[ ghc-Bugs-1219920 ] socketToHandle, hGetContents cause errors in ghci

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Bugs item #1219920, was opened at 2005-06-13 14:34
Message generated for change (Comment added) made by pimlott
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Category: libraries/network
Group: None
Status: Closed
Resolution: None
Priority: 5
Submitted By: Andrew Pimlott (pimlott)
Assigned to: Simon Marlow (simonmar)
Summary: socketToHandle, hGetContents cause errors in ghci

Initial Comment:
I wrote a server using socketToHandle and hGetContents.
 The first request worked fine, but the second request
crashed with "internal error: scavenge_stack: weird
activation record found on stack: 9".  I tried to
create a test case.  I couldn't reproduce the same
problem, but I did get an intermittent "Bad file
descriptor".  I think it may be a related problem, and
it may have to do with garbage collection (based on a
strace showing a call to getrusage).

The program is below, and when I run it with runghc and
feed it a line of input with nc, it crashes on the 17th
request.  It's not important to me that you fix it; in
fact I only tried this as an experiment to simplify
some working code.  But if you would like more
information, let me know.

I am using the Debian package ghc6 6.4-4.

import Network.Socket
import System.IO

main = do 
  s <- socket AF_INET Stream 0
  h <- inet_addr "127.0.0.1"
  bindSocket s (SockAddrInet 1236 h)
  listen s 5 
  loop s 

loop listenSock = do
  (s,_) <- accept listenSock
  h <- socketToHandle s ReadMode
  input <- hGetContents h
  putStrLn (take 10 input)
  send s "got it\n"
  sClose s
  loop listenSock


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>Comment By: Andrew Pimlott (pimlott)
Date: 2005-07-06 12:00

Message:
Logged In: YES 
user_id=498741

I should have detailed my test scenario, but what I see when
I run "echo hello world | nc localhost 1236" repeatedly is
that "hello worl" is printed 16 times, then

*** Exception: <socket: 5>: lazyRead: invalid argument (Bad
file descriptor)

So the error happens while trying to print out "input".  As
I understand, input should keep a reference to h, which
should keep a reference to s, so nothing should get garbage
collected too soon.  Moreover, something that happens later
(sClose) shouldn't be able to affect it.  That said, when I
use hClose, or remove the close, I can't reproduce the
problem.  So I'm perplexed.  Can I ask you to explain in
more detail?

----------------------------------------------------------------------

Comment By: Simon Marlow (simonmar)
Date: 2005-07-06 05:57

Message:
Logged In: YES 
user_id=48280

The "Bad file descriptor" error messages are to be expected:
when you turn a Socket into a Handle, it gets a finalizer
attached which closes the socket when the Handle is no
longer referenced.  You want to call hClose h rather than
sClose s in the code above.

I've added some documentation for socketToHandle to reflect
this.

If you can reproduce the "scavenge_stack" error, please
follow up with more info, for now I'll close this bug.

----------------------------------------------------------------------

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