An absent needle is normally indistinguishable from a vacuumed delete,
so check.disk skips it and replicas diverged by replication failures
cannot be reunited. But a replica with compaction revision 0 has never
been vacuumed: every delete it processed still holds its tombstone, so
a needle absent there is provably a missing write.
The new flag resurrects absent needles in exactly that case. The
receiving replica's live compaction revision is read after the index
snapshot and must be 0; vacuumed replicas keep the skip, now with the
revision in the message. The decision is passed per direction since
pass 2 runs pairs concurrently. Resurrected needles count toward
-nonRepairThreshold as before.
Chris Lu
013281b498