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

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

lib/osutil/lowprio_windows.go

@@ -0,0 +1,47 @@
+// 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/.
+
+package osutil
+
+import (
+	"syscall"
+
+	"github.com/pkg/errors"
+)
+
+var (
+	kernel32, _         = syscall.LoadLibrary("kernel32.dll")
+	setPriorityClass, _ = syscall.GetProcAddress(kernel32, "SetPriorityClass")
+)
+
+const (
+	// https://msdn.microsoft.com/en-us/library/windows/desktop/ms686219(v=vs.85).aspx
+	aboveNormalPriorityClass   = 0x00008000
+	belowNormalPriorityClass   = 0x00004000
+	highPriorityClass          = 0x00000080
+	idlePriorityClass          = 0x00000040
+	normalPriorityClass        = 0x00000020
+	processModeBackgroundBegin = 0x00100000
+	processModeBackgroundEnd   = 0x00200000
+	realtimePriorityClass      = 0x00000100
+)
+
+// SetLowPriority lowers the process CPU scheduling priority, and possibly
+// I/O priority depending on the platform and OS.
+func SetLowPriority() error {
+	handle, err := syscall.GetCurrentProcess()
+	if err != nil {
+		return errors.Wrap(err, "get process handler")
+	}
+	defer syscall.CloseHandle(handle)
+
+	res, _, err := syscall.Syscall(uintptr(setPriorityClass), uintptr(handle), belowNormalPriorityClass, 0, 0)
+	if res != 0 {
+		// "If the function succeeds, the return value is nonzero."
+		return nil
+	}
+	return errors.Wrap(err, "set priority class") // wraps nil as nil
+}