C finalizers are reversed during GC

(See also thread starting at http://www.haskell.org/pipermail/libraries/2012-August/018302.html )

The list of finalizers is reversed during GC (cf. rts/sm/MarkWeak.c), which may cause them to run in the wrong order. The following program reproduces this behaviour.

{-# LANGUAGE ForeignFunctionInterface, MagicHash #-}
import GHC.ForeignPtr
import GHC.Ptr
import System.Mem

-- one should really use own C function rather than this varargs one to avoid
-- possible ABI issues
foreign import ccall "&debugBelch" fun :: FunPtr (Ptr () -> Ptr () -> IO ())

new name = do
    p <- newForeignPtr_ (Ptr name)
    addForeignPtrFinalizerEnv fun (Ptr "finalizer 1 (%s)\n"#) p
    addForeignPtrFinalizerEnv fun (Ptr "finalizer 2 (%s)\n"#) p
    return p

main = do
    p <- new "p"#
    q <- new "q"#
    r <- new "r"#
    performGC -- collect p. finalizer order: 2, then 1.
    print q
    performGC -- collect q. finalizer order: 1, then 2.
              -- expected order: 2, then 1.
    print r
    performGC -- collect r. finalizer order: 2, then 1.

Trac metadata

Trac field	Value
Version	7.6.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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