8231259: (dc) DatagramChannel::disconnect re-binds socket to the wildcard address (macOS)

[Alan Bateman]

This issue is a corner case on macOS where a DatagramChannel is 
initially bound to a specific address, connected, and disconnect is 
called to dissolve the association. This scenario is problematic on BSD 
and macOS systems because disconnect re-binds the socket to the wildcard 
address. I brought up this issue on the bsd-port-dev a few months ago 
[1] where Kurt Miller replied to say that he has a patch for OpenBSD to 
fix this. Daniel Fuchs submitted a bug to Apple, tracked as FB7330899, 
on the macOS issue, but no news yet on if/when this will be fixed.

In the mean-time, we are continuing the effort to fix the various issues 
needed to replace the legacy implementation of DatagramSocket and 
MulticastSocket. I mentioned this in a recent mail [2] as it's the 
motivation for many of the recent changes/fixes to the DatagramChannel 
implementation and its adaptor. Those on net-dev will see related 
changes from Patrick Concannon and Daniel to DatagramSocket and 
MulticastSocket so that it will be ready for a new implementation. The 
overall effort runs into the macOS bug. The reason that it is not 
observed with legacy DatagramSocket is because it works around it by 
emulating connected sockets on macOS, an approach that doesn't make 
sense for DatagramChannel (as it is a selectable change). For 
DatagramChannel we considering a temporary workaround re-create the 
underlying socket on buggy kernels. "Re-create" means creating a new 
socket, using dup2 to re-plumb the original to the new socket, and then 
restoring the local address, multicast membership, selector 
registrations and socket options. Any of these steps can fail but this 
should be okay as the javadoc makes it clear that disconnect failing can 
leave the channel's socket in an unspecified state.

The webrev with the proposed change is here. The test that Daniel added 
as part of JDK-8231260 has been re-purposed to be a much broader test.

   http://cr.openjdk.java.net/~alanb/8231259/webrev/

-Alan

[1] 
https://mail.openjdk.java.net/pipermail/bsd-port-dev/2019-September/003215.html
[2] 
https://mail.openjdk.java.net/pipermail/nio-dev/2019-November/006775.html