A share handle added to multiple easy handles should be sufficient to share connections between each easy, and indeed this works when each easy is created fresh and attached to a share.

However I observed that when the share handle is added to a prototype easy, then new easy handles are created from that prototype using curl_easy_duphandle(), the share appears to be "lost" and must be reattached.

Attaching the share to the easy handle produced by curl_easy_duphandle() works as expected.

I traced the cause to

I did this

CURLSH *share = curl_share_init();
curl_share_setopt(share, CURLSHOPT_SHARE, CURL_LOCK_DATA_CONNECT);

CURL *base = curl_easy_init();
curl_easy_setopt(base, CURLOPT_SHARE, share);
curl_easy_setopt(base, CURLOPT_URL, "http://example.com");
// various other setopt calls

for(int i = 0; i < 10; ++i) {
  CURL *handle = curl_easy_duphandle(base);
  // child-specific setopt calls
  curl_easy_perform(handle);
  curl_easy_cleanup(handle);
}

I expected the following

One network connection to example.com, page fetched 10 times in a pipline.

curl/libcurl version

curl 7.58.0 (x86_64-pc-linux-gnu) libcurl/7.58.0 OpenSSL/1.1.0g zlib/1.2.11 libidn2/2.0.4 libpsl/0.19.1 (+libidn2/2.0.4) nghttp2/1.30.0 librtmp/2.3
Release-Date: 2018-01-24
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtmp rtsp smb smbs smtp smtps telnet tftp
Features: AsynchDNS IDN IPv6 Largefile GSS-API Kerberos SPNEGO NTLM NTLM_WB SSL libz TLS-SRP HTTP2 UnixSockets HTTPS-proxy PSL

operating system

Linux, Ubuntu 18.04