Synchronization tweaks for IPC services (#2028)

* using sequentally consistent checks

* making shutdown method synchronous

* redndant line

ipc/hypervisor/src/service.rs.in

@@ -42,7 +42,7 @@ pub struct ModuleState {
 
 #[derive(Ipc)]
 pub trait ControlService {
-	fn shutdown(&self);
+	fn shutdown(&self) -> bool;
 }
 
 #[derive(Ipc)]

parity/boot.rs

@@ -42,7 +42,7 @@ pub fn host_service<T: ?Sized + Send + Sync + 'static>(addr: &str, stop_guard: A
 		let mut worker = nanoipc::Worker::<T>::new(&service);
 		worker.add_reqrep(&socket_url).unwrap();
 
-		while !stop_guard.load(Ordering::Relaxed) {
+		while !stop_guard.load(Ordering::SeqCst) {
 			worker.poll();
 		}
 	});

parity/sync.rs

@@ -31,9 +31,10 @@ struct SyncControlService {
 }
 
 impl ControlService for SyncControlService {
-	fn shutdown(&self) {
+	fn shutdown(&self) -> bool {
 		trace!(target: "hypervisor", "Received shutdown from control service");
-		self.stop.store(true, ::std::sync::atomic::Ordering::Relaxed);
+		self.stop.store(true, ::std::sync::atomic::Ordering::SeqCst);
+		true
 	}
 }
 
@@ -75,14 +76,15 @@ pub fn main() {
 	let control_service = Arc::new(SyncControlService::default());
 	let as_control = control_service.clone() as Arc<ControlService>;
 	let mut worker = nanoipc::Worker::<ControlService>::new(&as_control);
+	let thread_stop = control_service.stop.clone();
 	worker.add_reqrep(
 		&service_urls::with_base(&service_config.io_path, service_urls::SYNC_CONTROL)
 	).unwrap();
 
-	while !control_service.stop.load(::std::sync::atomic::Ordering::Relaxed) {
+	while !thread_stop.load(::std::sync::atomic::Ordering::SeqCst) {
 		worker.poll();
 	}
-	service_stop.store(true, ::std::sync::atomic::Ordering::Relaxed);
+	service_stop.store(true, ::std::sync::atomic::Ordering::SeqCst);
 
 	hypervisor.module_shutdown(SYNC_MODULE_ID);
 	trace!(target: "hypervisor", "Sync process terminated gracefully");