[Haskell-cafe] addFinalizer in GHC 7.10

[Kazu Yamamoto (=?iso-2022-jp?B?GyRCOzNLXE9CSScbKEI=?=)]

Hi,

I registered this issue to network:

	https://github.com/haskell/network/issues/302

Viktor, thank you!

--Kazu

>> On Jan 29, 2018, at 10:25 PM, Kazu Yamamoto (山本和彦) <kazu at iij.ad.jp> wrote:
>> 
>>    socket family stype protocol = do
>>       fd <- c_socket ...
>>       ...
>>       let s = Socket fd
>>       addFinalizer s $ close s
>>       ruturn s
> 
> For the record, I think I've convinced Kazu Yamamoto that this is
> an anti-pattern.  Such a finalizer would easily end up closing
> already closed sockets, whose file-descriptors may already be
> associated with other open files or sockets.  That way lie all
> sorts of difficult to isolate race-condition bugs.  To make this
> safe, the close function would need to mutate the socket,
> invalidating the enclosed file-descriptor, and would then need to
> be a NOP or just raise an exception if the socket is closed again
> (the finalizer should invoke a close variant that just returns
> without raising exceptions if the socket is already closed).
> 
> There is, AFAIK still an unresolved bug along these lines somewhere
> in http-client and its dependencies.  So far no reproducing cases have
> been provided.  No very recent reports either, perhaps it went away,
> or people have just been more lucky lately:
> 
>    https://github.com/snoyberg/http-client/issues/252
>    https://github.com/vincenthz/hs-tls/issues/179
> 
> All that said, the original question about addFinalizer vs. GHC 7.10
> may still be worth exploring, even if its use-case for Sockets goes
> away.  So, please don't take this poset to mean that the original
> question should be ignored.
> 
> -- 
> 	Viktor.
> 
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