Understanding ResourceT

19 Jun 2017 Michael Snoyman

This blog post came out of two unrelated sets of questions I received last week about usage of the resourcet library. For those unfamiliar with it, the library is often used in combination with the Conduit streaming data library; basically every conduit tutorial will quickly jump into usage of the resourcet library.

Instead of just teaching you how to use the library, this post will demonstrate why you need it and how it works internally, to help you avoid some of the potential pitfalls of the library. And stay tuned in the next week or two for a fun debugging storing around resourcet, bracket, and monad-control.

Anyway, back to our topic. To start off, consider some code to read a file and print its size:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
import qualified Data.ByteString as B
import qualified System.IO as IO

main :: IO ()
main = do
  bs <- myReadFile "/usr/share/dict/words"
  print $ B.length bs

myReadFile :: FilePath -> IO B.ByteString
myReadFile fp = IO.withBinaryFile fp IO.ReadMode $ \h ->
  -- Highly inefficient, use a builder instead
  let loop front = do
        next <- B.hGetSome h 4096
        if B.null next
          then return front
          else loop $ B.append front next
   in loop B.empty

However, this is highly inefficient: it reads the entire contents of the file into memory at once, when we don't need that. Instead, let's calculate that in a streaming fashion:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE BangPatterns #-}
import qualified Data.ByteString as B
import qualified System.IO as IO

main :: IO ()
main = do
  len <- myFileLength "/usr/share/dict/words"
  print len

-- Yes, there's hFileSize... ignore that
myFileLength :: FilePath -> IO Int
myFileLength fp = IO.withBinaryFile fp IO.ReadMode $ \h ->
  let loop !total = do
        next <- B.hGetSome h 4096
        if B.null next
          then return total
          else loop $ total + B.length next
   in loop 0

Notice that in both of these implementations, we've used withBinaryFile to open the file in such a way that the handle will be closed when we're done with it, regardless of whether an exception is thrown.

Introduce continuations

But it's pretty unforunate that we've coupled together our file read logic with the logic that consumes the file. Let's make an abstraction similar to conduit to address that. We'll have an action which returns the next chunk of data from the file, and the following action to perform.

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE BangPatterns #-}
import qualified Data.ByteString as B
import qualified System.IO as IO

data IOSource a
  = IOChunk a (IO (IOSource a))
  | IODone

sourceHandle :: IO.Handle -> IO (IOSource B.ByteString)
sourceHandle h = do
  next <- B.hGetSome h 4096
  return $
    if B.null next
      then IODone
      else IOChunk next (sourceHandle h)

sourceFile :: FilePath -> IO (IOSource B.ByteString)
sourceFile fp = IO.withBinaryFile fp IO.ReadMode sourceHandle

sourceLength :: IO (IOSource B.ByteString) -> IO Int
sourceLength =
    loop 0
  where
    loop !total mnext = do
      next <- mnext
      case next of
        IOChunk bs mnext' -> loop (total + B.length bs) mnext'
        IODone -> return total

main :: IO ()
main = do
  len <- sourceLength $ sourceFile "/usr/share/dict/words"
  print len

Our IOSource is essentially a slimmed-down conduit which can't consume any input, only produce output. That's good enough for proving our point. The sourceHandle function has the same basic structure to what we were doing in our first two code examples: read a chunk of data, see if it's null, and if not, we return that chunk and then keep going. We then do a trivial wrapping up of sourceHandle with sourceFile, which uses the same withBinaryFile we had before. Finally, sourceLength just grabs the successive chunks from a given IOSource and counts the total bytes.

There's a major bug in this program. Try to spot it. Think through the control flow of this program. I encourage you to actually figure it out for yourself instead of just continuing to my explanation below.

Hint 1 This isn't a subtle exception-handling bug, it makes the program above completely broken in all cases (except, interestingly, the case of an empty file). You will never get a valid result, besides the empty file case.

Hint 2 The output when I run this program is /usr/share/dict/words: hGetBufSome: illegal operation (handle is closed).

Explanation When we enter the sourceFile function, we first call withBinaryFile. This opens up a file handle. We hand this file handle to sourceHandle, which reads the first chunk of data from the file, and returns an IOChunk value containing that chunk and a continuation, or instruction on what to do next. This continuation is an IO action, and it refers to that file handle we were given by sourceFile. (This bit is vital.) We then return this IOChunk value from sourceHandle to sourceFile. Inside sourceFile, we now trigger the cleanup bit of withBinaryFile, which closes the handle, and then return the IOChunk value back to the caller.

When we consume that IOChunk value, we will proceed to perform that continuation we were handed back. That continuation refers to the previously opened file handle, and will try to read from it. See the problem? We've already closed it! There is nothing we can do with it anymore.

Explicit close

Let's try rewriting this to delay the closing of the file handle until the handle is fully consumed. Also, let's replace our sourceLength function with a new function: it tells us what the first byte in the file is. I've also added a putStrLn to tell us when we're closing the file handle.

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE BangPatterns #-}
import qualified Data.ByteString as B
import qualified System.IO as IO
import Data.Word (Word8)

data IOSource a
  = IOChunk a (IO (IOSource a))
  | IODone

sourceHandle :: IO.Handle -> IO (IOSource B.ByteString)
sourceHandle h = do
  next <- B.hGetSome h 4096
  if B.null next
    then do
      putStrLn "Closing file handle"
      IO.hClose h
      return IODone
    else return $ IOChunk next (sourceHandle h)

sourceFile :: FilePath -> IO (IOSource B.ByteString)
sourceFile fp = do
  h <- IO.openBinaryFile fp IO.ReadMode
  sourceHandle h

firstByte :: IO (IOSource B.ByteString) -> IO (Maybe Word8)
firstByte mnext = do
  next <- mnext
  return $
    case next of
      IOChunk bs _mnext' -> Just $ B.head bs
      IODone             -> Nothing

main :: IO ()
main = do
  mbyte <- firstByte $ sourceFile "/usr/share/dict/words"
  print mbyte

OK, take a guess at the output. In particular, will our file handle be closed, and why?

It turns out that, when dealing with continuations, there is no way to guarantee that your continuation will ever get called. In our case, we're only interested in reading the first chunk of data from the file, and want to ignore the rest. As a result, our cleanup code will never get called. This doesn't even get into the fact that, if an exception is thrown, we have no exception handler in place to perform cleanup. The moral of the story:

Continuation based approaches, like conduit or ContT, cannot guarantee that cleanup code will be run.

(Side note: conduit actually adds a concept called finalizers to address the non-exception case and to ensure cleanup happens promptly. But that's not our topic today.)

So what's the right way to write this code? You have to use withBinaryFile outside of your sourceHandle call entirely, like this:

main :: IO ()
main = do
  mbyte <- IO.withBinaryFile "/usr/share/dict/words" IO.ReadMode
         $ \h -> firstByte $ sourceHandle h
  print mbyte

Why this is bad

Firstly, there's an aesthetic argument again the above code. A function like sourceFile is convenient, elegant, and simple to teach. Telling people that they need to open their file handles first can be confusing. But this isn't the only problem. Let's consider a few more complicated cases:

  1. I want to create an IOSource that reads from two files, not just one. Ideally, we would only keep one file handle open at a time. If you follow through on the withBinaryFile approach above, you'd realize you need to open up both files before you get started. This is a performance problem of using too many resources.
  2. Suppose you want to read a file, and each line in that file will tell you a new file to open and stream from. In this case, we won't know statically how many files to open, or even which files to open. Since these facts are dynamically determined, our withBinaryFile approach won't work at all.
  3. If the previous example seems a bit far-fetched, that's exactly the case when doing a deep directory traversal. We start with a top level directory, and for each entry, may or may not need to open up a new directory handle, depending on whether it's a directory or not.

In other words: this approach is a bit cumbersome to use, resource-inefficient, and prevents some programs from being written at all. We need something better.

Why withBinaryFile works

The reason that withBinaryFile solves our problems is that it lives outside of our continuation framework. It is not subject to the whims of whether a specific continuation will or will not be called. It lives in IO directly, and we know how to install a cleanup function which will always be called, regardless of whether an exception is thrown or not. Specifically: we can just use bracket.

We need some way to pair the control that bracket provides from outside our continuation with the dynamic allocations we want to perform inside our continuations.

A simplified ResourceT

In order to make this work, we'll implement a simplified version of ResourceT. We'll keep a list of file handles that need to be closed. But since we need to be able to update that list dynamically from within our continuation code, this will be a mutable list (wrapped in an IORef). Also, for simplicity, we'll make it ResourceIO instead of a proper monad transformer.

Note that, by sticking to just a list of file handles, we've simplified our work significantly. File handles can be closed multiple times, and closing a file handle is not supposed to throw an exception itself (though it can in some corner cases; we're ignoring that). The actual code for ResourceT ensures that cleanups only happen one time and explicitly deals with exceptions from cleanup code.

{-# LANGUAGE DeriveFunctor #-}
module ResourceIO
  ( ResourceIO
  , runResourceIO
  , openBinaryFile
  ) where

import Data.IORef
import qualified System.IO as IO
import Control.Exception
import Control.Monad
import Control.Monad.IO.Class

newtype ResourceIO a = ResourceIO (IORef [IO.Handle] -> IO a)
  deriving Functor

instance Applicative ResourceIO where
  pure x = ResourceIO $ \_ -> return x
  (<*>) = ap
instance Monad ResourceIO where
  return = pure
  ResourceIO f >>= g = ResourceIO $ \ref -> do
    x <- f ref
    let ResourceIO g' = g x
    g' ref
instance MonadIO ResourceIO where
  liftIO m = ResourceIO $ \_ref -> m

runResourceIO :: ResourceIO a -> IO a
runResourceIO (ResourceIO inner) = bracket
  (newIORef [])
  cleanup
  inner
  where
    cleanup ref = do
      handles <- readIORef ref
      mapM_ IO.hClose handles

openBinaryFile :: FilePath -> IO.IOMode -> ResourceIO IO.Handle
openBinaryFile fp mode = ResourceIO $ \ref -> mask $ \restore -> do
  h <- restore $ IO.openBinaryFile fp mode
  atomicModifyIORef' ref $ \hs -> (h:hs, ())
  return h

Most of the code here is involved in implementing a Monad/MonadIO interface for ResourceIO. If you focus on runResourceIO, you'll see that, as promised, we're using bracket. We create our shared mutable reference, ensure that cleanup is called regardless of exceptions, and then run the user-provided action.

openBinaryFile demonstrates how we would allocate resources. We open the file, and immediately modify our list of open handles to include the newly opened handle. In the real ResourceT, this is generalized to IO () actions to perform arbitrary cleanup.

Side note: if you're confused about the usage of mask here, it's to deal with the possibility of asynchronous exceptions, and to make sure an exception is not thrown between the call to openBinaryFile and atomicModifyIORef'. Proper async exception handling is a complicated topic, which is why it's best to stick to library functions like bracket and libraries like safe-exceptions that are designed to handle them.

Using it

We need to make some minor modifications to our program in order to use this. Firstly, we specialized IOSource to using IO actions only. We're now going to want this thing to run in ResourceIO, so let's add a type parameter to indicate the base monad (just like ConduitM has). And let's also call a spade a spade, and rename from IOSource to ListT. This is, after all, the correctly implemented list monad transformer. (Ignore the one from the transformers package, it's completely broken.)

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE BangPatterns #-}
import qualified Data.ByteString as B
import qualified System.IO as IO
import Data.Word (Word8)
import ResourceIO
import Control.Monad.IO.Class

data ListT m a
  = ConsT a (m (ListT m a))
  | NilT

sourceHandle :: MonadIO m => IO.Handle -> m (ListT m B.ByteString)
sourceHandle h = liftIO $ do
  next <- B.hGetSome h 4096
  if B.null next
    then do
      IO.hClose h
      return NilT
    else return $ ConsT next (sourceHandle h)

sourceFile :: FilePath -> ResourceIO (ListT ResourceIO B.ByteString)
sourceFile fp = do
  h <- openBinaryFile fp IO.ReadMode
  sourceHandle h

firstByte :: Monad m => m (ListT m B.ByteString) -> m (Maybe Word8)
firstByte mnext = do
  next <- mnext
  return $
    case next of
      ConsT bs _mnext' -> Just $ B.head bs
      NilT             -> Nothing

main :: IO ()
main = do
  mbyte <- runResourceIO $ firstByte $ sourceFile "/usr/share/dict/words"
  print mbyte

Note that there's no longer any call with withBinaryFile, and we have all of the exception safety guarantees we want. We can even implement something which reads two files in sequence, and have the desired behavior of only having one file open at a time:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
{-# LANGUAGE BangPatterns #-}
import qualified Data.ByteString as B
import qualified System.IO as IO
import Data.Word (Word8)
import ResourceIO
import Control.Monad.IO.Class

data ListT m a
  = ConsT a (m (ListT m a))
  | NilT

appendListT :: Monad m
            => m (ListT m a)
            -> m (ListT m a)
            -> m (ListT m a)
appendListT left0 right =
    loop left0
  where
    loop mnext = do
      next <- mnext
      case next of
        ConsT x mnext' -> return $ ConsT x $ loop mnext'
        NilT           -> right

sourceHandle :: MonadIO m => IO.Handle -> m (ListT m B.ByteString)
sourceHandle h = liftIO $ do
  next <- B.hGetSome h 4096
  if B.null next
    then do
      IO.hClose h
      return NilT
    else return $ ConsT next (sourceHandle h)

sourceFile :: FilePath -> ResourceIO (ListT ResourceIO B.ByteString)
sourceFile fp = do
  h <- openBinaryFile fp IO.ReadMode
  sourceHandle h

sourceLength :: Monad m => m (ListT m B.ByteString) -> m Int
sourceLength =
    loop 0
  where
    loop !total mnext = do
      next <- mnext
      case next of
        ConsT bs mnext' -> loop (total + B.length bs) mnext'
        NilT            -> return total

main :: IO ()
main = do
  len <- runResourceIO $ sourceLength $ appendListT
    (sourceFile "/usr/share/dict/words")
    (sourceFile "/usr/share/dict/words")
  print len

Concurrency

If you looked in the code above, I used atomicModifyIORef' to add a new file handle to the cleanup queue. You may think that this means we're concurrency-friendly. However, we aren't at all. Let's start by adding a new function to our ResourceIO interface:

asyncResourceIO :: ResourceIO a -> ResourceIO (Async a)
asyncResourceIO (ResourceIO f) = ResourceIO $ \ref -> async $ f ref

This uses the async library to fork a thread and provides an Async value to retrieve the value from that thread when it completes. Now let's naively use it in our main function:

main :: IO ()
main = do
  alen <- runResourceIO $ asyncResourceIO $ sourceLength $
    (sourceFile "/usr/share/dict/words")
  putStrLn "Do some other work in the main thread, may take a while..."
  threadDelay 100000
  len <- wait alen
  print len

With the ominous introduction I gave this, answer this question: do you think this is going to work? And why or why not?

Let's step through what's going to happen here:

  1. runResourceIO creates a mutable reference to hold onto file handles to be closed
  2. asyncResourceIO forks a child thread
  3. Child thread opens up a file handle and adds it to the mutable reference of things to clean up
  4. Parent thread finishes forking the child thread, and (from within runResourceIO) calls the cleanup action, closing the file handle
  5. Child thread continues to do work, but throws an exception trying to read from the (now closed) file handle

Actually, that's just one possible scenario. Another possibility is that the parent thread will call cleanup before the child thread grabs the file handle. In which case, the reads will succeed, but we'll have no guarantee that the file handle will be cleaned up. In other words, we have a race condition.

This should stress the important of getting concurrency and ResourceT correct. We need to make sure that runResourceT does not close any resources that are still being consumed by child threads. One way to do that is to use the resourceForkIO function, which introduces a reference counting scheme to ensure that resources are only closed when all threads are done with them.

Unfortunately, due to how the monad-control instances for ResourceT work, using concurrency functions from lifted-base or lifted-async will not use this reference counting behavior. Overall, my recommendation is: don't fork threads when inside ResourceT if you can avoid it.

Other ways to abuse ResourceT

There is no actual scoping of the resources you get from ResourceT to ensure that they are still alive. Such techniques do exist (e.g., regions), but the types are significantly more complicated, which is why the conduit ecosystem sticks to ResourceT.

The simplest demonstration of breaking this is:

main :: IO ()
main = do
  h <- runResourceIO $ openBinaryFile "/usr/share/dict/words" IO.ReadMode
  len <- sourceLength $ sourceHandle h
  print len

The handle we get back from openBinaryFile will be closed before we ever get a chance to pass it to sourceHandle. This code is just as broken as:

main :: IO ()
main = do
  h <- IO.withBinaryFile "/usr/share/dict/words" IO.ReadMode return
  len <- sourceLength $ sourceHandle h
  print len

But for many, the latter is more obviously wrong. The rule: make sure that your runResourceIO call lives around the entire scope that the resources will be used in.

As a more real-world example taken from a Twitter discussion, consider the following code that you might achieve by playing Type Tetris with Conduit:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
import Conduit

main :: IO ()
main = do
  len <- runConduit
       $ transPipe runResourceT (sourceFile "/usr/share/dict/words")
      .| lengthCE
  print len

transPipe applies some kind of a monad transformation at each step of the running of the given conduit. So each time we try to perform some action in sourceFile, we'll create a new mutable reference of cleanup actions, perform the action, and then immediately clean up the resources we allocated. In reality, we want those resources to persist through later continuations within the sourceFile. We would rewrite the code above to:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
import Conduit

main :: IO ()
main = do
  len <- runResourceT
       $ runConduit
       $ sourceFile "/usr/share/dict/words"
      .| lengthCE
  print len

Or, since runConduitRes = runResourceT . runConduit:

#!/usr/bin/env stack
-- stack --resolver lts-8.12 script
import Conduit

main :: IO ()
main = do
  len <- runConduitRes
       $ sourceFile "/usr/share/dict/words"
      .| lengthCE
  print len
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