This is what I've done, on the same CentOS 7.4.1708 (and libcurl 7.56.1-1.0.cf.rhel7) machine referred to in this thread.

- Checked out 071b85f66cabaa3a705a014b26b7c1eb31029b26 from branch 5.0 (which is the last one before Aug 18th).

- Used the latest http_async_client module

- make clean, make all, make install

- Launched tests at 10 cps for several seconds with:

modparam("http_async_client", "tls_ca_path", "/usr/local/etc/kamailio/ssl/ca")

modparam("http_async_client", "curl_verbose", 1)

...

$http_req(suspend) = 0;

http_async_query("$var(target_url)", "HTTP_REPLY");

but I wasn't able to reproduce the issue you reported.

Please consider upgrading to latest 5.0 (or master, if possible) and test again.

Regards,

Giacomo