event/SocketMonitor: add method Steal()

src/event/SocketMonitor.cxx

@@ -105,17 +105,25 @@ SocketMonitor::Open(int _fd)
 	g_source_add_poll(&source->base, &poll);
 }
 
-void
-SocketMonitor::Close()
+int
+SocketMonitor::Steal()
 {
 	assert(IsDefined());
 
 	Cancel();
 
-	close_socket(fd);
+	int result = fd;
 	fd = -1;
 
 	g_source_destroy(&source->base);
 	g_source_unref(&source->base);
 	source = nullptr;
+
+	return result;
+}
+
+void
+SocketMonitor::Close()
+{
+	close_socket(Steal());
 }

src/event/SocketMonitor.hxx

@@ -73,6 +73,12 @@ public:
 
 	void Open(int _fd);
 
+	/**
+	 * "Steal" the socket descriptor.  This abandons the socket
+	 * and puts the responsibility for closing it to the caller.
+	 */
+	int Steal();
+
 	void Close();
 
 	void Schedule(unsigned flags) {