On Fri, Nov 14, 2014 at 1:00 AM, Yuras Shumovich <shumovichy@gmail.com> wrote:

On Fri, 2014-11-14 at 00:33 +0200, Eyal Lotem wrote:
> On Fri, Nov 14, 2014 at 12:24 AM, Yuras Shumovich <shumovichy@gmail.com>
> wrote:
> > That fixes it w.r.t. sync *and* async exception without any special work
> > for async case.
> >
>
> NO: This new code is still broken. Async exception in close of h1 will just
> "jump" to block at h2, and yield the full side effect of closing h2 while
> skipping some of the side effect of closing h1.

Yes, but that is good default, I already explained why. If you care
about about side effect in case of async exception, when use
uninterruptibleMask explicitly, and explain in comments why you need
that.

I disagree that it is a good default. "withFile" combined with successful writes to the handle now no longer guarantee the writes occur. Your writes get lost forever despite diligently using the correct idioms, whenever async exceptions are involved.

I argue that it is *very* rare that your cleanups have anything sensible to do upon async exceptions (I don't think I've ever encountered a single case where that was desirable) and yet it is very common for cleanups to "forget" to uninterruptible-mask when they should (almost always). Again, this comes up when you review random uses of bracket, which are broken in virtually every case of an interruptible cleanup.

The sane default is to guarantee that the cleanup happens even if there is an async exception. Then someone who has some sensible thing to do when an async exception occurs during cleanup can go ahead and use the rarely-useful bracketInterruptible.

>
> All my code used these kind of idioms for sync-exception-safety but was
> still ridden with bugs w.r.t async exceptions.

Several options:
a) your code is buggy, fix it
b) you rely on buggy code, pester it's author to fix it (and temporary
use uninterruptibleMask)
c) you really need uninterruptibleMask here, go ahead and use it

Why don't we consider some realistic examples?

withFile => you want it to flush successful writes => You want uninterruptibleMask.

withMVar => you want it to guarantee the MVar doesn't remain in an inconsistent state => You want uninterruptibleMask

createProcess/kill+wait => you want to guarantee that the process-bracket actually maintains an invariant about the process's state => you want uninterruptibleMask

withSemaphore => you want to guarantee that the semaphore doesn't remain in an inconsistent state => you want uninterruptibleMask

See a pattern here?

>
> The kill&wait for process are another example, but I have multiple examples
> -- all of which become more reasonable when the cancellation is
> uninterruptible.

Then just use uninterruptibleMask if it is reasonable in your case.

>
> >
> > Almost nobody test code for case when file is deleted, so it is unlikely
> > to discover the bug in the first version in case of uninterruptibleMask.
> > So the proposal *hides* the bug, probably for 99% cases, but the bug is
> > here, and it will bit you sooner or later.
> >
>
> Are you talking about Windows or POSIX? With POSIX, file deletion has
> nothing to do with hClose.

Ok, Almost nobody test code for case when <insert a case when your
cleanup action throws sync exception>

sync exceptions are irrelevant here.

>
>
> >
> >
> > > The proposal makes it easer to continue ignoring async exceptions. That
> > > is why I'm arguing here against it. (Possible breakage if existing code
> > > worries me too, but much less)
> > >
> > > I think it's a good thing to make it easier to ignore async exceptions.
> >
> > It is already easy -- just don't use them.
> >
>
> Whenever you use the async library, for example, you use async exceptions.
> And then all your bracket invariants are broken *by-default*.

Don't use async then.

Whereas with this proposal, people can safely use async and bracket will work correctly.

>
>
> > Or wrap our main into uninterruptibleMask :)
> >
> > main :: IO ()
> > main = uninterruptibleMask_ $ do
> > ....
> >
>
> But cancelling/killing non-cleanups is not problematic in general.

Did you read "Dealing with Asynchronous Exceptions during Resource
Acquisition" article?

Yes.

I didn't say it's not tricky -- I said it's not problematic. i.e: It's very possible to make resource acquisition cancelable - and it is often critical that we can abort threads that are still at resource acquisition stage.

> >
> > It is a myth that handling async exceptions in cleanup is much harder
> > then handling sync exceptions. Async exceptions are hard to deal with
> > because they can be raised at any point (even between points :) ). But
> > in cleanup action async exceptions are masked, and can be raised only in
> > well known points, just like regular sync exceptions.
> >
>
> Sync exceptions are under your control. You can make sure the preconditions
> of the operation are met so that they are not raised. If they are raised,
> they are related to the operation at hand so there may be something that
> you can do. Async exceptions are fundamentally different, so please stop
> mishmashing these two dissimilar things together.
>

So you want to ban sync exceptions in cleanup actions?

What?? No.

I want to write my code in such a way that eliminates as many sync exceptions as possible. For example, I will avoid passing invalid handles to hClose - and I have a guarantee that one sync exception will not happen.

Eyal