What happens during the reload case for nginx? At reload time, the previous-generation worker that bound to the socket with SO_REUSEPORT will need to close its listener, just like haproxy.
When reloading, nginx starts up a new set of workers and duplicates the sockets across to them with SCM_RIGHTS. None of the sockets is closed - only duplicate file descriptors.
In nginx worker processes don't open and don't close sockets. They inherit them from the master process.
But isn't that what reuseport changes? Each worker opening it's own socket?
No, they don't (and they can't, since they usually don't have such privileges). The master process opens all sockets for each worker process.