[Haskell-cafe] Mutable data structures and asynchronous exceptions

[Станислав Черничкин]

It's quite hard to implement mutable data structures in presence of
asynchronous exceptions. Since exception can arise at any point, it is not
possible to guarantee atomicity of operation, hence mutable data structure
may remain in incorrect state in case of interruption. One can certainly
use maskAsyncExceptions# and friends to protect critical regions, but
masking function are living in IO, mutable data structures on other hand
trend to be state-polymorphic (to allow it usage in ST).

This lead to conflicting requirements:
- One should not care about asynchronous exceptions inside ST (it is not
possible to catch exception in ST, hence not possible to use something in
invalid state). More over, it is not even possible to do write
“exception-safe” code, because masking functions not available.
- One should provide accurate masking then using same data structures in IO.

So I want do discuss several questions topics on this case.

1. Impact. Are async exceptions really common? Would not be easier to say:
“ok, things can go bad if you combine async exceptions with mutable data
structures, just don't do it”.

2. Documentation. Should library authors explicitly mention async
exceptions safety? For example
https://hackage.haskell.org/package/hashtables – is it async exceptions
safe when used in IO? Or even worse
https://hackage.haskell.org/package/ghc-prim-0.5.1.0/docs/GHC-Prim.html#v:resizeMutableByteArray-35-
- what will happened in case of async exception? This functions is
sate-polimorphic, will it implicitly mask exceptions if used from IO?

3. Best practices. How should we deal with problem? Is creating separate
versions of  code for ST and IO is the only way? Probably it is possible to
add “mask” to something like
https://hackage.haskell.org/package/primitive-0.6.2.0/docs/Control-Monad-Primitive.html#t:PrimMonad
emit mask in IO instance and NOOP in ST version? Or maybe somebody know
better patterns for async exeption safe code?

-- 
Sincerely, Stanislav Chernichkin.
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