Ctrl-Break handler doesn't work in forked threads in "-threaded" RTS

[bulatz@…]

The following program demonstrates bug in 6.4.1 (6.4.0 was ok). if you compile it with "-threaded", run and press Ctrl-Break, you will see message "main.EXE: <stdin>: hGetLine: end of file" and program blocks (because forked thread is dead). the problem is that installed ctrl-break handler don't called in forked threads

if you compile program without "-threaded" or with 6.4.0, it works as expected - pressing Ctrl-Break runs my handler so you will see "Break! main.EXE: thread killed" message

import Control.Concurrent
import Control.Exception
import GHC.ConsoleHandler

handleCtrlBreak action = do
  myThread <- myThreadId
  let onBreak event = do
        putStrLn " ^Break!"
        killThread myThread
  bracket (installHandler$ Catch onBreak) (installHandler) $  \oldHandler -> do
    action

main =  do
  handleCtrlBreak $ do
    x <- newEmptyMVar
    forkIO$ do a <- getLine
               print a
               putMVar x ()
               return ()
    takeMVar x
    return ()

[guest]

The following program demonstrates bug in 6.4.1 (6.4.0 was ok).

if you compile it with "-threaded", run and press Ctrl-Break, you will see message "main.EXE: <stdin>: hGetLine: end of file" and program blocks (because forked thread is dead). the problem is that installed ctrl-break handler don't called in forked threads

if you compile program without "-threaded" or with 6.4.0, it works as expected - pressing Ctrl-Break runs my handler so you will see "^{Break! main.EXE: thread killed" message}

 import Control.Concurrent
 import Control.Exception
 import GHC.ConsoleHandler

 handleCtrlBreak action = do
   myThread <- myThreadId
   let onBreak event = do
         putStrLn " ^Break!"
         killThread myThread
   bracket (installHandler$ Catch onBreak) (installHandler) $  \oldHandler -> do
     action

 main =  do
   handleCtrlBreak $ do
     x <- newEmptyMVar
     forkIO$ do a <- getLine
                print a
                putMVar x ()
                return ()
     takeMVar x
     return ()

[simonmar]

Verified. I'm not sure why it worked in 6.4.0, perhaps it was by accident. The console handler function, generic_handler in ConsoleHandler?.c, has no way to wake up an RTS thread - it tries to ping the async IO system, but async IO isn't used in -threaded on Windows now (since 6.2).

The proper fix for this is for Windows to use the I/O manager with -threaded, as Unix does. On Unix, when a signal arrives, a byte is sent down a pipe to the I/O manager thread which launches the appropriate signal handler.

I'll punt on this for 6.4.2.

[simonmar]

Punt.

[simonmar]

Fixed.