OpenSSL: Add support for TLS ASYNC state.

[bermuell]

In case that the TLS connection is paused using openssl ASYNC_pause_job()
current implementation of libcurl handles it as a connection error.

The proposed change adds support in case the TLS connections is paused that way.

[bagder]

I checked. The define SSL_ERROR_WANT_ASYNC was added in openssl's commit openssl/openssl@07bbc92ccb96d48044, which then has been in releases since 1.1.0.

[jay]

I checked. The define SSL_ERROR_WANT_ASYNC was added in openssl's commit openssl/openssl@07bbc92, which then has been in releases since 1.1.0.

note that commit doesn't mention SSL_connect for the async

[bagder]

@bermuell do you have a particular use case where you can trigger this failure and the fix then also fixes it?

[bermuell]

The use case is when libCurl's SSL_CTX_set_cert_verify_callback is used to verify a cert-chain and the verify procedure is a async call to a PKI instance.

1. Send request
2. verify_callback: get server certificates
3. Pause TLS connection (ASYNC_pause_job)
4. Send PKI_verify request (async)
5. Callback with PKI_verify_result
6. Unpause TLS connection
7. return result (SUCCESS/FAIL) from callback (of SSL_CTX_set_cert_verify_callback)

Without this fix the connection is closed by libCurl and reporting related 'WANT_ASYNC' error message.

[bagder]

Thanks!