Weak reference semantics - why does a dead weak ref keep its value alive?

[Luite Stegeman]

Hi all,

I'm reviewing and improving my weak references implementation for GHCJS,
among other things to make sure that the profiling/stack trace support,
currently being implemented by Ömer Sinan Ağacan as a GSoC project has the
correct heap information to work with.

JavaScript does not have weak references with 'observable deadness', so we
have to walk the heap data structures from time to time to test
reachability, schedule finalizers and break the weak links. I'm trying to
find a design that keeps GHC's semantics, but where JavaScript's own
garbage collector can get rid of as much data as possible, even before we
run our own heap scan.

Now I ran into the following peculiarity, from the semantics
(System.Mem.Weak documentation):

A heap object is reachable if:
- It is a member of the root set.
- It is directly pointed to by a reachable object, other than a weak
pointer object.
- It is a weak pointer object whose key is reachable.
- It is the value or finalizer of a weak pointer object whose key is
reachable.

This says that even if nothing has a reference to a weak pointer, as long
as there are references to its key, the value is considered to be
reachable. For example in this program

import Data.Maybe
import System.Mem.Weak
import System.Mem
import Control.Exception
import System.IO
import Control.Concurrent

gc = performGC >> threadDelay 1000000

main = do
  hSetBuffering stdout NoBuffering
  k <- evaluate "k"
  v <- evaluate "v"
  addFinalizer v (putStrLn "fv")
  w <- evaluate =<< mkWeak k v (Just $ putStrLn "fkv")
  putStrLn =<< fmap (fromMaybe ".") (deRefWeak w)
  addFinalizer w (putStrLn "fw")
  gc
  putStrLn k
  gc

there is no way to reach 'v' after `w` has been finalized, but still it's
kept alive. This agrees with the semantics in the documentation, but what's
the reason for this?

luite
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