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.