C finalizer may be called on re-used memory

It seems that the runtime system sometimes reuses memory referred to by ForeignPtr values before the associated C finalizers have been run. At that's what it looks like to me. Maybe I'm interpreting the behavior of GHC's runtime system incorrectly, or it's a defect in my test program.

To reproduce the defect, compile finalizertest.hs and finalizerlib.c, e.g. with

ghc finalizertest.hs finalizerlib.c -threaded

Run the resulting program as

./finalizertest +RTS -N2

After a while it prints a message like

finalize_value: 80f69dc != 11223344 after 47393 calls
Aborted

The C code prints this if the pointer passed to the finalizer does not point to the expected value.

It's not necessary to link with the threaded runtime, nor is it necessary to run with more than one CPU. Doing so increases the likelihood of the defect to occur substantially, though.

I've observed this with several different combinations of GHC and host-systems:

• GHC 7.10.2 on a 64 bit Linux (Debian jessie)

• GHC 7.10.1 on a 32 bit Linux (Debian wheezy)

• GHC 7.4.1 on a 32 bit Linux (Debian wheezy)

  This GHC is the one packaged by Debian

It's crucial for the defect that the memory is allocated with mallocForeignPtrBytes. Using mallocBytes instead and building a ForeignPtr with finalizerFree avoids the defect.

I came across this defect while trying to debug a segmentation fault in the zlib package. This defect was reported on the libraries mailing list https://mail.haskell.org/pipermail/libraries/2015-June/025829.html (corresponding Agda ticket: https://github.com/agda/agda/issues/1518). I came across it in one of my own projects last week.

My test program basically does what zlib does when allocating and initializing the z_stream value: it allocates memory with mallocForeignPtrBytes and later adds a finalizer with addForeignPtrFinalizer.

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture