EINPROGRESS (WSAEINPROGRESS for Windows) indicates that a non-blocking socket's connection couldn't be established immediately – the connection is still being established at the point-in-time of the error. The manpages state that the proper next step when receiving this error is to call select on the socket to verify that a connection has been established. After this point, any such operation can continue as expected.

As of current-day MongoDB, connection establishment code automatically fails on EINPROGRESS (as seen in the linked BFs). This ticket is to audit all places where we call connect() on a non-blocking socket and add the logic to catch EINPROGRESS and verify the socket's connectedness.