[Haskell-cafe] Conduit missing closing file handles

30 Apr 2026

      I have a pretty simple program for converting ZIP to TAR archives using 
the 'zip' and 'tar-conduit' packages, both using Conduit for streaming 
data. In principle the program works, but if the ZIP archive contains a 
non-trivial number of files, the program breaks because of exceeded limit 
of open file handles:

$ zip2tar /usr/share/libreoffice/share/config/images_colibre.zip >/tmp/images_colibre.tar
zip2tar: Uncaught exception 
ghc-internal:GHC.Internal.IO.Exception.IOException:

/usr/share/libreoffice/share/config/images_colibre.zip: openFile: resource 
exhausted (Too many open files)

While handling /usr/share/libreoffice/share/config/images_colibre.zip: 
openFile: resource exhausted (Too many open files)

HasCallStack backtrace:
   throwIO, called at ./Control/Monad/Trans/Resource.hs:195:13 in 
resourcet-1.3.0-8b355f9483371034fb75ff40adb2e07157502721c941fbf8a6f607df6dd81546:Control.Monad.Trans.Resource

The main function is essentially this one: [1]

convertZipToTar ::
    (Zip.EntrySelector -> Zip.EntryDescription -> CTar.FileInfo) ->
    Sort -> FilePath -> IO ()
convertZipToTar fileInfoFromZipEntry_ sortMode zipPath =
    Zip.withArchive zipPath $ do
       entries <- Map.toList <$> Zip.getEntries
       files <-
          for entries $ \(selector, descr) ->
             flip fmap (Zip.getEntrySource selector) $ \src ->
                src
                .|
                (C.yield (Left $ fileInfoFromZipEntry_ selector descr)
                 >>
                 C.mapC Right)
       C.liftIO $ runConduitRes $
          sequence_ files
          .|
          void CTar.tar
          .|
          C.stdoutC

'Zip.getEntrySource selector' generates a conduit for reading a file from 
the ZIP archive. It opens the archive anew for every accessed file, but it 
calls Conduit.Binary.sourceIOHandle, which promises to close a file 
promptly after reading. [2]

It seems that files are not immediately closed after reading, but I cannot 
spot the cause. I suspected that "sequence_ files" might concatenate the 
Conduits while accumulating open file handles, but I think it should not 
and I did not find a matching alternative to sequence_.

[1] https://hackage-content.haskell.org/package/zip2tar-0.0/src/src/Main.hs
[2] https://hackage-content.haskell.org/package/zip-2.2.1/docs/src/Codec.Archive...

[Haskell-cafe] Conduit missing closing file handles

Henning Thielemann