Seek problem with curl_formadd with CURLFORM_STREAM

From: Jeff Mears via curl-library <curl-library_at_lists.haxx.se>
Date: Mon, 29 Jan 2024 19:33:59 +0000

I have code that's attempting to use CURLFORM_STREAM with curl_formadd, and it is getting a CURLE_SEND_FAIL_REWIND error from the library.

Looking at the libcurl code, it looks like it might be a bug, but it's hard for me to tell for sure. A full example of how the library is being used would take a while to construct.

curl_formadd(&download->m_postMulti, &download->m_lastMulti,
                CURLFORM_COPYNAME, uploadName,
                CURLFORM_FILENAME, uploadFilename,
                CURLFORM_CONTENTTYPE, uploadContentType,
                CURLFORM_STREAM, download,
                CURLFORM_CONTENTSLENGTH, static_cast<long>(download->m_uploadSize),
                CURLFORM_END);

I expected my read callbacks to get called, but instead, the transfer fails with CURLE_SEND_FAIL_REWIND. I have a seek callback set in addition to a read callback.

Tracing through the code, I find that it's because curl_mimepart::seekfunc is set to null. In formdata.c there is this:

        else if(post->flags & HTTPPOST_CALLBACK) {
          /* the contents should be read with the callback and the size is set
             with the contentslength */
          if(!clen)
            clen = -1;
          result = curl_mime_data_cb(part, clen,
                                     fread_func, NULL, NULL, post->userp);
        }

There is no seek function being given to curl_mime_data_cb in Curl_getformdata, resulting in mime_part_rewind failing, finally causing a CURLE_SEND_FAIL_REWIND.

Is this a bug in libcurl? My code is fairly old at this point, and wasn't written to use curl_mime_*. Using curl_mime_data_cb solves the problem, because I can provide the seek callback.

Thanks~

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Received on 2024-01-29