#2558: re-throwing an asynchronous exception throws it synchronously
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Reporter: simonmar | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: Compiler | Version: 6.8.3
Severity: normal | Keywords:
Difficulty: Unknown | Testcase:
Architecture: Unknown | Os: Unknown
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If you catch and re-throw an asynchronous exception, the second time it
will be thrown synchronously. This only makes a difference when using
`unsafePerformIO` or `unsafeInterleaveIO`, because otherwise the `catch`
will not be inside any thunks.
This could have the unfortunate effect of updating an `unsafePerformIO`
thunk with `Timeout`, or `KillThread`, or some other asychronous exception
that should not be recorded as the value of the thunk.
We believe that exceptions thrown by an exception handler should be thrown
in the same mode (synchronous or asynchronous) as the original exception.
This means that we need to
* keep track of the current exception-throwing mode
* push a frame when in an exception handler (we already do this, because
the
exception handler is implicitly wrapped in `block`)
* respect this mode when throwing exceptions.
This raises another issue: catching an exception and recovering by tail-
calling out of the exception handler is not a good thing to do. Currently
it has the undesirable effect that the thread remains in `block` mode, and
an explicit `unblock` is required. With the changes above, the thread may
also remain in "asynchronous exception" mode.
The right way to solve this is just "don't do that". Here's how to use
the exception API:
* For cleanup, use `finally`, `bracket`, and `onException`.
* To modify the exception, use `mapException`.
* For recovery, use `try`.
Perhaps we should be deprecating `catch`? Or perhaps `catch` should be
implemented in terms of `try`, so it doesn't have the implicit `block` /
re-throw async exceptions behaviour?
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2558>
GHC <http://www.haskell.org/ghc/>
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