While investigating why nonblocking does not seem to work exactly right for unix sockets, I found that the flags seem to be getting garbled by the existing setBlocking logic.

For logic that sets it to nonblock, the flags start as 2, then with O_NONBLOCK they become 6 and are set back into fcntl. By the time it tries to set it blocking again, the flags are nonsense. Once I saw 194, and another time 491294 or something like that.

By contrast a version using the fcntl bound by jnr-posix seems to properly set it to 6 and back to 2.