lib/osutil: Fix priority lowering on Windows

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4686

lib/osutil/lowprio_windows.go

@@ -6,8 +6,44 @@
 
 package osutil
 
+import (
+	"syscall"
+
+	"github.com/pkg/errors"
+)
+
+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 {
-	return nil
+	modkernel32 := syscall.NewLazyDLL("kernel32.dll")
+	setPriorityClass := modkernel32.NewProc("SetPriorityClass")
+
+	if err := setPriorityClass.Find(); err != nil {
+		return errors.Wrap(err, "find proc")
+	}
+
+	handle, err := syscall.GetCurrentProcess()
+	if err != nil {
+		return errors.Wrap(err, "get process handler")
+	}
+	defer syscall.CloseHandle(handle)
+
+	res, _, err := setPriorityClass.Call(uintptr(handle), belowNormalPriorityClass)
+	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
 }