The fingerprint check added to PeerSync in
SOLR-9446 works fine when all servers are running 6.3 but this means it's hard to do a rolling upgrade from e.g. 6.2.1 to 6.3 because the 6.3 server sends a request to a 6.2.1 server to get a fingerprint and then gets a NPE because the older server doesn't return the expected field in its response.
This leads to the PeerSync completely failing, and results in a full index replication from scratch, copying all index files over the network. We noticed this happening when we tried to do a rolling upgrade on one of our 6.2.1 clusters to 6.3. Unfortunately this amount of replication was hammering our disks and network, so we had to do a full shutdown, upgrade all to 6.3 and restart, which was not ideal for a production cluster.
The attached patch should behave more gracefully in this situation, as it will typically return false for alreadyInSync() and then carry on doing the normal re-sync based on versions.