# Handling (async) exceptions in haskell: pushing bracket to the limits

November 20, 2014
Yuras Shumovich

(Originally posted here)

Probably bracket is the most widely used approach to resource management in haskell. There are other, probably more convenient, but bracket covers most of use cases. However it is common to claim, that haskell is broken with respect to exceptions. Here we’ll try to play with bracket and it’s implementation in order to find out, what exactly is wrong with exceptions in haskell and how to fix them. I’ll assume you know how bracket is implemented and why (see here for details) and understand basic principles of exception handling.

The post is organized into a series of examples, each subsequent extends the previous one.

## example #1: Playground

To study internal implementation of bracket we’ll need a convenient way to test it. For example, we’ll need to run sample code and check whether any resource leaked. Lets use file I/O as a model and create custom Handle-like API

First, imports (I’ll usually skip unimportant details, but link to full source code at the end of section):

import Data.IORef
import Control.Exception (SomeException)
import qualified Control.Exception as E
import System.IO.Unsafe (unsafePerformIO)

To account allocated resources, lets use global variable. Right now we care only about total number of live handles:

{-# NOINLINE numHandles #-}
numHandles :: IORef Int
numHandles = unsafePerformIO $newIORef 0 The Handle and basic operation: data Handle = Handle openFile :: FilePath -> IO Handle openFile _ = do modifyIORef' numHandles succ return Handle hClose :: Handle -> IO () hClose _ = modifyIORef numHandles pred hPutStr :: Handle -> String -> IO () hPutStr _ _ = return () Note, that openFile simply increments global resource counter, and hClose decrements it. We also want to know whether our sample throws an exception and how many resources leaked. The following helper executes sample action and outputs any exception and content of the resource counter on exit: test :: IO () -> IO () test action = do action E.catch \e -> do putStrLn$ "exception: " ++ show (e :: SomeException)
readIORef numHandles >>= putStrLn . ("Number of open handles: " ++) . show

That is all for now. Lets test it with the following example:

main :: IO ()
main = test example

example :: IO ()
example = do
h <- openFile "path"
hPutStr h "Hello"
hClose h

It is obviously not exception-safe, but if you run it, you’ll see the next:

Number of open handles: 0

No resource leak, and that it great. But what if something goes wrong before we close the handle?

example = do
h <- openFile "path"
hPutStr h "Hello"
error "something went wrong"
hClose h

exception: something went wrong

Number of open handles: 1

So the exception escaped to top level, and the handle leaked, as expected. You can find full source code here

## example #2: bracket

The issue in the example #1 is pretty clear – we should ensure the resource is freed even in case of exception. That is exactly the use case bracket exists for:

bracket :: IO a -> (a -> IO ()) -> (a -> IO b) -> IO b
bracket allocate release use =
E.mask $\restore -> do resource <- allocate restore (use resource) E.finally release resource example = bracket (openFile "path") hClose$ \h -> do
hPutStr h "Hello"
error "something went wrong"
hPutStr h "World"

(Here the implementation of bracket follows the standard implementation.) Try it and check that the resource doesn’t leak. It looks good enough. It even handles asynchronous exceptions (You if are not sure, please read more). Lets check that. The easiest way is to use timeout to raise async exception and threadDelay to make sure it will be delivered exactly in the point we need it. Here is an example:

import System.Timeout (timeout)

example = void $timeout (1 * 1000 * 1000)$
bracket (openFile "path") hClose $\h -> do hPutStr h "Hello" hPutStr h "World" threadDelay (2 * 1000 * 1000) Note: both functions accept time in nanoseconds. We choose timeout to be 1 sec, and the delay is 2 secs. That way we ensure the async exception will interrupt the delay. Try it yourself to see that it finishes in ~1 sec without exception or resource leak. But what if the exception will interrupt cleanup action, hClose? First of all, hClose is interruptible action – it can be interrupted by async exception even if they are masked. Why? For example, because it needs to flush internal buffers before closing the file. Lets model that: To track, what was actually written to file, we’ll flush data to global IORef. {-# NOINLINE dataWritten #-} dataWritten :: IORef [String] dataWritten = unsafePerformIO$ newIORef []

Handle will have internal buffer, and hFlush will move it’s content (if any) to the dataWritten variable:

data Handle = Handle (IORef (Maybe String))

hFlush :: Handle -> IO ()
hFlush (Handle ref) = do
case val of
Just str -> modifyIORef dataWritten (str :)
_ -> return ()
writeIORef ref Nothing

Now the hPutStr. It will flush the old buffer and replace it with new value:

hPutStr :: Handle -> String -> IO ()
hPutStr h@(Handle ref) str = do
hFlush h
writeIORef ref (Just str)

And finally hClose. If flushes the buffer and closes the handle. Our implementation roughly models the implementation from ghc:

hClose :: Handle -> IO ()
hClose h = hFlush h
E.finally modifyIORef numHandles pred

Note: hClose guarantees, that the handle will be closed even if hClose fails (see docs). Here we use finally to make sure resource doesn’t leaks even if hFlush fails.

Note2: the current implementation in ghc is a bit broken – there are rare cases where hClose fails to free resource. This issue is well known and easy to fix (but nobody cares enough even to open a ticket :) ), We’ll ignore that unfortunate fact.

Lets also print content of dataWritten on exit:

test action = do
action E.catch \e -> do
putStrLn $"exception: " ++ show (e :: SomeException) readIORef numHandles >>= putStrLn . ("Number of open handles: " ++) . show readIORef dataWritten >>= putStrLn . ("Data writtern to file: " ++) . show If you run the last example in the new playground, you’ll see the output: Number of open handles: 0 Data writtern to file: [“World”,“Hello”] Looks good. Now we can check how it works if hFlush fails on cleanup: hClose :: Handle -> IO () hClose h = hFlushFailing h E.finally modifyIORef numHandles pred hFlushFailing :: Handle -> IO () hFlushFailing _ = error "hFlush failed" We replaces hFlush with it’s failing variant. Note: hPutStr uses the initial version still. The example is the same: example = void$ timeout (1 * 1000 * 1000) $bracket (openFile "path") hClose$ \h -> do
hPutStr h "Hello"
hPutStr h "World"
threadDelay (2 * 1000 * 1000)

exception: hFlush failed

Number of open handles: 0

Data writtern to file: [“Hello”]

Exception escapes to top level, but the resource was released correctly. Also some data was flush to file, but other wasn’t. That is ok, basic exception safely guarantees are satisfied: we left the file in valid (closed in that case) state. The state is probably unknown, because we don’t know exactly what was written to the file, but that is ok. We don’t really care about file content here because the whole operation fails anyway.

So, everything is good? timeout uses async exception to interrupt the action, but catches it internally, and the program can continue execution. In our case, async exception interrupted threadDelay, but bracket catches the exception and run cleanup action. Cleanup fails, throwing other exception. Now bracket has to decide what to do – rethrow the original exception or let the new one to propagate. The standard implementation chooses the later solution, it lets new exception propagate. The async exception was silently ignored. That is the instance of double-throw issue.

Homework: construct double-throw issue not using async exceptions.

The source code so far.

## example #3: fixing bracket

The previous example was interrupted by timeout, so it already failed, and we don’t care about file’s content anymore. We want to prevent resource leak, that is all. There is no any reason to propagate exception from cleanup action here. So lets fix bracket:

bracket :: IO a -> (a -> IO ()) -> (a -> IO b) -> IO b
bracket allocate release use =
E.mask $\restore -> do resource <- allocate restore (use resource) E.finally (E.catch (release resource) (\e -> putStrLn$ "Ignoring: " ++ show (e :: SomeException)))

We guarded cleanup action with catch and ignore any exceptions. If you run the example, you’ll see:

Ignoring: hFlush failed

Number of open handles: 0

Data writtern to file: [“Hello”]

That makes perfect sense, but totally wrong. Lets remove the timeout:

example =
bracket (openFile "path") hClose $\h -> do hPutStr h "Hello" hPutStr h "World" threadDelay (2 * 1000 * 1000) Ignoring: hFlush failed Number of open handles: 0 Data writtern to file: [“Hello”] The exception is ignored. Note the difference, now the example succeeded, but internal buffer was not flushed. We silently lost data. We should ignore exception only if the inner action fails: bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c bracket allocate release use = E.mask$ \restore -> do
resource <- allocate
result <- E.catch (restore $use resource)$ \e -> do
void (release resource) E.catch \e' ->
putStrLn ("Ignoring exception: " ++ show (e' :: SomeException))
E.throw (e :: SomeException)
void (release resource)
return result

Try this with and without the timeout. [The source code] (https://github.com/Yuras/io-region/blob/master/misc/imask/example3.hs)

## example #4: kill them all

The implementation is still problematic. To show that, lets try to kill the main thread while it is executing the example:

hClose :: Handle -> IO ()
hClose h = hFlushFailing h
E.finally modifyIORef numHandles pred

hFlushFailing :: Handle -> IO ()
hFlushFailing _ = do
error "hFlush failed"

example :: IO ()
example = void $timeout (2 * 1000 * 1000)$
bracket (openFile "path") hClose $\h -> do hPutStr h "Hello" hPutStr h "World" threadDelay (2 * 1000 * 1000) main = do pid <- myThreadId void$ forkIO $do threadDelay (1 * 1000 * 1000) killThread pid test example Ignoring exception: hFlush failed exception: thread killed Number of open handles: 0 Data writtern to file: [“Hello”] The delay is interrupted by killThread (it used async exception for that). hClose failed to flush buffers, but the exception was ignored, and the thread was successfully killed. Perfect. Now lets give timeout a chance to fire. We can add delay in hFlushFailing, so that it will block waiting for the timeout. Obviously async exception will be ignored, just like in previous case with sync exception. But what if we change the order? Let timeout fire before killing thread: hFlushFailing :: Handle -> IO () hFlushFailing _ = do threadDelay (2 * 1000 * 1000) error "hFlush failed" example :: IO () example = void$ timeout (1 * 1000 * 1000) $bracket (openFile "path") hClose$ \h -> do
hPutStr h "Hello"
hPutStr h "World"
threadDelay (2 * 1000 * 1000)

main = do
void $forkIO$ do
threadDelay (2 * 1000 * 1000)
test example

Number of open handles: 0

Data writtern to file: [“Hello”]

We failed to kill the thread! It is probably clear now, that there is no way for bracket to choose what exception to propagate. Either way will break one of this two examples. What makes the difference compared to sync exceptions? Async exceptions are external, so neither hClose no bracket know their semantics, and have to be exception-neutral to them: always propagate instead of handling or ignoring. But clearly there is no way to propagate two exceptions at once.

Source code

We just showed that async exceptions are problematic when come paired. Then we should disable them using uninterruptibleMask. Lets try the previous example:

bracket :: IO a -> (a -> IO b) -> (a -> IO c) -> IO c
bracket allocate release use =
E.mask $\restore -> do resource <- allocate result <- E.catch (restore$ use resource) $\e -> do -- I added uninterruptibleMask below: void (E.uninterruptibleMask_$ release resource) E.catch \e' ->
putStrLn ("Ignoring exception: " ++ show (e' :: SomeException))
E.throw (e :: SomeException)
void (release resource)
return result

hFlushFailing :: Handle -> IO ()
hFlushFailing _ = do
-- delay set to 20 seconds to make the issue clear
threadDelay (20 * 1000 * 1000)
error "hFlush failed"

\$ time ./example5

Ignoring exception: hFlush failed

Number of open handles: 0

Data writtern to file: [“Hello”]

real 0m21.017s

user 0m0.000s

sys 0m0.004s

OK, nobody will say that we failed to kill the thread, because we finally killed it. After 20 seconds of waiting….

Flushing buffers usually is fast. Until you are working with network filesystem, or ftp server via FUSE, or TCP socket, or…. But if you are only using the local filesystem, then you are probably OK :)

Source code

## example #6: pushing to the limits

Double throw issue is unsolvable in current haskell without introducing unbound blocking in cleanup actions. I hope the examples convinced you. Is haskell broken? Probably. Is there anything we can do? Definitely.

Lets think about the semantics. We are going to kill the thread, but we can’t because we are blocked on hFlush. Note that we don’t need to flush buffers here, we are not interested in side effect on the action. We just want to kill it. We are blocked on the action, that we don’t need at all. We only want to ensure no resources leak.

(From theoretical point of view we should not flush buffers here because in case of failure we should minimize side effects of our action. Here flushing buffers is unnecessary side effect. Ideally we should undo all writes to the file to achieve strong exception safety, but unfortunately it is not possible here, so we minimize side effects and guarantee basic exception safety, see here)

hClose provides a way to stop it from doing unnecessary work: it is interruptible, but guarantees resource cleanup even in case of exception. But to interrupt action we need async exception, which is harmful because of double throw. We need a way to disable async exceptions but interrupt any interruptible action on the way. Something like a fast way that skips all optional work. Lets call it interruptibleMask.

IMO it is not possible to properly implement it in user space, minimal support from RTS is necessary here. But the example6 contains pure mans implementation, so that you can try it out, play with it and get the idea. No resource leaks, KillThread exception is not ignored, there is no double-throw issue. (The implementation is not thread safe and pretty limited, so you’ll probably break it easily.)

## Notes: uninterruptibleMask in bracket proposal

The idea of the post is 2 or 3 month old. I came to the double throw issue half a year ago. Unfortunately nobody answered my questions, either nobody knows about the issue or nobody cares.

So the post is not directly related to the recent proposal to introduce uninterruptibleMask in bracket, but the proposal forced me to publish this writeup earlier. I didn’t have time to ensure it is sound and doesn’t contain flaws. So I assume it does.

But I hope to convince people that the design space is pretty wide right now, there can be much better solutions to exception handling in haskell. The proposal is probably a local optimum, but it is hard to revert later because it will break correct code. There will be no way to escape the local optimum even if we find better one. We should explore the design space before closing the door.

Thanks to Eyal Lotem for very useful discussion.

I forgot to describe one important idea, but it doesn’t fit to the post naturally anymore. So I’ll just append it to the end.

The semantics of the example6 will not change if you move hFlush from hClose to the end of bracket’s 3d argument and use ordinary uninterruptibleMask in cleanup. So withFile will look like the next:

withFile :: FilePath -> (Handle -> IO a) -> IO a
withFile path action =
bracket (openFile path)
return r)