This came up in this patch build for PHPC-2047.

In that patch build, I neglected to specify the correct mongo orchestration config, so a sharded cluster (with SSL) was launched with two mongos servers listening on ports 27017 and 27018. The load balancer on port 8000 was proxying a single mongos on port 27050 (which did not actually exist).

The connection string was: mongodb://127.0.0.1:8000/?loadBalanced=true&ssl=true&sslallowinvalidcertificates=true

Most tests in the patch build were skipped due to the following error:

Could not establish stream for node 127.0.0.1:8000: [TLS handshake failed: error:00000000:lib(0):func(0):reason(0) calling hello on '127.0.0.1:8000']

openssl/openssl#1272 suggests that "error:00000000:lib(0):func(0):reason(0)" indicates that there is no OpenSSL error to report. Looking at the format of that error message, I believe this topology scanner error message originates from this line in _mongoc_stream_tls_openssl_handshake when _mongoc_stream_tls_openssl_cert_verify_failed returns false.

I imagine some error is to be expected here, since the server is clearly inaccessible and cannot complete a handshake, but it seems odd that libmongoc would report an empty OpenSSL error.