[GHC] #5902: Cannot tell from an exception handler whether the exception was asynchronous

[GHC]

#5902: Cannot tell from an exception handler whether the exception was
asynchronous
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        Reporter:  simonmar          |            Owner:  simonmar
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Compiler          |          Version:  7.4.1
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonmar):

 `newCatch` doesn't work for `bracket`.  Consider:

 {{{
 bracket before after thing =
   mask $ \restore -> do
     a <- before
     r <- restore (thing a) `onException` after a
     _ <- after a
     return r
 }}}

 Now, when the async exception is received, `onException` runs the
 exception handler (`after a`), which cleans up whatever resource was
 allocated by `before`.  But when this this resumed after the async
 exception, if `onException` is using `newCatch`/`newThrow`, it would re-
 run `restore (thing a)`, which assumes that the resource is present and
 accessible.

 So while it might make sense to re-run the whole `unsafePerformIO` when we
 resume after an async exception, it certainly doesn't make sense to do it
 at a finer granularity.  We don't want most exception-handlers to be
 resumable.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5902#comment:16>
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