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/pipewire.go

@@ -191,36 +191,30 @@ func (ctx *Context) queueFiles(fds ...int) (offset Fd) {
 	return
 }
 
+// InconsistentFilesError describes an implementation error where an incorrect amount
+// of files is queued between two messages.
+type InconsistentFilesError [2]Int
+
+func (e *InconsistentFilesError) Error() string {
+	return "queued " + strconv.Itoa(int(e[0])) + " files instead of the expected " + strconv.Itoa(int(e[1]))
+}
+
 // writeMessage appends the POD representation of v and an optional footer to buf.
-func (ctx *Context) writeMessage(
-	Id Int, opcode byte,
-	v KnownSize,
-) (err error) {
+func (ctx *Context) writeMessage(Id Int, v Message) (err error) {
+	if fileCount := Int(len(ctx.pendingFiles) - ctx.headerFiles); fileCount != v.FileCount() {
+		return &InconsistentFilesError{fileCount, v.FileCount()}
+	}
+
 	if ctx.pendingFooter == nil && ctx.deferredPendingFooter != nil {
 		ctx.pendingFooter, ctx.deferredPendingFooter = ctx.deferredPendingFooter, nil
 	}
 
-	size := v.Size()
-	if ctx.pendingFooter != nil {
-		size += ctx.pendingFooter.Size()
-	}
-	if size&^SizeMax != 0 {
-		return ErrSizeRange
-	}
-
-	ctx.buf = slices.Grow(ctx.buf, int(SizeHeader+size))
-	ctx.buf = (&Header{
-		ID: Id, Opcode: opcode, Size: size,
-		Sequence:  ctx.sequence,
-		FileCount: Int(len(ctx.pendingFiles) - ctx.headerFiles),
-	}).append(ctx.buf)
-	ctx.headerFiles = len(ctx.pendingFiles)
-	ctx.buf, err = MarshalAppend(ctx.buf, v)
-	if err == nil && ctx.pendingFooter != nil {
-		ctx.buf, err = MarshalAppend(ctx.buf, ctx.pendingFooter)
+	ctx.buf, err = MessageEncoder{v}.AppendMessage(ctx.buf, Id, ctx.sequence, ctx.pendingFooter)
+	if err == nil {
+		ctx.headerFiles = len(ctx.pendingFiles)
 		ctx.pendingFooter = nil
+		ctx.sequence++
 	}
-	ctx.sequence++
 	return
 }