Redirect overwrites `CURLOPT_URL` option value when `CURLOPT_FOLLOWLOCATION` used.

[rpv-tomsk]

Hi.

In Collectd project (http://collectd.org, https://github.com/collectd/collectd/) there are several plugins which uses libcurl.

Libcurl is used by the following scheme:

1. Libcurl object initialized and configured once, by calling curl_easy_init() and curl_easy_setopt().
2. Requests will be done later by calling curl_easy_perform() on the same libcurl object many times.

The code uses CURLOPT_FOLLOWLOCATION option set to 1 at initialization.
The CURLOPT_URL option set at the same time (at initialization).

After some request was finished by redirect, subsequent requests are done to new location, not to location set by CURLOPT_URL.

That is unexpected and undocumented behaviour.

It is expected that subsequent calls to curl_easy_perform() will be done with same options set, and to the location, set by CURLOPT_URL.

IMHO, something required to be changed - code or documentation.

What do you think?

curl/libcurl version

7.38.0 (yes, it is quite old, but I found no issue-related changes in current 7.54.1 version too)

operating system

Debian jessie

Related Collectd issue: collectd/collectd#2328

[rpv-tomsk]

The curl-7.38.0/lib/url.c has a code which sets value for CURLOPT_URL option:

  case CURLOPT_URL:
    /*
     * The URL to fetch.
     */
    if(data->change.url_alloc) {
      /* the already set URL is allocated, free it first! */
      Curl_safefree(data->change.url);
      data->change.url_alloc = FALSE;
    }
    result = setstropt(&data->set.str[STRING_SET_URL],
                       va_arg(param, char *));
    data->change.url = data->set.str[STRING_SET_URL];
    break;

The data->set.str[STRING_SET_URL] is not used nothere in libcurl code.
All the work is done using data->change.url value. (Which represents CURLINFO_EFFECTIVE_URL option of getinfo_char(), see curl-7.38.0/lib/getinfo.c)

But in Curl_follow() of curl-7.38.0/lib/transfer.c there is an assignment to data->change.url, which overwrites the value set previously.
There is also some other assignments exists too.

At the https://curl.haxx.se/libcurl/c/curl_easy_perform.html page there is the following note:

Just note that you will have to use curl_easy_setopt between the invokes to set options for the following curl_easy_perform.

But it is unclear if it is general requirement or only related to 'more than one file transfer', which requires to change file URL.

[bagder]

This is definitely a bug, still present in 7.54.1... Setting the URL in the handle should make it remain set like that for subsequent transfers done on the same handle.

[bagder]

My proposed fix for this bug and test case to verify it, are in #1632