[GHC] #15417: Memory leaks because of too conservative isAlive check in the GC

[GHC]

#15417: Memory leaks because of too conservative isAlive check in the GC
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           Reporter:  osa1           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  Research needed
          Component:  Runtime        |           Version:  8.5
  System                             |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 (See also the mailing list [https://mail.haskell.org/pipermail/ghc-
 devs/2018-July/016027.html thread])

 Static objects with no SRTs and pointer fields are currently ignored by
 `evacuate()`. This is efficient but causes trouble when checking alive-
 ness of static objects in `isAlive()`, which is used for (among other
 things) checking whether a weak's key has died. This causes leaks as value
 of a weak with static key will always be kept alive.

 Here's a demonstration:

 {{{#!haskell
 module Main where

 import System.Mem.Weak (mkWeak, deRefWeak)
 import System.Mem (performMajorGC)

 mkKey :: IO String
 mkKey = readFile "/dev/random"

 -- mkKey :: IO Int
 -- mkKey = return 1

 main :: IO ()
 main = do
     w <- mkKey >>= \k -> mkWeak k () Nothing
     performMajorGC
     performMajorGC
     performMajorGC
     deRefWeak w >>= print
 }}}

 The idea is that the first `mkKey` function returns a non-static string
 and this program prints `Nothing`. If I enable the second `mkKey` function
 the key is now a static object and the program prints `Just ()`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15417>
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