[GHC] #13439: ForeignPtr finalizers not searched for reachable objects?

[GHC]

#13439: ForeignPtr finalizers not searched for reachable objects?
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        Reporter:                    |                Owner:  (none)
  facundo.dominguez                  |
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  libraries/base    |              Version:  8.0.2
      Resolution:                    |             Keywords:  ForeignPtr
                                     |  finalizers
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by facundo.dominguez):

 The documentation of [http://hackage.haskell.org/package/base-4.9.1.0/docs
 /Foreign-ForeignPtr.html#v:touchForeignPtr touchForeignPtr] mentions that
 it can't be used in finalizers to keep the pointer alive. But the program
 fails the same if one replaces it with
 {{{
 main = do
     rFinalized <- newIORef Set.empty
     forM_ [0..50000] $ \i -> do
       fp0 <- newForeignPtr nullPtr $
                atomicModifyIORef' rFinalized (\s -> (Set.insert i s, ()))
 +     mv <- newMVar fp0
       newForeignPtr nullPtr $ do
         finalized <- atomicModifyIORef' rFinalized
                        (\s -> (Set.delete i s, Set.member i s))
         when finalized $
           putStrLn "fp0 was prematurely finalized"
 -       touchForeignPtr fp0
 +       void $ takeMVar mv
     performMajorGC
     threadDelay 1000000
 }}}

 Which means that the references in finalizers are peculiar in some sense
 beyond the specifics of `touchForeignPtr`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13439#comment:1>
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