#7170: Foreign.Concurrent finalizer called twice in some cases -------------------------------------+------------------------------------- Reporter: joeyadams | Owner: Type: bug | Status: new Priority: high | Milestone: 7.6.1 Component: | Version: 7.8.3 libraries/base | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: crash | Test Case: | ffi/should_run/7170 | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Changes (by arybczak): * cc: hvr, ekmett (added) * owner: simonmar => * status: closed => new * resolution: fixed => * version: 7.4.2 => 7.8.3 Comment: This is still happening, albeit is much harder to reproduce (I wasn't able to write simple testcase for that). Steps to reproduce (hopefully): 1. git clone https://github.com/scrive/hpqtypes.git 2. git checkout 8b4d57876dc573759c2935064306806b793844c5 (this is the last commit before the workaround I applied to solve the issue) 3. git apply custom_finalizer.diff (to be sure it's haskell that tries to free the same memory twice) 4. Change tests/Main.hs to the following code: {{{ module Main where import Control.Applicative import Control.Concurrent import Control.Exception import Control.Monad import Debug.Trace import System.Environment import qualified Data.ByteString.Char8 as BS import Database.PostgreSQL.PQTypes mkConnSource :: String -> ConnectionSource mkConnSource ci = defaultSource $ defaultSettings { csConnInfo = BS.pack ci } main :: IO () main = do cs <- mkConnSource . head <$> getArgs let dts = defaultTransactionSettings flip finally (runDBT cs dts $ return ()) $ do void . forkIO $ do forM_ [1..100000::Int] $ \n -> trace (show n) $ return () return () threadDelay 1000000 }}} 4. cabal sandbox init 5. cabal install --dependencies-only 6. cabal configure -ftests --ghc-options="-O0" 7. cabal build 8. cd dist/build/tests 9. valgrind ./tests user=postgres The last step must be repeated a couple of times to get the error we are interested in. I attach the output of running it on my machine (valgrind- output.txt). If you look at the output file, at line 1112 I pressed Ctrl-C to interrupt the program, then the finalizer was called for the first time at line 1880 and then for the second time at line 2255, which results in tons of output from valgrind about invalid reads of already freed memory. Relevant file: https://github.com/scrive/hpqtypes/blob/8b4d57876dc573759c2935064306806b7938... runDBT calls withConnection of the defaultSource (line 89), which is a simple wrapper for {{{bracket connect disconnect}}}. In disconnect (line 152) the destruction of the database connection is forced by calling finalizeForeignPtr. When finalizeForeignPtr is removed, the bug disappears. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7170#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler