event/SocketMonitor: add method Open()

Allow creating a closed SocketMonitor instance.

src/event/SocketMonitor.cxx

@@ -74,16 +74,11 @@ static GSourceFuncs socket_monitor_source_funcs = {
 };
 
 SocketMonitor::SocketMonitor(int _fd, EventLoop &_loop)
-	:fd(_fd), loop(_loop),
+	:fd(-1), loop(_loop),
-	 source((Source *)g_source_new(&socket_monitor_source_funcs,
+	 source(nullptr) {
-				       sizeof(*source))),
+	assert(_fd >= 0);
-	 poll{fd, 0, 0} {
-	assert(fd >= 0);
 
-	source->monitor = this;
+	Open(_fd);
-
-	g_source_attach(&source->base, loop.GetContext());
-	g_source_add_poll(&source->base, &poll);
 }
 
 SocketMonitor::~SocketMonitor()
@@ -92,6 +87,24 @@ SocketMonitor::~SocketMonitor()
 		Close();
 }
 
+void
+SocketMonitor::Open(int _fd)
+{
+	assert(fd < 0);
+	assert(source == nullptr);
+	assert(_fd >= 0);
+
+	fd = _fd;
+	poll = {fd, 0, 0};
+
+	source = (Source *)g_source_new(&socket_monitor_source_funcs,
+					sizeof(*source));
+	source->monitor = this;
+
+	g_source_attach(&source->base, loop.GetContext());
+	g_source_add_poll(&source->base, &poll);
+}
+
 void
 SocketMonitor::Close()
 {

src/event/SocketMonitor.hxx

@@ -54,6 +54,9 @@ public:
 	static constexpr unsigned ERROR = G_IO_ERR;
 	static constexpr unsigned HANGUP = G_IO_HUP;
 
+	SocketMonitor(EventLoop &_loop)
+		:fd(-1), loop(_loop), source(nullptr) {}
+
 	SocketMonitor(int _fd, EventLoop &_loop);
 
 	~SocketMonitor();
@@ -68,6 +71,8 @@ public:
 		return fd;
 	}
 
+	void Open(int _fd);
+
 	void Close();
 
 	void Schedule(unsigned flags) {