Since d65f2c932 ("Bump Error Prone version to 2.3.4 and fix new bug
patterns") the channel selected callback is no longer a final field of
the connection instance, hence it may be come null even if the
connection instance is still strongly referenced. Also the
ConnectionAttemptState class uses simply a lambda as callback, which
is also not strongly referenced otherwise.
The "channel selected" callback was wrapped in weak reference, so that
connection instances could get gc'ed if they are still connected but
the user lost all references to them. In this case, the weak reference
to the connection instance would become 'null' and
selectionKey.cancel() would be called.
This change means that a socket and its selection key of a "leaked"
connected connection instance continues to be part of the reactor. But
this may not be that bad: first, users are expected to manager their
connection instances, and disconnect them before they are
discarded. And secondly, at some point the connection likely will get
disconnected, and in this case, the socket and its selection key will
be removed from the reactor.
b1a4ccfae8