internal/pipewire: implement message header

Test cases are from interactions between pw-container and PipeWire. Results are validated against corresponding body.

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

1 files changed, 73 insertions, 0 deletions

diff --git a/internal/pipewire/header.go b/internal/pipewire/header.go
new file mode 100644
index 00000000..ceb4cde6
--- /dev/null
+++ b/internal/pipewire/header.go
@@ -0,0 +1,73 @@
+package pipewire
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+const (
+	// HeaderSize is the fixed size of [Header].
+	HeaderSize = 16
+	// SizeMax is the largest value of [Header.Size] that can be represented in its 3-byte segment.
+	SizeMax = 0x00ffffff
+)
+
+var (
+	// ErrSizeRange indicates that the value of [Header.Size] cannot be represented in its 3-byte segment.
+	ErrSizeRange = errors.New("size out of range")
+	// ErrBadHeader indicates that the header slice does not have length [HeaderSize].
+	ErrBadHeader = errors.New("incorrect header size")
+)
+
+// A Header is the fixed-size message header described in protocol native.
+type Header struct {
+	// The message id this is the destination resource/proxy id.
+	ID uint32 `json:"Id"`
+	// The opcode on the resource/proxy interface.
+	Opcode byte `json:"opcode"`
+	// The size of the payload and optional footer of the message.
+	// Note: this value is only 24 bits long in the format.
+	Size uint32 `json:"size"`
+	// An increasing sequence number for each message.
+	Sequence uint32 `json:"seq"`
+	// Number of file descriptors in this message.
+	FileCount uint32 `json:"n_fds"`
+}
+
+// append appends the protocol native message header to data.
+//
+// Callers must perform bounds check on [Header.Size].
+func (h *Header) append(data []byte) []byte {
+	data = binary.NativeEndian.AppendUint32(data, h.ID)
+	data = binary.NativeEndian.AppendUint32(data, uint32(h.Opcode)<<24|h.Size)
+	data = binary.NativeEndian.AppendUint32(data, h.Sequence)
+	data = binary.NativeEndian.AppendUint32(data, h.FileCount)
+	return data
+}
+
+// MarshalBinary encodes the protocol native message header.
+func (h *Header) MarshalBinary() (data []byte, err error) {
+	if h.Size&^SizeMax != 0 {
+		return nil, ErrSizeRange
+	}
+	return h.append(make([]byte, 0, HeaderSize)), nil
+}
+
+// unmarshalBinary decodes the protocol native message header.
+func (h *Header) unmarshalBinary(data [HeaderSize]byte) {
+	h.ID = binary.NativeEndian.Uint32(data[0:4])
+	h.Size = binary.NativeEndian.Uint32(data[4:8])
+	h.Opcode = byte(h.Size >> 24)
+	h.Size &= SizeMax
+	h.Sequence = binary.NativeEndian.Uint32(data[8:])
+	h.FileCount = binary.NativeEndian.Uint32(data[12:])
+}
+
+// UnmarshalBinary decodes the protocol native message header.
+func (h *Header) UnmarshalBinary(data []byte) error {
+	if len(data) != HeaderSize {
+		return ErrBadHeader
+	}
+	h.unmarshalBinary(([HeaderSize]byte)(data))
+	return nil
+}