catch change in security permission of file without blocking

Question

I want to catch change in security permission of file without blocking till the change is made, like an event that popup when the change is made.

I also don't want to install any third party modules or softwares.

The main requirement of this is to be in some of win32 modules or built-in modules.

I'm currently watching for security change by this function:

    import win32con, win32file
    def security_watcher():

        specific_file = "specific.ini"
        path_to_watch = "." 
        FILE_LIST_DIRECTORY = 0x0001
        hDir = win32file.CreateFile(path_to_watch,
                                    FILE_LIST_DIRECTORY,
                                    win32con.FILE_SHARE_READ | 
                                    win32con.FILE_SHARE_WRITE | 
                                    win32con.FILE_SHARE_DELETE,
                                    None,
                                    win32con.OPEN_EXISTING,
                                    win32con.FILE_FLAG_BACKUP_SEMANTICS,
                                    None)
        results = win32file.ReadDirectoryChangesW(hDir,
                                                  1024,
                                                  False,
                                                  win32con.FILE_NOTIFY_CHANGE_SECURITY,
                                                  None,
                                                  None)
        print results
        for action, file_name in results:
            if file_name == specific_file:
               # wake another function to do something about that

Note: I need it non blocking because I use this function in GUI Application and it freezes the GUI.

What OS? Clearly Windows, but what version? How many files do you need to watch? (If it's just one, you could always launch a separate blocking process that writes to a pipe or such.) — alexis, Aug 31 '16 at 12:06
And if I write to pipe it anyway blocks, because if the GUI process wait for reading from the pipe I came into blocking issue if nothing changed yet. — justlittlequestion, Aug 31 '16 at 12:46
Non-blocking reads on a pipe are easy to do (don't know the Windows syntax otomh, unfortunately). So is checking a log file to see if it has grown since you last checked. — alexis, Aug 31 '16 at 12:58
But of course the ideal solution would be a non-blocking version of the call you have, as you propose. — alexis, Aug 31 '16 at 12:59
The problem is if I do that I'll need thread that watching for any data on the pipe. when I get that data I need to modify some label in the GUI Application, which will make some unusual errors since the element isn't modified by the mainloop of the GUI. EDIT: I'll try it anyway and see if I can make it works, thanks. — justlittlequestion, Aug 31 '16 at 13:24

score 0 · Accepted Answer · edited May 23 '17 at 11:51

If you don't mind (or can't avoid) adding some threading overhead, you can launch a separate process or thread that waits on the blocking call to win32.ReadDirectoryChangesW() that you already have. When it receives a change, it writes the result to a pipe shared with the main thread of your GUI.

Your GUI can periodically execute a non-blocking read at the appropriate point in your code (presumably, where you now call win32file.ReadDirectoryChangesW()). Do make sure to wait a bit between reads, or your app will spend 100% of its time on non-blocking reads.

You can see in this answer how to set up a non-blocking read on a pipe in an OS-independent way. The final bit will look like this:

try:  
    results = q.get_nowait()
except Empty:
    pass
else:  
    for line in results.splitlines():
        # Parse and use your result format
        ...
        if file_name == specific_file:
            ...

Thanks! I use that and I find a hacky way to use the thread also! — justlittlequestion, Aug 31 '16 at 17:02

catch change in security permission of file without blocking

1 Answers1