On UNIX, the initial socket connection can be synchronous (that is, no trial connection any more, but it is guaranteed to make successful connection at the first "connect").
See the patch D510: Connection to gpg-agent/dirmngr/etc., which was applied to master. For Windows, it keeps old code.
By making it synchronous, we can remove a race condition. It matters only in heavy loads, but it occurs, like the case of Debian buildd.