[Haskell-cafe] hGetLine problem

[Keean Schupke]

What is happening is that the socket is closed after the accept
but before the hGetLine, so the handle is invalid (there is no
socket any more)... This is correct behaviour when the client
closes the connection whilst you are writing to it...

The answer is just to catch the exception.

    Keean.


Michael Walter wrote:

>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
>>>>
>>>>
>>>>        
>>>>
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