In #4858 I implemented close_on_exec logic, but the original script continued to fail because our TCPServer.for_fd (and all other JDK-based sockets) is not fully functional. Specifically, it can't reconstitute a JDK wrapper object around a real fileno, so the child fails to produce a working TCPServer socket.

IO.for_fd produces a working IO, but none of the socket methods are defined on it.

This will not be possible to fix until all socket IO is done via native streams. This would be (I believe) the last major stream type to move to fully-native IO.