Modifying TVar after calling always causes freeze/spin in GHCi

[Baughn]

The attached code, if run under GHCi, causes GHCi to freeze while spinning the CPU. It works correctly when compiled, or if the ">> modifyTVar" part of f is commented out.

(For the curious: Yes, I believed always does what check does. Thankfully nothing had made its way into production code, because of this bug. The documentation should probably be clarified.)

[igloo]

Thanks for the report.

I can reproduce this 99% of the time. Here's a version that doesn't need anything outside of validate:

import Control.Monad
import GHC.Conc
import System.IO

modifyTVar :: TVar Integer -> (Integer -> Integer) -> STM ()
modifyTVar t f = readTVar t >>= writeTVar t . f

main :: IO ()
main = do
  hSetBuffering stdout LineBuffering
  t <- newTVarIO 0
  let f = atomically $ do always (liftM2 (<=) (readTVar t) (return 5))
                          modifyTVar t succ
  putStrLn "f1"
  f
  putStrLn "f2"
  f
  putStrLn "v"
  v <- atomically $ readTVar t
  print v

*Main> main
f1
f2
^CInterrupted.
*Main> main
f1
f2
^CInterrupted.
*Main> 

[simonmar]

Fixed:

Tue Jun 15 13:36:43 BST 2010  Simon Marlow <marlowsd@gmail.com>
  * stmAddInvariantToCheck: add missing init of invariant->lock (#4057)