Add IPC support (#6348)

This is useful for both security and performance reasons. IPC is faster
than TCP, and it is subject to OS access controls.

substrate/client/service/src/lib.rs

@@ -510,6 +510,16 @@ mod waiting {
 		}
 	}
 
+	pub struct IpcServer(pub Option<sc_rpc_server::IpcServer>);
+	impl Drop for IpcServer {
+		fn drop(&mut self) {
+			if let Some(server) = self.0.take() {
+				server.close_handle().close();
+				let _ = server.wait();
+			}
+		}
+	}
+
 	pub struct WsServer(pub Option<sc_rpc_server::WsServer>);
 	impl Drop for WsServer {
 		fn drop(&mut self) {
@@ -555,6 +565,7 @@ fn start_rpc_servers<H: FnMut(sc_rpc::DenyUnsafe) -> sc_rpc_server::RpcHandler<s
 	}
 
 	Ok(Box::new((
+		config.rpc_ipc.as_ref().map(|path| sc_rpc_server::start_ipc(&*path, gen_handler(sc_rpc::DenyUnsafe::No))),
 		maybe_start_server(
 			config.rpc_http,
 			|address| sc_rpc_server::start_http(