wiki:Exceptions/StackTraces

Plan for providing exception stack-traces

See also: #12096

Exception handling in modern Haskell programs revolves around the Control.Exception module provided by base. This module provides a mechanism where any type belonging to the Exception typeclass can be thrown as an exception and potentially caught by a handler.

The goal of this work is to augment this mechanism with the ability to transparently attach to an exception a stack trace locating the point where it was thrown. This should be done in such a way that backwards compatibility is preserved while providing a way for exception handlers who want a backtrace to get one.

In order to see how we might accomplish this it will be helpful to review how exception handling currently works in GHC Haskell.

Primitive operations

All (synchronous) exception handling in GHC is built on a few primitive operations,

catch# :: (State# RealWorld -> (# State# RealWorld, a #))
       -> (b -> State# RealWorld -> (# State# RealWorld, a #))
       -> State# RealWorld
       -> (# State# RealWorld, a #)

raise# :: a -> b

raiseIO# :: a -> State# RealWorld -> (# State# RealWorld, b #)

Which we can roughly interpret as,

catch# :: IO a
       -> (b -> IO a)
       -> IO a

raise# :: a -> b

raiseIO# :: a -> IO b

Note how, like many primops, none of these care in the slightest about the types you give them. In fact, it is trivial to realize unsafePerformIO equipped with them,

λ> IO $ catch# (raiseIO# 48) (\a s1 -> (# s1, a::Char #))
'0'
λ> IO $ catch# (raiseIO# [1,2,3::Int]) (\a s1 -> (# s1, a::Int #))
8646911832613230923

For this reason, base implements a far safer interface on top of this mechanism.

Control.Exception

Control.Exception provides a type-guided mechanism for exception handling,

class (Typeable e, Show e) => Exception e where ...

-- | A value of any type in @Exception@ can be raised as an exception
throw :: Exception e => e -> a

-- | Run a potentially failing computation, catching and handling
-- exceptions of a particular type @e@.
catch :: Exception e
      => IO a         -- ^ potentially failing computation
      -> (e -> IO a)  -- ^ a handler
      -> IO a

The Exception typeclass itself demands very little of its members, requiring only a Show instance (merely so we have something useful to show the user) and a runtime type representation (i.e. belonging to the Typeable class).

To make this polymorphic handling scheme work we need to have some way to preserve information about the type of the exception that was thrown, lest the handler has no idea how to interpret the closure it is handed. For this, we introduce a type allowing us to box the exception value itself along with an Exception dictionary,

data SomeException = forall e. Exception e => SomeException e

Since Exception implies Typeable this gives us the ability to identify the type of the exception contained within. We can now safely define throw without having to worry about being unable to reconstruct the type of the thrown exception later,

class (Typeable e, Show e) => Exception e

throw :: Exception e => e -> a
throw e =
    let e' = SomeException e
    in raise# e'

Now when we catch an exception, we can use the fact that we know the value thrown was of type SomeException. This provides us the ability to determine whether a given exception is appropriate for a given handler using Typeable's cast function,

-- | From 'Data.Typeable'
cast :: (Typeable a, Typeable b) => a -> Maybe b

catchException :: Exception e => IO a -> (e -> IO a) -> IO a
catchException (IO io) handler = IO $ catch# io handler'
  where
    handler' :: SomeException -> IO a
    handler' se@(SomeException e) =
      case cast e of
        Just e' -> unIO (handler e')
        Nothing -> raiseIO# se

Note how in the event that cast returns Nothing we rethrow the exception. This will be an important consideration on our treatment of stack traces.

The plan for stack traces

Having examined the current scheme, we can now discuss how to add persist stack trace information through this entire process. It should be said that this stack trace support must remain optional: not only does stack trace support pose a tricky engineering challenge for second tier platforms, but backtrace collection itself can often represent a significant runtime cost which users may seek to avoid.

Feeding a stack trace to the handler

For the moment ignore the question of *how* we actually collect a stack trace; instead we'll consider how we can get a backtrace safely to the handler once it has been collected.

We begin by modifying SomeException to add an optional StackTrace field, using pattern synonyms to preserve the existing interface,

-- | Some notion of a stack trace
data StackTrace

data SomeException = forall e. Exception e => SomeExceptionWithStack !(Maybe StackTrace) e

-- | Preserve compatibility for existing users
pattern SomeException e <- SomeExceptionWithStack _ e where
    SomeException e = SomeExceptionWithStack Nothing e

We can now easily introduce a variant of catch which can provide this optional stack trace to the handler,

catchExceptionWithStack :: Exception e
                        => IO a
                        -> (Maybe StackTrace -> e -> IO a)
                        -> IO a
catchExceptionWithStack (IO io) handler = IO $ catch# io handler'
  where
    handler' :: SomeException -> IO a
    handler' se@(SomeExceptionWithStack stack e) =
      case cast e of
        Just e' -> unIO (handler stack e')
        Nothing -> raiseIO# se

Note how both catchException and catchExceptionWithStack both handle rethrowing correctly; since they are rethrowing the same SomeException, the stack trace to the original throw-site will be preserved even as the exception bubbles up through the handler stack.

Producing a stack trace

Now we come to the question of how one actually produces a stack trace. Despite the perceptions of many users, Haskell actually has a wealth of potential stack trace sources,

  • The cost-center stack may be available if the program was built with profiling enabled
  • The (relatively new) implicit-parameter-based stacktrace facility may be able to offer a bit of local context
  • DWARF-based stack unwinding may be available depending upon the platform and availability of debugging information

Minimizing unnecessary stack collection

Sadly, all of these mechanisms come at some cost. For instance, GHC's newly-integrated libdw-based stack unwinder appears to require on the order of one microsecond per unwound frame (although this can probably be improved by implementing a fast-path for traversing the STG stack). For this reason, we ideally want to minimize unnecessary stack collections: if none of the handlers on the stack want a backtrace when an exception is thrown, we should avoid collecting one at all.

To accomplish this, we introduce a piece of per-Haskell-thread state which records the location of the outer-most stack-frame wanting a backtrace (if any). This pointer would be set by the RTS on the first call to catchExceptionWithStack in a thread's lifetime. Conversely it would be cleared when this catch frame is popped from the stack.

When the thread raises an exception, throw would check this state and collect a stack trace if a parent frame has reported interest in one.

While this reduces the number of unnecessary stack collections, it does not eliminate them entirely. If a handler with no interest in a stack frame handles the exception before it bubbles up to the stack-trace-desiring handler, the effort spent in collecting the trace will have been wasted. Unfortunately, this may be a rather common occurrence, as we would likely want the top-level exception handler to provide a stack trace if possible.

Alternatives

A more sophisticated (and complicated) scheme for reducing the cost of stack collection would be to utilize memory management trickery to preserve the stack immutably when an exception is thrown. However, given the complexity this involves and the cost of setting up memory mappings, this is unlikely to be worth exploring.

Moreover, given that exceptions are relatively rare in Haskell code, it's unclear whether any of the effort described in the above two sections is worthwhile. Perhaps we just accept a high exception cost and enable/disable stacktrace collection explicitly on a per-thread or even per-process basis.

Asynchronous exceptions

Up until this point we have been ignoring asychronous exceptions, which build upon the killThread# primitive operation,

killThread# :: ThreadId# -> a
            -> State# RealWorld -> State# RealWorld

throwTo :: Exception e => ThreadId -> e -> IO ()
throwTo (ThreadId tid) ex = IO $ \ s ->
   case (killThread# tid (toException ex) s) of s1 -> (# s1, () #)

This is a bit tricky as we (probably) want the stack returned to be that of thread subject to the exception. While scheme for handling synchronous exceptions presented above can be implemented almost entirely in library code, asynchronous exceptions will require a bit more support from the RTS for this reason.

Unfortunately, this ends up making the RTS's handling of asynchronous exceptions a bit messier than it is currently. As it stands, the story is quite simple: killThread# takes the closure it is passed, packages it up in a message, and sends it to the target thread. The target then simply takes the exception and begins the usual unwinding process (TODO check this).

The easiest way to fit stack traces into this story would be to introduce a variant of killThread#,

killThreadWithStack# :: ThreadId# -> (Maybe StackTrace -> a)
                     -> State# RealWorld -> State# RealWorld

This would be handled in much the same way but the RTS would send the function provided in a message indicating that it wants to be applied to a stack trace. The RTS would then (possibly) check whether any handlers may want a stack trace, collect one if necessary, apply the supplied function to the result, and unwind as usual.

Given that many exceptions frequently encountered in the wild are asynchronous (e.g. StackOverflow and HeapOverflow), I believe this added complexity will pull its weight.

Last modified 8 months ago Last modified on Apr 10, 2017 8:50:22 PM