libcurl 8.18.0 returns "out-of-memory" error on set accept encoding option with all value if built with no compression

[Dmitry Karpov via curl-library]

Hi All,

When trying libcurl 8.18.0 in our application, I noticed that 
curl_easy_setopt() returns CURLE_OUT_OF_MEMORY for  CURLOPT_ACCEPT_ENCODING 
option with “” value (all available)
libcurl was built without compression libraries (zlib etc).

curl_easy_setopt(easy, CURLOPT_ACCEPT_ENCODING, ""); // returns 
CURLE_OUT_OF_MEMORY

The previous releases (i.e. 8.6.0) returned CURLE_OK, and the performed 
transfers had "Accept-Encoding: identity" header.

I looked into the code and the issue is in the following code in \lib\setopt.c

case CURLOPT_ACCEPT_ENCODING:
 ...
  if(ptr && !*ptr) {
    ptr = Curl_get_content_encodings();
    if(ptr) {
      curlx_free(s->str[STRING_ENCODING]);
      s->str[STRING_ENCODING] = ptr;
    }
    else
      result = CURLE_OUT_OF_MEMORY;
    return result;
  }
  return Curl_setstropt(&s->str[STRING_ENCODING], ptr);

The problem is that the Curl_get_content_encodings() function returns NULL if 
libcurl is built without compression libraries because it skips over the 
CONTENT_ENCODING_DEFAULT("identity"),

If we look into the Curl_get_content_encodings() code:

char *Curl_get_content_encodings(void)
{
  struct dynbuf enc;
  const struct Curl_cwtype * const *cep;
  CURLcode result = CURLE_OK;
  curlx_dyn_init(&enc, 255);

  for(cep = general_unencoders; *cep && !result; cep++) {
    const struct Curl_cwtype *ce = *cep;
    if(!curl_strequal(ce->name, CONTENT_ENCODING_DEFAULT)) {
      if(curlx_dyn_len(&enc))
        result = curlx_dyn_addn(&enc, ", ", 2);
      if(!result)
        result = curlx_dyn_add(&enc, ce->name);
    }
  }
  if(!result)
    return curlx_dyn_ptr(&enc);
  return NULL;
}

we will see that if the general_unencoders contains only " identity", then the 
curlx_dyn_ptr(&enc) will return NULL.

To fix the problem, we need to add "identity" to the enc buffer if no encoding 
strings were added to it in the for-loop, like:

if(!result) {
  if (!enc.bufr)
    curlx_dyn_add(&enc, CONTENT_ENCODING_DEFAULT);
  return curlx_dyn_ptr(&enc);
}

Thanks,
Dmitry Karpov
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