[Haskell-cafe] Timing out a pure evaluation of an expression I did not write myself

[Ian Denhardt]

That should have the same problem; the exceptions can't be delivered
until the target thread reaches a yield. 'timeout' uses the same
underlying mechanism.

Obvious worrisome question: is it possible to use this to DoS
lambdabot/mueval?

Quoting Erik Hesselink (2018-11-29 05:15:49)
>    Hi Ryan,
>    I'm not quite sure of your use case, but lambdabot/mueval do something
>    like this I think. From a brief look at the source [1] it seems they
>    compute in a separate thread, then in another thread, wait until the
>    timeout then kill the computation thread. Would something like that
>    work for you?
>    Erik
>    [1]
>    [1]https://github.com/gwern/mueval/blob/09b6a7aa5a25c4115442ea2e6ae0c2d
>    b557007f8/Mueval/Parallel.hs
>
>    On Thu, 29 Nov 2018 at 03:41, Ryan Reich <[2]ryan.reich at gmail.com>
>    wrote:
>
>    I expected something like that. I'm all the way back in ghc-8.2.2, but
>    I think what this really shows is that the flag is unreliable and
>    version-dependent. Unfortunately there doesn't seem to be a precise
>    specification of where yield points should appear, and therefore, where
>    they might be not-omitted.
>
>    On Wed, Nov 28, 2018, 17:10 Viktor Dukhovni <[3]ietf-dane at dukhovni.org
>    wrote:
>
>      On Wed, Nov 28, 2018 at 09:39:01AM -0800, Ryan Reich wrote:
>      > Thanks for the suggestion, which Arjen made also.�  Unfortunately,
>      it does
>      > not appear to help.�  See this simple program:
>      >
>      > -- Loop.hs
>      > import Control.Exception
>      > import System.Timeout
>      >
>      > main :: IO (Maybe Integer)
>      > main = timeout 100000 $ evaluate $ last $ repeat 0
>      > -- end
>      >
>      > With either GHC invocation "stack exec ghc Loop[ --
>      -fno-omit-yields]",
>      > running ./Loop fails to terminate (it should do so in 0.1s).
>      >
>      > Based only on the very terse description of that flag in the
>      User's Guide,
>      > and its name, I think the problem is simply that GHC doesn't
>      normally
>      > *generate* yields in that loop, so there's nothing not to omit.
>      It times out for me with GHC 8.4.4 on FreeBSD 11.2, and "ghc -O
>      -fno-omit-yields"
>      and does�  not time out with "ghc -O":
>      �  �  $ cat /tmp/foo.hs
>      �  �  import Control.Exception
>      �  �  import System.Timeout
>      �  �  main :: IO (Maybe Integer)
>      �  �  main = timeout 1000000 $ evaluate $ last $ repeat 0
>      �  �  $ ghc -O -fno-omit-yields /tmp/foo.hs
>      �  �  [1 of 1] Compiling Main�  �  �  �  �  �  � ( /tmp/foo.hs,
>      /tmp/foo.o ) [Optimisation flags changed]
>      �  �  Linking /tmp/foo ...
>      �  �  $ time /tmp/foo
>      �  �  real�  �  0m1.033s
>      �  �  user�  �  0m1.025s
>      �  �  sys�  �  � 0m0.008s
>      �  �  $ rm /tmp/foo
>      �  �  $ ghc -O /tmp/foo.hs
>      �  �  [1 of 1] Compiling Main�  �  �  �  �  �  � ( /tmp/foo.hs,
>      /tmp/foo.o ) [Optimisation flags changed]
>      �  �  Linking /tmp/foo ...
>      �  �  $ time /tmp/foo
>      �  �  ^C^C
>      �  �  real�  �  0m5.864s
>      �  �  user�  �  0m5.857s
>      �  �  sys�  �  � 0m0.000s
>      On MacOS X with GHC 7.10.3, it does not time out either way.�
>      Perhaps
>      some versions of GHC don't make the timeout possible.
>      --
>      �  �  �  �  Viktor.
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> Verweise
>
>    1. https://github.com/gwern/mueval/blob/09b6a7aa5a25c4115442ea2e6ae0c2db557007f8/Mueval/Parallel.hs
>    2. mailto:ryan.reich at gmail.com
>    3. mailto:ietf-dane at dukhovni.org
>    4. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>    5. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe