UNSUPPORTED_PROTOCOL from curl_easy_send on handle removed from curl_multi

[redbaron]

I did this

If CURLOPT_CONNECT_ONLY socket was connected using mutli interface, then curl_easy_send returns CURLE_UNSUPPORTED_PROTOCOL. Here is minimal reproducer.

main.cpp:

#include <curl/curl.h>
#include <cassert>


int main(int, char**) {
    CURL* e = curl_easy_init();
    curl_easy_setopt(e, CURLOPT_URL, "http://google.com/");
    curl_easy_setopt(e, CURLOPT_CONNECT_ONLY, 1);
    curl_easy_setopt(e, CURLOPT_VERBOSE, 1);

    CURLM* m = curl_multi_init();
    curl_multi_add_handle(m, e);

    int count;
    do {
        curl_multi_perform(m, &count);
    } while (count > 0);

    CURLMsg* msg = curl_multi_info_read(m, &count);
    assert(msg);
    assert(count == 0);
    assert(msg->easy_handle == e);
    assert(msg->data.result == CURLE_OK);

    curl_multi_remove_handle(m, e);

    const char buf[] = "XYZ";
    size_t sent;
    CURLcode res = curl_easy_send(e, buf, sizeof(buf), &sent);
    assert(res == CURLE_OK); // CURLE_UNSUPPORTED_PROTOCOL is returned

    curl_easy_cleanup(e);
    curl_multi_cleanup(m);
    return 0;
}

I expected the following

curl_easy_send is usable after curl_multi_remove_handle returns;

curl/libcurl version

7.84.0

operating system

Linux WSL2

[redbaron]

Hm, commenting curl_multi_remove_handle call makes it work. Reading curl_multi_add_handle doc I was under impression that I shouldn't call any curl_easy_* IO functions until handle is removed from multi, even though docs talks about curl_easy_perform only.

I am not sure if not calling curl_multi_remove_handle is an intended use of CURLOPT_CONNECT_ONLY sockets or just a workaround sidestepping legitimate bug.

[bagder]

I think this is mostly a case where we should update/clarify the documentation.

[bagder]

#9262 is my attempt to clarify how to use this feature with the multi interface