GHCi doesn't run computations in a new thread

[dons]

A broken QuickCheck property cause ghci to panic after
reloading the module. Seen in stable and head branch.

paprika$ ghci T.hs
*Main> do_test
                     test : *                         
      (0)
*Main> :reload
ghc-6.5: panic! (the `impossible' happened, GHC version
6.5):
        <<loop>>

Please report it as a compiler bug to
glasgow-haskell-bugs@haskell.org,
or http://sourceforge.net/projects/ghc/.

Changing the property to check for empty lists causes
the test to pass, and reload to work fine.

-- Don Stewart

[dons]

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Here's the test case, since uploading files is annoying in
sourceforge:

import Test.QuickCheck.Batch

prop_silly :: [()] -> Bool
prop_silly xs = head xs == head xs 

do_test = runTests "test" defOpt [ run prop_silly ]

[simonpj]

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Test.Quickcheck.Batch.run forks a "watcher" thread that 
sends a NonTermination exception to the parent; the idea is 
to time-out tests that run too long.  The parent kills the 
watcher when the test completes.

Sadly, the parent doesn't kill the watcher when the test itself 
throws an exception.

So, two problems:

1.  The "run" in QuickCheck.Batch is wrong and should be 
fixed.  I'm not sure who wrote it.

2.  GHCi itself can be killed by a thread spawned by an 
evaluation started by GHCi.   That's really a bug; but I'm not 
sure about the best way to fix it.

I'm going to leave this until Simon gets back from paternity 
leave!

Simon

[simonmar]

Fixed, test is ghci/should_run/ghci014.

Related to #488.