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.

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

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.