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When GC runs a number of finalizers in a row, and the first of them throws an exception, then other finalizers are ignored. The relevant piece of code is here.

The following program reproduces the issue:

import Data.IORef
import Control.Monad
import Control.Exception
import System.Mem

main :: IO ()
main = do
  run
  run
  run
  run
  performMajorGC
  performMajorGC

run :: IO ()
run = do
  ref <- newIORef ()
  void $ mkWeakIORef ref $ do
    putStr "."
    throwIO $ ErrorCall "failed"

I expect it to output "....", but I get only "."

The issue makes it unsafe to rely on finalizer for resource cleanup because unrelated finalizer (e.g. from some other library) may prevent your finalizer from running.

Actually I was sure the issue is known, but today I tried to find a reference to it, and failed.

If it is by design, then it should be documented somewhere.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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