File Copy Based Initial Sync requires the majority point of the set to advance in order to start an extension round. This means if the set is unhealthy the sync will not succeed. We should fix this, either by

1) If extension fails before we've copied anything, logging an error and continuing with the sync. This may leave the node more lagged than desired, but it will catch up with normal replication (and perhaps make the set healthy again, if this was a re-sync)

and/or

2) Remove the requirement of majority point advance when extended the backup cursor. To get a valid extension, it is only necessary that the no-holes point be advanced beyond the extension time, not the majority. However, doing this may affect backup which may be expecting extensions to be majority committed, so we should check this; if that is the case we will need to parameterize .