cmd/syncthing, lib/config, lib/osutil: Lower process priority (fixes #4628)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4675

lib/osutil/lowprio_unix.go

@@ -0,0 +1,24 @@
+// Copyright (C) 2018 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build !windows,!linux
+
+package osutil
+
+import (
+	"syscall"
+
+	"github.com/pkg/errors"
+)
+
+// SetLowPriority lowers the process CPU scheduling priority, and possibly
+// I/O priority depending on the platform and OS.
+func SetLowPriority() error {
+	// Process zero is "self", niceness value 9 is something between 0
+	// (default) and 19 (worst priority).
+	err := syscall.Setpriority(syscall.PRIO_PROCESS, 0, 9)
+	return errors.Wrap(err, "set niceness") // wraps nil as nil
+}