Re: [Viking-devel] [ETag] Memcheck error

[Guilhem B. <gui...@gm...>]

2010/9/26 Jocelyn Jaubert <joc...@gm...>:
> Le 26 septembre 2010, Jon Burgess a écrit :
>> On Sat, 2010-09-25 at 23:30 +0200, Guilhem Bonnefille wrote:
>
>> What I think happens is:
>> 1) We first do a request for a tile with an ETag and apply a custom
>> header, this gets set into the conn->data->set.headers pointer
>> 2) The header gets freed, but the set.headers pointer is left as a
>> dangling reference to the memory
>> 3) A subsequent request is generated for a tile without an etag so we
>> do not overwrite the set.headers pointer and it keeps the old, invalid
>> value and the HTTP request code tries to reference it.
>
> Yes, that makes sense, and that explains Guilhem's failure.
>
> I think that I never hit the bug because I'm using only Mapnik tiles,
> which uses etag, and not any other tile server that doesn't use etag.
>
> Guilhem: to make things clearer, curl uses the option
> CURLOPT_HTTPHEADER to add any header, and expect a valid string there.
> Viking also reuse curl from the previous call of curl_download(), which
> means that CURLOPT_HTTPHEADER wasn't cleared.

Here is my understanding.
We use curl in a multi-thread context. To optimize connection, we also
reuse curl's handles. We reuse the handle in the context of a single
thread. And a thread is fired for a single map source.

So, the matter occured only when, in a single request, we request a
fresh version of an already downloaded tile (with local etag value)
and a new tile (without local etag value).

Am I right?

> Btw, shouldn't we initialize _all_ curl options at each pass ? I think
> we can the same problem with other curl options that are initialized
> each time - like CURLOPT_TIMECONDITION or CURLOPT_TIMEVALUE. I will try
> to provide a patch to initialize everything.

If we are true, we have to reset all CURLOPT that can be set or not
set between two consecutive downloads.

-- 
Guilhem BONNEFILLE
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