[GHC] #14503: Killing a thread will block if there is another process reading from a handle

21 Nov 2017

      #14503: Killing a thread will block if there is another process reading from a
handle
----------------------------------------+---------------------------------
           Reporter:  dnadales          |             Owner:  (none)
               Type:  bug               |            Status:  new
           Priority:  normal            |         Milestone:
          Component:  Compiler          |           Version:  8.0.2
           Keywords:                    |  Operating System:  Windows
       Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
          Test Case:                    |        Blocked By:
           Blocking:                    |   Related Tickets:
Differential Rev(s):                    |         Wiki Page:
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 When trying to kill a thread, the program (which uses a thread) hangs if
 there is another process trying to read from a handle. This bug can be
 reproduced with using
 [https://github.com/capitanbatata/sandbox/tree/master/dangling-connections
 this sample code]. I'll explain the relevant details below.

 I have the following Haskell code:

 {{{#!hs
 someFuncWithChans :: IO ()
 someFuncWithChans = withSocketsDo $ do
     h <- connectTo "localhost" (PortNumber 9090)
     hSetBuffering h NoBuffering
     ch <- newChan
     putStrLn "Starting the handler reader"
     readerTid <- forkIO $ handleReader h ch
     cmdsHandler h ch
     putStrLn "Killing the handler reader"
     killThread readerTid
     putStrLn "Closing the handle"
     hClose h

 cmdsHandler :: Handle -> Chan Action -> IO ()
 cmdsHandler h ch = do
     act <- readChan ch
     case act of
       Quit      -> putStrLn "Bye bye"
       Line line -> do
             hPutStrLn h (reverse line)
             cmdsHandler h ch

 handleReader :: Handle -> Chan Action -> IO ()
 handleReader h ch = forever $ do
     line <- strip <$> hGetLine h
     case line of
         "quit" -> writeChan ch Quit
         _      -> writeChan ch (Line line)

 data Action = Quit | Line String

 }}}

 Is the function `someFuncWithChans` is run along with the following Java
 program, then the former will block while killing the handler reader
 (`readerTid`).

 {{{#!java
     public static void main(String[] args) throws IOException,
 InterruptedException {

         ServerSocket serverSock = new ServerSocket(9090);

         Socket sock = serverSock.accept();

         InputStream inStream = sock.getInputStream();
         BufferedReader sockIn = new BufferedReader(new
 InputStreamReader(inStream));

         OutputStream outStream = sock.getOutputStream();
         PrintWriter sockOut = new PrintWriter(new
 OutputStreamWriter(outStream));

         while (true) {
             Thread.sleep(1000);
             System.out.println("Sending foo");
             sockOut.println("foo");
             sockOut.flush();
             String s = sockIn.readLine();
             System.out.println("Got " + s );
             Thread.sleep(1000);
             System.out.println("Sending bar");
             sockOut.println("bar");
             sockOut.flush();
             s = sockIn.readLine();
             System.out.println("Got " + s );
             Thread.sleep(1000);
             System.out.println("Sending quit");
             sockOut.println("quit");
             sockOut.flush();
             // This will cause someFuncWithChans to block when killing the
             // reader thread.
             s = sockIn.readLine();
             System.out.println("Got " + s );
         }
     }
 }}}

 If the `sockIn.readLine()` is commented out, then killing the thread will
 succeed. This problem appears only on my Windows machine (at work),
 whereas it does not on my personal Linux machine.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14503
GHC http://www.haskell.org/ghc/
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