.
+type Message interface {
+	// Opcode returns the opcode of this message.
+	Opcode() byte
+	// FileCount returns the number of files associated with this message.
+	FileCount() Int
+
+	KnownSize
+}
+
+// A MessageEncoder provides methods for encoding a [Message].
+type MessageEncoder struct{ Message }
+
+// SizeMessage returns the size of Message transmitted over protocol native.
+func (m MessageEncoder) SizeMessage(footer KnownSize) (size Word) {
+	size = SizeHeader + m.Message.Size()
+	if footer != nil {
+		size += footer.Size()
+	}
+	return
+}
+
+// AppendMessage appends the protocol native encoding of Message to dst and returns the appended slice.
+func (m MessageEncoder) AppendMessage(dst []byte, Id, sequence Int, footer KnownSize) (data []byte, err error) {
+	size := m.SizeMessage(footer)
+	if size&^SizeMax != 0 {
+		return dst, ErrSizeRange
+	}
+
+	data = slices.Grow(dst, int(size))
+	data = (&Header{
+		ID:        Id,
+		Opcode:    m.Message.Opcode(),
+		Size:      size - SizeHeader,
+		Sequence:  sequence,
+		FileCount: m.Message.FileCount(),
+	}).append(data)
+	data, err = MarshalAppend(data, m.Message)
+	if err == nil && footer != nil {
+		data, err = MarshalAppend(data, footer)
+	}
+
+	return
+}