[Haskell-cafe] hGetLine problem

[Michael Walter]

Yep - this program "sometimes" fails for me with such an error message:

  Fail: <socket: 8>: hGetLine: invalid argument (Invalid argument)

If I omit the forkIO and do synchronous processing, I noticed that
Apache Benchmark connects a second time but closes the connection
immediately. When I added a `catch` \e -> mainLoop socket at the end
of the main loop, it worked fine.

But when I reintroduced the forkIO, it seemed to be "hanging" in the
main process's accept (when the fork failed with above's error
message).

Any ideas?

Confusingly yours,
Michael


On Thu, 9 Dec 2004 14:29:10 -0500, Michael Walter
<michael.walter at gmail.com> wrote:
> Hello again,
> 
> this test program should do the relevant parts for the bug -- on
> Windows it works fine, though, I'll have to check at home whether it's
> reproducable using it.
> 
> - Michael
> 
> module Test where
> 
> import Control.Concurrent
> import Network
> import System.IO
> 
> mainLoop socket = do
>         (handle, _, _) <- accept socket
>         forkIO $ do
>                 line <- hGetLine handle
>                 putStrLn line
>                 hPutStrLn handle "HTTP/1.0 200 OK\r"
>                 hPutStrLn handle "Content-Type: text/html\r"
>                 hPutStrLn handle "\r"
>                 hPutStrLn handle "Test\r"
>                 hClose handle
>         mainLoop socket
> 
> main = withSocketsDo $ listenOn (PortNumber 8000) >>= mainLoop
> 
> 
> 
> 
> On Thu, 9 Dec 2004 11:44:16 -0500, Michael Walter
> <michael.walter at gmail.com> wrote:
> > I'll do so at lunch break/from home tonight.
> >
> > Thanks,
> > Michael
> >
> >
> >
> >
> > On Thu, 9 Dec 2004 16:19:20 +0000, Jules Bean <jules at jellybean.co.uk> wrote:
> > >
> > > On 9 Dec 2004, at 15:30, Michael Walter wrote:
> > > > I continued toying with my toy web server, and I'm "sometimes" getting
> > > > "Invalid argument" errors in hGetLine for a handle I retrieved from
> > > > Network.listenOn.
> > > >
> > >
> > > My first guess would be that hGetLine would return invalid argument if
> > > it was called on a handle which represents a file which is now closed.
> > > However, testing in GHCI suggests that returns "illegal operation
> > > (handle is closed)". Could be different for network sockets, I suppose.
> > >
> > >
> > >
> > > > "Sometimes", because it works fine in the browser, except when I
> > > > reload very frequently -- maybe it's related to some concurrency
> > > > issues (I'm using forkIO to spawn off the handler process from the
> > > > main server loop). When I wanted to benchmark the server using "ab"
> > > > (Apache Benchmark), it didn't even "survive" one request but Fail'ed
> > > > using that error message - "<handle: n>: hGetLine: Invalid argument".
> > > >
> > > > I tried adjusting the buffering mode.
> > > >
> > > > Do you have any ideas?
> > > >
> > >
> > > Sounds like a double-close error or something like that. Hard to say
> > > without seeing the code. Can you make a minimal example?
> > >
> > > Jules
> > >
> > >
> >
>