Asynchronous exception wormholes kill modularity

[basvandijk]

Functions like finally create, what I call, an asynchronous exception wormhole because they unblock asynchronous exceptions even if you call them in a blocked scope:

finally :: IO a -> IO b -> IO a
a `finally` sequel = block $ do
  r <- unblock a `onException` sequel
  _ <- sequel
  return r

This hurts modularity.

I  proposed solving this as follows:

finally :: IO a -> IO b -> IO a
a `finally` sequel = do 
  b <- blocked
  block $ do
    r <- (if b then unblock a else a) `onException` sequel
    _ <- sequel
    return r

Besides finally the following functions also have a wormhole:

• Control.Exception.finally/bracket/bracketOnError
• Control.Concurrent.MVar.withMVar/modifyMVar_/modifyMVar
• Foreign.Marshal.Pool.withPool

In the interesting discussion that followed several other solutions were proposed (for example a block and unblock that count nesting levels).

Later, Simon Marlow  proposed an even nicer solution:

mask :: ((IO a -> IO a) -> IO b) -> IO b
mask io = do
   b <- blocked
   if b
      then io id
      else block $ io unblock

to be used like this:

a `finally` sequel =
   mask $ \restore -> do
     r <- restore a `onException` sequel
     sequel
     return r

I created this ticket so we won't forget about this problem.

(These are related bugs: #3944 and #3945)

[simonmar]

Fixed; see #1036.