[GHC] #12547: Concurrent.ForeignPtr needs to access a C-ForeignPtr, but this is already gone

[GHC]

#12547: Concurrent.ForeignPtr needs to access a C-ForeignPtr, but this is already
gone
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        Reporter:  Lemming           |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Runtime System    |              Version:  8.0.1
      Resolution:                    |             Keywords:
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 Lemming):

 Replying to [comment:1 simonmar]:
 > Currently all `ForeignPtrs` that are unreachable get finalized at the
 same time.  This is how it's intended to work.
 >
 > We *could* refine it so that a finalizer from a `Concurrent.ForeignPtr`
 can keep a C `ForeignPtr` alive.  It would mean processing the two kinds
 of weak pointers in separate batches (and possibly keeping them in
 separate lists, I'm not sure).

 You gave me privately the tip to use `StablePtr`. So I added a
 `freeStablePtr` after `withForeignPtr`. This works nicely!

 Thus I think it is enough to add a warning to `touchForeignPtr` and
 `withForeignPtr` that they have no effect in a concurrent finalizer and
 that you should consider `StablePtr` instead.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12547#comment:3>
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