DatagramChannel send/receive can be used to send and receive zero-length datagrams when the channel's socket is not connected. When connected there are several issues:

1. The read methods return -1 rather than 0
2. The send method and the write methods do nothing

The API docs need to be updated to document the support for zero-length datagrams, including the API hazards that arise when the DatagramChannel is configured non-blocking (a return value of 0 is ambiguous).

The updates to the API docs will include the changes to the read return value, track previously as JDK-8234756, so we can close that issue. In addition, the implementation changes address a residual issue with closing a channel around the time that a zero-length datagram is received so we can probably close JDK-8233198 too (JDK-8233198 was originally observed with the receive method but the implementation and should be limited to the read methods since JDK 14).