Race-condition in alternative 'System.Timeout.timeout' implementation

[Herbert Valerio Riedel]

Hello *,

I've been experimenting with an alternative implementation of
'System.Timeout.timeout'[1] which avoids the overhead of spawning a new
thread for each invocation.

Part of my motivation is to see if I can implement a faster version of

    threadWaitReadTimeout :: Int -> Fd -> IO Bool
    threadWaitReadTimeout to = liftM (maybe False (const True)) 
                               . timeout to . threadWaitRead

and thus exploit GHC's event notification system instead of having to
reimplement a timeout-manager myself (like popular HTTP server libraries
such as Snap or Warp do currently).


The following Haskell program shows a proof-of-concept implementation
derived directly from 'System.Timeout.timeout' together with a Criterion
benchmark comparing the performance between the original and the
alternative 'timeout' function wrapping a 'readMVar' call.

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On a i7-3770 with GHC-7.6.2/Linux/64bit ran with "+RTS -A4m -N4", the
benchmark shows a 15x improvement for the new implementation (below 1
uS) compared to the original implementation (~13 uS):

,----
| benchmarking id
| mean: 22.60933 ns, lb 22.50331 ns, ub 22.73515 ns, ci 0.950
| std dev: 591.0383 ps, lb 509.6189 ps, ub 663.2670 ps, ci 0.950
| found 17 outliers among 100 samples (17.0%)
|   17 (17.0%) high mild
| variance introduced by outliers: 19.992%
| variance is moderately inflated by outliers
| 
| benchmarking timeout_1ms
| mean: 13.79584 us, lb 13.62939 us, ub 13.92814 us, ci 0.950
| std dev: 756.3080 ns, lb 524.7628 ns, ub 1.068547 us, ci 0.950
| found 14 outliers among 100 samples (14.0%)
|   4 (4.0%) low severe
|   5 (5.0%) high mild
|   5 (5.0%) high severe
| variance introduced by outliers: 52.484%
| variance is severely inflated by outliers
| 
| benchmarking timeout2_1ms
| mean: 879.8152 ns, lb 874.5223 ns, ub 885.9759 ns, ci 0.950
| std dev: 29.31963 ns, lb 25.65941 ns, ub 32.98116 ns, ci 0.950
| found 9 outliers among 100 samples (9.0%)
|   9 (9.0%) high mild
| variance introduced by outliers: 28.734%
| variance is moderately inflated by outliers
| ...
`----

Alas there's a race-condition hidden somewhere I'm struggling with; When
the timeout is set low enough, the internal 'Timeout2' exceptions leaks
outside the 'timeout2' wrapper:

,----
| ...
| benchmarking timeout2_10us
| newtimeout: <<timeout2>>
`----

I've tried rewriting the code but couldn't figure out a way to keep the
exception from escaping 'timeout2'. Does the race-condition actually lie
in the 'timeout2' implementation -- and if so, is it possible to rewrite
'timeout2' to solve it?


 [1]: http://hackage.haskell.org/packages/archive/base/latest/doc/html/System-Timeout.html#v:timeout

cheers,
  hvr