Jay Satiro 4f99efb192 easy: allow connect-only handle reuse with easy_perform

- Detach and disconnect an attached connection before performing.

Prior to this change it was not possible to safely reuse an easy handle
with an attached connection in a second call to curl_easy_perform. The
only known case of this is a connect-only type handle where the
connection was detached when curl_easy_perform returned, only to be
reattached by either curl_easy_send/recv.

This commit effectively reverts 2f8ecd5d and be82a360, the latter of
which treated the reuse as an error. Prior to that change undefined
behavior may occur in such a case.

Bug: https://curl.se/mail/lib-2025-01/0044.html
Reported-by: Aleksander Mazur

Closes https://github.com/curl/curl/pull/16008

2025-01-28 03:27:04 -05:00

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SPDX-License-Identifier

Title

Section

Source

See-also

Protocol

Added-in

curl

CURLOPT_CONNECT_ONLY

libcurl

CURLOPT_HTTPPROXYTUNNEL (3)

CURLOPT_VERBOSE (3)

curl_easy_recv (3)

curl_easy_send (3)

All

7.15.2

NAME

CURLOPT_CONNECT_ONLY - stop when connected to target server

SYNOPSIS

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_CONNECT_ONLY, long only);

DESCRIPTION

Pass a long. If the parameter equals 1, it tells the library to perform all the required proxy authentication and connection setup, but no data transfer, and then return.

The option can be used to simply test a connection to a server, but is more useful when used with the CURLINFO_ACTIVESOCKET(3) option to curl_easy_getinfo(3) as the library can set up the connection and then the application can obtain the most recently used socket for special data transfers.

Since 7.86.0, this option can be set to '2' and if HTTP or WebSocket are used, libcurl performs the request and reads all response headers before handing over control to the application.

Transfers marked connect only do not reuse any existing connections and connections marked connect only are not allowed to get reused.

If the connect only transfer is done using the multi interface, the particular easy handle must remain added to the multi handle for as long as the application wants to use it. Once it has been removed with curl_multi_remove_handle(3), curl_easy_send(3) and curl_easy_recv(3) do not function.

DEFAULT

%PROTOCOLS%

EXAMPLE

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode ret;
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
    curl_easy_setopt(curl, CURLOPT_CONNECT_ONLY, 1L);
    ret = curl_easy_perform(curl);
    if(ret == CURLE_OK) {
      /* only connected */
    }
  }
}

HISTORY

WS and WSS support added in 7.86.0.

%AVAILABILITY%

RETURN VALUE

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors(3).

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