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Network.Curl cookie jar madness

Michael Orlitzky
I'm one bug away from a working program and need some help. I wrote a
little utility that logs into LWN.net, retrieves an article, and creates
an epub out of it. Full code here:

  git clone http://michael.orlitzky.com/git/lwn-epub.git

This is the code that gets the login cookie:

  cj <- make_cookie_jar
  li_result <- log_in cj uname pword

  case li_result of
    Left err -> do
      let msg = "Failed to log in. " ++ err
      hPutStrLn stderr msg
    Right response_body -> do
      hPutStrLn stderr response_body

  return $ cfg { C.cookie_jar = Just cj }

Curl is making the request, but if I remove the (hPutStrLn stderr
response_body), it doesn't work! What's even more insane is, this works:

  hPutStrLn stderr response_body

and this doesn't:

  hPutStrLn stdout response_body

whaaaaaaatttttttt? I really don't want to dump the response body to
stderr, but I can't even begin to imagine what's going on here. Has
anyone got Network.Curl working with a cookie jar?

For anyone with an LWN account: you can set,

  username = foo
  password = bar

in ~/.lwn-epub/lwn-epub.conf and attempt to run with,

  lwn-epub -o current.epub current/bigpage

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Re: Network.Curl cookie jar madness

Iustin Pop
On Sat, Aug 18, 2012 at 08:52:00PM -0400, Michael Orlitzky wrote:

> I'm one bug away from a working program and need some help. I wrote a
> little utility that logs into LWN.net, retrieves an article, and creates
> an epub out of it. Full code here:
>
>   git clone http://michael.orlitzky.com/git/lwn-epub.git
>
> This is the code that gets the login cookie:
>
>   cj <- make_cookie_jar
>   li_result <- log_in cj uname pword
>
>   case li_result of
>     Left err -> do
>       let msg = "Failed to log in. " ++ err
>       hPutStrLn stderr msg
>     Right response_body -> do
>       hPutStrLn stderr response_body
>
>   return $ cfg { C.cookie_jar = Just cj }
>
> Curl is making the request, but if I remove the (hPutStrLn stderr
> response_body), it doesn't work! What's even more insane is, this works:
>
>   hPutStrLn stderr response_body
>
> and this doesn't:
>
>   hPutStrLn stdout response_body
>
> whaaaaaaatttttttt? I really don't want to dump the response body to
> stderr, but I can't even begin to imagine what's going on here. Has
> anyone got Network.Curl working with a cookie jar?

Is this perchance due to laziness? And the fact that stderr is not
buffered by default, so all output is forced right then (forcing the
evaluation), whereas stdout is buffered, so the output might only be
made later (or even after you to an hFlush).

I'd try to make sure that response_body is fully evaluated before
returning from the function.

Or I might be totally wrong, in which case sorry :)

regards,
iustin

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Re: Network.Curl cookie jar madness

Michael Orlitzky
On 08/18/2012 09:00 PM, Iustin Pop wrote:

> On Sat, Aug 18, 2012 at 08:52:00PM -0400, Michael Orlitzky wrote:
>>
>> Curl is making the request, but if I remove the (hPutStrLn stderr
>> response_body), it doesn't work! What's even more insane is, this works:
>>
>>   hPutStrLn stderr response_body
>>
>> and this doesn't:
>>
>>   hPutStrLn stdout response_body
>>
>> whaaaaaaatttttttt? I really don't want to dump the response body to
>> stderr, but I can't even begin to imagine what's going on here. Has
>> anyone got Network.Curl working with a cookie jar?
>
> Is this perchance due to laziness? And the fact that stderr is not
> buffered by default, so all output is forced right then (forcing the
> evaluation), whereas stdout is buffered, so the output might only be
> made later (or even after you to an hFlush).
>
> I'd try to make sure that response_body is fully evaluated before
> returning from the function.
>
> Or I might be totally wrong, in which case sorry :)
>

I thought so at first, but I've tried every trick I know to avoid it. If
I add an hFlush to the stdout version, it still fails. If I deepseq the
response_body (it's just a string, after all), it still fails.

In the case statement, we've already made the request and received a
response (the Left/Right is based on the response code). I'm thinking
the cookie jar should be full at that point whether or not I use the
response body.

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Re: Network.Curl cookie jar madness

Michael Orlitzky
In reply to this post by Michael Orlitzky
On 08/18/2012 08:52 PM, Michael Orlitzky wrote:
> I'm one bug away from a working program and need some help. I wrote a
> little utility that logs into LWN.net, retrieves an article, and creates
> an epub out of it.

I've created two pages where anyone can test this. The first just takes
any username and password via post and sets a session variable. The
second prints "Success." if the session variable is set, and "Failure."
if it isn't. The bash script,

  #!/bin/bash

  COOKIE_JAR='/tmp/network-curl-test-bash.txt'
  POST_DATA='username=foo&password=bar'
  URL1='http://michael.orlitzky.com/tmp/network-curl-test1.php'
  URL2='http://michael.orlitzky.com/tmp/network-curl-test2.php'

  echo 'Logging in...'
  curl --cookie-jar "${COOKIE_JAR}" \
       --data "${POST_DATA}" \
       "${URL1}"

  echo 'Retrieving second page...'
  curl --cookie "${COOKIE_JAR}" \
       "${URL2}"

works:

  $ ./bash-test.sh
  Logging in...
  Retrieving second page...
  Success.

The attached haskell program using Network.Curl, doesn't:

  $ runghc haskell-test.hs
  Logged in...
  Failure.

Any help is appreciated =)

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Re: Network.Curl cookie jar madness

Brandon Allbery
In reply to this post by Michael Orlitzky
On Sat, Aug 18, 2012 at 8:52 PM, Michael Orlitzky <[hidden email]> wrote:
Curl is making the request, but if I remove the (hPutStrLn stderr
response_body), it doesn't work! What's even more insane is, this works:

  hPutStrLn stderr response_body

and this doesn't:

  hPutStrLn stdout response_body

whaaaaaaatttttttt? I really don't want to dump the response body to

At a guess, this is laziness and buffering interacting:  stderr is usually unbuffered since it's error or log output that one usually wants to see immediately; stdout is usually line buffered unless redirected, in which case it's block buffered.

The real issue is that you (or perhaps Curl) is being too lazy and not running the log_in until the result is actually needed; hPutStrLn is forcing it, but incompletely when it's buffered.  (Which strikes me as weird unless Curl is using unsafeInterleaveIO somewhere....)  You will need to force it or hold the handle open until the content is fully evaluated; if it's in the half-closed state that hGetContents sets, it's usually best to not close the handle explicitly but let the implicit lazy close do it.

--
brandon s allbery                                      [hidden email]
wandering unix systems administrator (available)     (412) 475-9364 vm/sms


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Re: Network.Curl cookie jar madness

Iustin Pop
In reply to this post by Michael Orlitzky
On Sun, Aug 19, 2012 at 12:45:47AM -0400, Michael Orlitzky wrote:
> On 08/18/2012 08:52 PM, Michael Orlitzky wrote:
> > I'm one bug away from a working program and need some help. I wrote a
> > little utility that logs into LWN.net, retrieves an article, and creates
> > an epub out of it.
>
> I've created two pages where anyone can test this. The first just takes
> any username and password via post and sets a session variable. The
> second prints "Success." if the session variable is set, and "Failure."
> if it isn't. The bash script,

[…]

> The attached haskell program using Network.Curl, doesn't:
>
>   $ runghc haskell-test.hs
>   Logged in...
>   Failure.
>
> Any help is appreciated =)

So, take this with a grain of salt: I've been bitten by curl (the
haskell bindings, I mean) before, and I don't hold the quality of the
library in great regard.

The libcurl documentation says: "When you set a file name with
CURLOPT_COOKIEJAR, that file name will be created and all received
cookies will be stored in it when curl_easy_cleanup(3) is called" (i.e.
at the end of a curl handle session). But even though the curl bindings
seem to run easy_cleanup on handles (initialize → mkCurl →
mkCurlWithCleanup), they don't do this correctly:

DEBUG: ALLOC: CURL
DEBUG: ALLOC: /tmp/network-curl-test-haskell20417.txt
DEBUG: ALLOC: username=foo&password=bar
DEBUG: ALLOC: http://michael.orlitzky.com/tmp/network-curl-test1.php
DEBUG: ALLOC: WRITER
DEBUG: ALLOC: WRITER

Note there's no "DEBUG: FREE: CURL" as the code seems to imply there
should be. Hence, the handle is never cleaned up (do the curl bindings
leak handles?), so the cookie file is never written.

Side note: by running the same program multiple times, sometimes you see
DEBUG: FREE: CURL, sometimes no FREE actions. I believe there's
something very wrong in the curl bindings with regard to cleanups.

If I modify curl to export a "force cleanup" function, I can make the
program work (but not always; my patch is a hack).

Alternatively, as the curl library doesn't need a cookie jar to use
cookies in the same handle, by modifying your code to reuse the same
curl handle (returning it from log_in and reusing the same in get_page)
gives me a success code. But the cookie file is still not filled, since
the curl handle is never properly terminated.

Since the curl bindings also have problems in multi-threaded programs
when SSL is enabled (as it doesn't actually setup the curl library
correctly with regards to multi-threaded memory allocation), I would
suggest you try to use the http conduit library, since that's a pure
haskell library that should work as well, if not better.

Happy to be proved wrong, if I'm just biased against curl :)

regards,
iustin

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Re: Network.Curl cookie jar madness

Iustin Pop
On Sun, Aug 19, 2012 at 06:06:53PM +0200, Iustin Pop wrote:

> On Sun, Aug 19, 2012 at 12:45:47AM -0400, Michael Orlitzky wrote:
> > On 08/18/2012 08:52 PM, Michael Orlitzky wrote:
> > > I'm one bug away from a working program and need some help. I wrote a
> > > little utility that logs into LWN.net, retrieves an article, and creates
> > > an epub out of it.
> >
> > I've created two pages where anyone can test this. The first just takes
> > any username and password via post and sets a session variable. The
> > second prints "Success." if the session variable is set, and "Failure."
> > if it isn't. The bash script,
>
> […]
>
> > The attached haskell program using Network.Curl, doesn't:
> >
> >   $ runghc haskell-test.hs
> >   Logged in...
> >   Failure.
> >
> > Any help is appreciated =)
>
> So, take this with a grain of salt: I've been bitten by curl (the
> haskell bindings, I mean) before, and I don't hold the quality of the
> library in great regard.
>
> The libcurl documentation says: "When you set a file name with
> CURLOPT_COOKIEJAR, that file name will be created and all received
> cookies will be stored in it when curl_easy_cleanup(3) is called" (i.e.
> at the end of a curl handle session). But even though the curl bindings
> seem to run easy_cleanup on handles (initialize → mkCurl →
> mkCurlWithCleanup), they don't do this correctly:
>
> DEBUG: ALLOC: CURL
> DEBUG: ALLOC: /tmp/network-curl-test-haskell20417.txt
> DEBUG: ALLOC: username=foo&password=bar
> DEBUG: ALLOC: http://michael.orlitzky.com/tmp/network-curl-test1.php
> DEBUG: ALLOC: WRITER
> DEBUG: ALLOC: WRITER
>
> Note there's no "DEBUG: FREE: CURL" as the code seems to imply there
> should be. Hence, the handle is never cleaned up (do the curl bindings
> leak handles?), so the cookie file is never written.
>
> Side note: by running the same program multiple times, sometimes you see
> DEBUG: FREE: CURL, sometimes no FREE actions. I believe there's
> something very wrong in the curl bindings with regard to cleanups.

On more investigation, this seems to be due to the somewhat careless use
of Foreign.Concurrent; from the docs:

  “The finalizer will be executed after the last reference to the
  foreign object is dropped. There is no guarantee of promptness, and in
  fact there is no guarantee that the finalizer will eventually run at
  all.”

Also, see http://hackage.haskell.org/trac/ghc/ticket/1364.

So it seems that the intended way of cleaning up curl handles is all
fine and dandy if one doesn't require timely cleanup; in most cases,
this is not needed, but for cookies it is broken.

I don't know what the proper solution is; either way, it seems that
there should be a way to force the cleanup to be run, via
finalizeForeignPtr, or requiring full manual handling of curl handles
(instead of via finalizers).

Gah, native libs++.

regards,
iustin

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Re: Network.Curl cookie jar madness

Michael Orlitzky
On 08/19/2012 12:58 PM, Iustin Pop wrote:

>
> On more investigation, this seems to be due to the somewhat careless use
> of Foreign.Concurrent; from the docs:
>
>   “The finalizer will be executed after the last reference to the
>   foreign object is dropped. There is no guarantee of promptness, and in
>   fact there is no guarantee that the finalizer will eventually run at
>   all.”
>
> Also, see http://hackage.haskell.org/trac/ghc/ticket/1364.
>
> So it seems that the intended way of cleaning up curl handles is all
> fine and dandy if one doesn't require timely cleanup; in most cases,
> this is not needed, but for cookies it is broken.
>
> I don't know what the proper solution is; either way, it seems that
> there should be a way to force the cleanup to be run, via
> finalizeForeignPtr, or requiring full manual handling of curl handles
> (instead of via finalizers).
>
> Gah, native libs++.
>

Wow, thanks for the in-depth analysis. I'll just switch to
Network.Browser or its conduit counterpart.

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