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/securitycontext.go

@@ -61,6 +61,12 @@ type SecurityContextCreate struct {
 	Properties *SPADict `json:"props"`
 }
 
+// Opcode satisfies [Message] with a constant value.
+func (c *SecurityContextCreate) Opcode() byte { return PW_SECURITY_CONTEXT_METHOD_CREATE }
+
+// FileCount satisfies [Message] with a constant value.
+func (c *SecurityContextCreate) FileCount() Int { return 2 }
+
 // Size satisfies [KnownSize] with a value computed at runtime.
 func (c *SecurityContextCreate) Size() Word {
 	return SizePrefix +
@@ -106,7 +112,6 @@ func (securityContext *SecurityContext) Create(listenFd, closeFd int, props SPAD
 	offset := securityContext.ctx.queueFiles(closeFd, listenFd)
 	return securityContext.ctx.writeMessage(
 		securityContext.ID,
-		PW_SECURITY_CONTEXT_METHOD_CREATE,
 		&SecurityContextCreate{ListenFd: offset + 1, CloseFd: offset + 0, Properties: &props},
 	)
 }