Proposal: Use uninterruptibleMask for cleanup actions in Control.Exception

Eyal Lotem

3 Sep 2014 3 Sep '14

4:56 p.m.

I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost. For example, bracket is defined as: bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup. This means bracket does not in fact guarantee the cleanup occurs. For example: readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken! withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle! Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants. I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically: *bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException* should all simply wrap their exception/cancellation handler with uninterruptibleMask. The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup. -- Eyal

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Gregory Collins

3 Sep 3 Sep

8:01 p.m.

Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop. The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads. G On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...

I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins

Merijn Verstraaten

8:03 p.m.

Handles use MVar’s internally, therefore any operation on a Handle can potentially block, making the operation interruptible. FWIW, I’m +1 on this. It’s been a pain to get this correct in my code. Cheers, Merijn On 03 Sep 2014, at 17:01 , Gregory Collins wrote:

...

Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote: I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as: bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

Eyal Lotem

4 Sep 4 Sep

3:29 a.m.

In addition to hClose, the majority of cleanup handlers in my programs turned out to be interruptible. Moreover, whether something is interruptible is unclear. You can easily add a putStrLn to a cleanup handler and now it is accidentally interruptible. I'd love to see examples of some code where interruptible cleanup handlers are not a bug. Every single one in my programs that I examined was a bug. Is withMVar also a primop? Because it's buggy in the same way as withFile currently is. The current situation is that virtually all uses of bracket in the entire Haskell ecosystem are subtle bugs. On Sep 4, 2014 3:01 AM, "Gregory Collins" wrote:

...

Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...
I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins

John Lato

3:46 a.m.

Being a primop or not has nothing to do with whether an MVar operation is interruptible. What matters is whether or not the operation will block. withMVar (or readMVar in any incarnation) on an empty MVar is interruptible. If the MVar happened to be full, it's not interruptible. I agree this is a problem. I don't think the proposed solution is perfect, but I do think it's possibly better than the status quo. Perhaps the user should be required to use uninterruptibleMask_ on the cleanup action if necessary? I've long thought that using an interruptible operation in a cleanup handler is a programming error. John L. On Thu, Sep 4, 2014 at 12:29 AM, Eyal Lotem wrote:

...

In addition to hClose, the majority of cleanup handlers in my programs turned out to be interruptible. Moreover, whether something is interruptible is unclear. You can easily add a putStrLn to a cleanup handler and now it is accidentally interruptible.

I'd love to see examples of some code where interruptible cleanup handlers are not a bug.

Every single one in my programs that I examined was a bug.

Is withMVar also a primop? Because it's buggy in the same way as withFile currently is.

The current situation is that virtually all uses of bracket in the entire Haskell ecosystem are subtle bugs. On Sep 4, 2014 3:01 AM, "Gregory Collins" wrote:

...
Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...
I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

Eyal Lotem

10:01 a.m.

The problem with requiring the user to use uninterruptibleMask_ on their cleanup is how error-prone it is. If we examine some uses of bracket in the GHC repo: compiler/main/GhcMake.hs:992: let withSem sem = bracket_ (waitQSem sem) (*signalQSem sem*) *signalQSem is interruptible, this is a bug!* bracket_ (hSetBinaryMode h False) (*hSetBinaryMode h True*) Is the hSetBinaryMode operation interruptible? Is this a bug? Hard to tell! withExtendedLinkEnv new_env action = gbracket (liftIO $ extendLinkEnv new_env) (\_ -> *reset_old_env*) (\_ -> action) *reset_old_env *uses MVars so is probably interruptible, probably a bug. Lots of manual openFile/hClose duplications of withFile, which are all buggy due to *hClose* being interruptible. InteractiveUI.doLoad: gbracket (liftIO $ do hSetBuffering stdout LineBuffering hSetBuffering stderr LineBuffering) (\_ -> liftIO $ do* hSetBuffering stdout NoBuffering* hSetBuffering stderr NoBuffering) $ \_ -> do hSetBuffering uses hFlush and is thus interruptible(?). If many uses of bracket by the GHC developers are broken, is it not a clear indication that the default behavior is too error prone? If someone wants an interruptible cleanup handler (Very weird thing to want!) they can use "mask" and "onException" directly. Or variants of all the cleanup functions with an "interruptible" suffix can be exported too (bracketInterruptible, catchInterruptible, etc). Keeping the current behavior has only one benefit: Use cases that need to have interruptible cleanups. Does anyone have any such use case at all? I can't imagine one... On Thu, Sep 4, 2014 at 10:46 AM, John Lato wrote:

...

Being a primop or not has nothing to do with whether an MVar operation is interruptible. What matters is whether or not the operation will block. withMVar (or readMVar in any incarnation) on an empty MVar is interruptible. If the MVar happened to be full, it's not interruptible.

I agree this is a problem. I don't think the proposed solution is perfect, but I do think it's possibly better than the status quo. Perhaps the user should be required to use uninterruptibleMask_ on the cleanup action if necessary? I've long thought that using an interruptible operation in a cleanup handler is a programming error.

John L.

On Thu, Sep 4, 2014 at 12:29 AM, Eyal Lotem wrote:

...
In addition to hClose, the majority of cleanup handlers in my programs turned out to be interruptible. Moreover, whether something is interruptible is unclear. You can easily add a putStrLn to a cleanup handler and now it is accidentally interruptible.

I'd love to see examples of some code where interruptible cleanup handlers are not a bug.

Every single one in my programs that I examined was a bug.

Is withMVar also a primop? Because it's buggy in the same way as withFile currently is.

The current situation is that virtually all uses of bracket in the entire Haskell ecosystem are subtle bugs. On Sep 4, 2014 3:01 AM, "Gregory Collins" wrote:

...
Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...
I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Eyal

Roman Cheplyaka

10:46 a.m.

I find your arguments quite convincing. Count that as +1 from me. Roman

John Lato

7:28 p.m.

I suspect the main reason someone would want an interruptible cleanup handler is because if the action blocks, it's the only way to kill the thread. I'm not sure how common this case is though. IMHO I think the real problem is that the current exception implementation is far too complex to reason about. Do other languages have this interruptible/uninterruptible distinction? If so, I've yet to encounter it. I'm starting to think that throwing an exception to a thread is the wrong way to kill it, and it should use something other than the exception mechanism. Then I think we could discard the notion of interruptible actions entirely. I guess count me as +0.5 for this proposal. John L. On Thu, Sep 4, 2014 at 10:01 PM, Eyal Lotem wrote:

...

The problem with requiring the user to use uninterruptibleMask_ on their cleanup is how error-prone it is.

If we examine some uses of bracket in the GHC repo:

compiler/main/GhcMake.hs:992: let withSem sem = bracket_ (waitQSem sem) (*signalQSem sem*) *signalQSem is interruptible, this is a bug!*

bracket_ (hSetBinaryMode h False) (*hSetBinaryMode h True*) Is the hSetBinaryMode operation interruptible? Is this a bug? Hard to tell!

withExtendedLinkEnv new_env action = gbracket (liftIO $ extendLinkEnv new_env) (\_ -> *reset_old_env*) (\_ -> action)

*reset_old_env *uses MVars so is probably interruptible, probably a bug.

Lots of manual openFile/hClose duplications of withFile, which are all buggy due to *hClose* being interruptible.

InteractiveUI.doLoad: gbracket (liftIO $ do hSetBuffering stdout LineBuffering hSetBuffering stderr LineBuffering) (\_ -> liftIO $ do* hSetBuffering stdout NoBuffering* hSetBuffering stderr NoBuffering) $ \_ -> do

hSetBuffering uses hFlush and is thus interruptible(?).

If many uses of bracket by the GHC developers are broken, is it not a clear indication that the default behavior is too error prone?

If someone wants an interruptible cleanup handler (Very weird thing to want!) they can use "mask" and "onException" directly. Or variants of all the cleanup functions with an "interruptible" suffix can be exported too (bracketInterruptible, catchInterruptible, etc).

Keeping the current behavior has only one benefit: Use cases that need to have interruptible cleanups. Does anyone have any such use case at all? I can't imagine one...

On Thu, Sep 4, 2014 at 10:46 AM, John Lato wrote:

...
Being a primop or not has nothing to do with whether an MVar operation is interruptible. What matters is whether or not the operation will block. withMVar (or readMVar in any incarnation) on an empty MVar is interruptible. If the MVar happened to be full, it's not interruptible.

I agree this is a problem. I don't think the proposed solution is perfect, but I do think it's possibly better than the status quo. Perhaps the user should be required to use uninterruptibleMask_ on the cleanup action if necessary? I've long thought that using an interruptible operation in a cleanup handler is a programming error.

John L.

On Thu, Sep 4, 2014 at 12:29 AM, Eyal Lotem wrote:

...
In addition to hClose, the majority of cleanup handlers in my programs turned out to be interruptible. Moreover, whether something is interruptible is unclear. You can easily add a putStrLn to a cleanup handler and now it is accidentally interruptible.

I'd love to see examples of some code where interruptible cleanup handlers are not a bug.

Every single one in my programs that I examined was a bug.

Is withMVar also a primop? Because it's buggy in the same way as withFile currently is.

The current situation is that virtually all uses of bracket in the entire Haskell ecosystem are subtle bugs. On Sep 4, 2014 3:01 AM, "Gregory Collins" wrote:

...
Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...
I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Eyal

Bertram Felgenhauer

6 Sep 6 Sep

3:03 p.m.

Eyal Lotem wrote:

...

The problem with requiring the user to use uninterruptibleMask_ on their cleanup is how error-prone it is.

If we examine some uses of bracket in the GHC repo:

compiler/main/GhcMake.hs:992: let withSem sem = bracket_ (waitQSem sem) (*signalQSem sem*) *signalQSem is interruptible, this is a bug!*

Not really, but signalQSem is a rare exception from the rule. signalQSem :: QSem -> IO () signalQSem (QSem m) = uninterruptibleMask_ $ do -- Note [signal uninterruptible] r <- takeMVar m r' <- signal r putMVar m r' It's tempting to suggest that every cleanup type function should just mask exceptions itself like this. However, that does not solve the problem when several cleanup actions are required: at the end of the first uninterruptibleMask_, any pending exceptions will be delivered. Overall I agree with your assessment that a general uninterruptibleMask_ for cleanup handlers avoids more trouble than it causes. Cheers, Bertram

Eric Mertens

4 Sep 4 Sep

11:16 a.m.

It's hasn't been noted or I haven't noticed that putMVar is only interruptible when the MVar is full. This means that there isn't a problem in cases like withMVar because the MVar is empty due to the establishing takeMVar, and there is no leak. For hClose if you are closing the handle, presumably the contained MVar is full and there is no block and therefore no interruption. Only if another thread is actively using the handle and you are trying to close the handle out from under it is there potential for failure. In the exotic cases like this there is already a way to make an interruptible operation uninterruptible. I'm -1 until it becomes clear that it is actually an issue in common code. On Sep 4, 2014 12:47 AM, "John Lato" wrote:

...

Being a primop or not has nothing to do with whether an MVar operation is interruptible. What matters is whether or not the operation will block. withMVar (or readMVar in any incarnation) on an empty MVar is interruptible. If the MVar happened to be full, it's not interruptible.

I agree this is a problem. I don't think the proposed solution is perfect, but I do think it's possibly better than the status quo. Perhaps the user should be required to use uninterruptibleMask_ on the cleanup action if necessary? I've long thought that using an interruptible operation in a cleanup handler is a programming error.

John L.

On Thu, Sep 4, 2014 at 12:29 AM, Eyal Lotem wrote:

...
In addition to hClose, the majority of cleanup handlers in my programs turned out to be interruptible. Moreover, whether something is interruptible is unclear. You can easily add a putStrLn to a cleanup handler and now it is accidentally interruptible.

I'd love to see examples of some code where interruptible cleanup handlers are not a bug.

Every single one in my programs that I examined was a bug.

Is withMVar also a primop? Because it's buggy in the same way as withFile currently is.

The current situation is that virtually all uses of bracket in the entire Haskell ecosystem are subtle bugs. On Sep 4, 2014 3:01 AM, "Gregory Collins" wrote:

...
Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...
I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Gregory Collins

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

Eyal Lotem

12:03 p.m.

On Thu, Sep 4, 2014 at 6:16 PM, Eric Mertens wrote:

...

It's hasn't been noted or I haven't noticed that putMVar is only interruptible when the MVar is full.

This means that there isn't a problem in cases like withMVar because the MVar is empty due to the establishing takeMVar, and there is no leak.

It is plausible that another thread will put into the MVar in the mean time.

...

For hClose if you are closing the handle, presumably the contained MVar is full and there is no block and therefore no interruption. Only if another thread is actively using the handle and you are trying to close the handle out from under it is there potential for failure.

Only in rare cases is this a bug, I agree. But rare bugs are in many ways even worse than frequent bugs. They are harder to debug, detect, etc.

...

In the exotic cases like this there is already a way to make an interruptible operation uninterruptible.

I'm -1 until it becomes clear that it is actually an issue in common code.

I think you're missing an important point. A) Cases that were not interruptible will remain the same. B) Cases that were interruptible *were bugs* and will be fixed. You're claiming that B is rare, but I don't think it is a valid argument against this change, because whether or not you agree B is frequent or not -- the change only affects B and not A. So the question is whether the change is desirable in this circumstance. As an empirical point: My project (buildsome) was very buggy with a lot of leaks and deadlocks, until I replaced my use of Control.Exception with a wrapper that properly used uninterruptible-mask.

...

On Sep 4, 2014 12:47 AM, "John Lato" wrote:

...
Being a primop or not has nothing to do with whether an MVar operation is interruptible. What matters is whether or not the operation will block. withMVar (or readMVar in any incarnation) on an empty MVar is interruptible. If the MVar happened to be full, it's not interruptible.

I agree this is a problem. I don't think the proposed solution is perfect, but I do think it's possibly better than the status quo. Perhaps the user should be required to use uninterruptibleMask_ on the cleanup action if necessary? I've long thought that using an interruptible operation in a cleanup handler is a programming error.

John L.

On Thu, Sep 4, 2014 at 12:29 AM, Eyal Lotem wrote:

...
In addition to hClose, the majority of cleanup handlers in my programs turned out to be interruptible. Moreover, whether something is interruptible is unclear. You can easily add a putStrLn to a cleanup handler and now it is accidentally interruptible.

I'd love to see examples of some code where interruptible cleanup handlers are not a bug.

Every single one in my programs that I examined was a bug.

Is withMVar also a primop? Because it's buggy in the same way as withFile currently is.

The current situation is that virtually all uses of bracket in the entire Haskell ecosystem are subtle bugs. On Sep 4, 2014 3:01 AM, "Gregory Collins" wrote:

...
Unless I'm mistaken, here the "mask" call inside bracket already makes sure you don't receive asynchronous exceptions unless you call a function that is interruptible (i.e. goes back into the runtime system). The hClose example you give doesn't fall in this category, as something inside the RTS needs to call "allowInterrupt" (or otherwise unmask exceptions) in order for async exceptions to be delivered. The "readMVar" example you give *does* have this issue (because putMVar does an implicit allowInterrupt) but in recent GHC readMVar has been redefined as a primop.

The danger of deadlock is *not* minimal here, doing what you suggest will transform many valid programs (i.e. if you block on a "takeMVar" in the cleanup action) into ones that have unkillable orphan threads.

G

On Wed, Sep 3, 2014 at 1:56 PM, Eyal Lotem wrote:

...
I'd like to propose a change in the behavior of Control.Exception to help guarantee cleanups are not accidentally lost.

For example, bracket is defined as:

bracket before after thing = mask $ \restore -> do a <- before r <- restore (thing a) `onException` after a _ <- after a return r

This definition has a serious problem: "after a" (in either the exception handling case, or the ordinary case) can include interruptible actions which abort the cleanup.

This means bracket does not in fact guarantee the cleanup occurs.

For example:

readMVar = bracket takeMVar putMVar -- If async exception occurs during putMVar, MVar is broken!

withFile .. = bracket (openFile ..) hClose -- Async exception during hClose leaks the file handle!

Interruptible actions during "before" are fine (as long as "before" handles them properly). Interruptible actions during "after" are virtually always a bug -- at best leaking a resource, and at worst breaking the program's invariants.

I propose changing all the cleanup handlers to run under uninterruptibleMask, specifically:

*bracket, bracketOnError, bracket_, catch, catchJust, finally, handle, handleJust, onException*

should all simply wrap their exception/cancellation handler with uninterruptibleMask.

The danger of a deadlock is minimal when compared with the virtually guaranteed buggy incorrect handling of async exceptions during cleanup.

-- Eyal

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-- Gregory Collins

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-- Eyal

Bardur Arantsson

12:09 p.m.

On 2014-09-04 18:03, Eyal Lotem wrote:

...

On Thu, Sep 4, 2014 at 6:16 PM, Eric Mertens wrote:

[--snip--]

...

...
I'm -1 until it becomes clear that it is actually an issue in common code.

I think you're missing an important point.

A) Cases that were not interruptible will remain the same. B) Cases that were interruptible *were bugs* and will be fixed.

You're claiming that B is rare, but I don't think it is a valid argument against this change, because whether or not you agree B is frequent or not -- the change only affects B and not A. So the question is whether the change is desirable in this circumstance.

I'm in *no* way an expert on the finer points of concurrency in Haskell, but if the above is true, then +1. Regards,

Felipe Lessa

12:51 p.m.

On 04-09-2014 13:09, Bardur Arantsson wrote:

...

I'm in *no* way an expert on the finer points of concurrency in Haskell, but if the above is true, then +1.

My thoughts, exactly. Eyal's arguments look solid to me, I can't see any flaw, so +1 with the caveat that I'm not an expert here. Assuming he's right, there is a *lot* of broken Haskell code out there! -- Felipe.

Herbert Valerio Riedel

5 Sep 5 Sep

4:37 a.m.

Hello, I'm forwarding this on behalf of Edsko: -------------------- Start of forwarded message -------------------- [...] Hi Herbert,

...

...and I'd like to point your attention to yet another related proposal that came in yesterday:

Subject: Proposal: Use uninterruptibleMask for cleanup actions in Control.Exception

http://thread.gmane.org/gmane.comp.lang.haskell.libraries/22775

Hmmm, I thought I was subscribed to libraries@. It seems I wasn’t. I subscribed earlier today but haven’t got the confirmation email yetI (?). Perhaps you can forward my reply for now? Anyhow, I’m not convinced about this proposal. uninterruptibleMask is rather crude. It would be one thing to block, say, timeout signals during resource cleanup (perhaps), but uninterruptibleMask blocks _all_ asynchronous exceptions, including stack overflow and heap overflow. I don’t think that the _default_ behaviour of bracket should be to disable those. Note also that in the bracket takeMVar putMVar example, the putMVar is _NOT_ interruptible. By definition, putMVar is interruptible _only_ if the resource is not available. Since the takeMVar succeeded, the MVar must be empty at the time of the putMVar and hence it cannot be interrupted. (This is assuming of course that there are no putMVars in the body of the bracket, but if you’re doing that, you have bigger problems..). Incidentally, if that behavior of putMVar is not in fact the case, then that is a bug in putMVar, not in bracket. The hClose example ironically _is_ an example, though it doesn’t look like one — Merijn is right, it uses an MVar under the hood and in fact it does a withMVar, which is interruptible, of course — in case somebody else happens to have the file handle at that time. But again, I’m not sure what the right answer is. I sort of feel that these file operations should _not_ be interruptible. The fact that they use MVars under the hood makes them so, but perhaps that could be considered a bug (and they should do some uninterruptibleMasking of their own) — certainly, as far as I know, it’s nowhere actually _documented_ that these operations are interruptible and most people don’t think of them as such. I’m not sure to be honest. I would like Simon Marlow’s opinion here. But my feeling right now is that I’d vote against this, but to try and make sure that the operations we use in cleanup handlers are, in fact, not interruptible. -E -------------------- End of forwarded message --------------------

Eyal Lotem

5:12 a.m.

On Fri, Sep 5, 2014 at 11:37 AM, Herbert Valerio Riedel wrote:

...

Hello,

I'm forwarding this on behalf of Edsko:

-------------------- Start of forwarded message -------------------- [...]

Hi Herbert,

...
...and I'd like to point your attention to yet another related proposal that came in yesterday:

Subject: Proposal: Use uninterruptibleMask for cleanup actions in Control.Exception

http://thread.gmane.org/gmane.comp.lang.haskell.libraries/22775

Hmmm, I thought I was subscribed to libraries@. It seems I wasn’t. I subscribed earlier today but haven’t got the confirmation email yetI (?). Perhaps you can forward my reply for now?

Anyhow, I’m not convinced about this proposal. uninterruptibleMask is rather crude. It would be one thing to block, say, timeout signals during resource cleanup (perhaps), but uninterruptibleMask blocks _all_ asynchronous exceptions, including stack overflow and heap overflow. I don’t think that the _default_ behaviour of bracket should be to disable those.

I just tested a little program that stack-overflows within an uninterruptibleMask_. The exception doesn't seem to be blocked. Can you(he?) give an example of the actual problem?

...

Note also that in the

bracket takeMVar putMVar

example, the putMVar is _NOT_ interruptible. By definition, putMVar is interruptible _only_ if the resource is not available. Since the takeMVar succeeded, the MVar must be empty at the time of the putMVar

Of course not! Different threads may have putMVar during the bracket. Typical MVar use will not do this, but it's certainly a possible use case that is broken. I think the fact people are missing this bug is more evidence that this behavior is a problem. and hence it cannot be interrupted. (This is assuming of course that

...

there are no putMVars in the body of the bracket, but if you’re doing that, you have bigger problems..). Incidentally, if that behavior of putMVar is not in fact the case, then that is a bug in putMVar, not in bracket.

The hClose example ironically _is_ an example, though it doesn’t look like one — Merijn is right, it uses an MVar under the hood and in fact it does a withMVar, which is interruptible, of course — in case somebody else happens to have the file handle at that time. But again, I’m not sure what the right answer is. I sort of feel that these file operations should _not_ be interruptible. The fact that they use MVars under the hood makes them so, but perhaps that could be considered a bug (and they should do some uninterruptibleMasking of their own) — certainly, as far as I know, it’s nowhere actually _documented_ that these operations are interruptible and most people don’t think of them as such.

But this seems like a weird position to have. On the one hand, you are claiming that making the cleanup uninterruptible is too crude. On the other hand, you're claiming that all cleanup handlers that happen to be interruptible are a bug, each. These positions contradict.

...

I’m not sure to be honest. I would like Simon Marlow’s opinion here. But my feeling right now is that I’d vote against this, but to try and make sure that the operations we use in cleanup handlers are, in fact, not interruptible.

The question is what do you want to do with cleanup handlers that are still interruptible? I think the only valid solutions are: A) Mask them uninterruptible as I suggest B) Raise a runtime error (or at least a warning) that this is a bug that should be fixed by making sure that the cleanup handlers are not interruptible, manually. Leaving the current behavior is the worst of all worlds.

...

-E -------------------- End of forwarded message --------------------

-- Eyal

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participants (10)

Bardur Arantsson
Bertram Felgenhauer
Eric Mertens
Eyal Lotem
Felipe Lessa
Gregory Collins
Herbert Valerio Riedel
John Lato
Merijn Verstraaten
Roman Cheplyaka