[Haskell-cafe] inotify-alike for mac os x?

[Ross Mellgren]

On Dec 4, 2009, at 5:30 PM, Bryan O'Sullivan wrote:

> On Fri, Dec 4, 2009 at 8:39 AM, Svein Ove Aas <svein.ove at aas.no>  
> wrote:
>
> That said.. you say you have to handle the events "fast". What happens
> if you don't?
>
> If you don't handle events quickly, they're typically thrown away by  
> the kernel without you ever getting to read them. That is, for  
> instance, what happens on Linux with inotify. Throwing away events  
> means that your app's internal mirror of the filesystem state  
> becomes wrong, which is Very Bad for most applications that care.  
> (i.e. Ross's assertion than nothing bad will happen is generally not  
> true.)

Ah hah yeah, I meant in the context of it won't block the kernel or  
cause your computer to melt. It varies between applications whether  
dropping events is bad or not so I wasn't commenting there.

> However, with inotify you also can't afford to perform a single read  
> system call per event, because that will cause your "watch the  
> filesystem" event to soak up most of the system's CPU time. So what  
> you have to do is select to listen for "there's an event ready to be  
> read", then sleep a little while, then read in the hope that many  
> (but not too many!) events will have been queued that you can all  
> read at once.
>
> And at that point, you'll be getting a stale notification about a  
> file or directory that may no longer even exist, or may have changed  
> type. Consider: I create a file f, write data into it, rename it to  
> g, then create a directory named f. You wake up 10 milliseconds  
> later, and the first event you hear about is that a file named f was  
> created.
>
> This is all by way of saying that working with filesystem change  
> notification interfaces is extremely subtle and tricky, enormously  
> more so than you'd think on casual inspection. It's very easy to  
> write a program that uses these interfaces in ways that will make it  
> either generate garbage or consume huge amounts of CPU, and in fact  
> the common case is to write a program that does both.

Amen. I've written an application that does this kind of work using  
inotify and it was a nightmare. I think this is why fseventsd was  
invented for OS X, and I'm not sure if there's any linux equivalent.

However, if someone were to write a library that uses kqueue /  
inotify / win32-call-I-forget-the-name-of-from-earlier-post in a way  
that is both efficient and correct, that would be totally awesome.

-Ross


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