Similar to JGRP-2904: investigate whether there is a case where a sender can block.
E.g.:

• A sends to B
• B crashes and is excluded from the view: the entry for B in A is marked as closed
• A sends a spurious message (e.g. failure detection) to B and re-opens (or creates a new) entry to B
• This entry will never receive acks from B, so over time it might block...