Final bikeshedding call: Fixing Control.Exception.bracket

[Eyal Lotem]

On Fri, Nov 14, 2014 at 1:00 AM, Yuras Shumovich <shumovichy at 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 at 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
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