fix: during upgrades, a storage server which does not have data for a shard could be in the source servers, so as a fallback if a fetch keys fails long enough disable locality based load balancing to allow the storage server to peek from someone different than itself

fdbserver/storageserver.actor.cpp

@@ -1908,6 +1908,11 @@ ACTOR Future<Void> fetchKeys( StorageServer *data, AddingShard* shard ) {
 					// Throw away deferred updates from before fetchVersion, since we don't need them to use blocks fetched at that version
 					while (!shard->updates.empty() && shard->updates[0].version <= fetchVersion) shard->updates.pop_front();
 
+					//FIXME: remove when we no longer support upgrades from 5.X
+					if(debug_getRangeRetries >= 100) {
+						data->cx->enableLocalityLoadBalance = false;
+					}
+
 					debug_getRangeRetries++;
 					if (debug_nextRetryToLog==debug_getRangeRetries){
 						debug_nextRetryToLog += std::min(debug_nextRetryToLog, 1024);
@@ -1922,6 +1927,9 @@ ACTOR Future<Void> fetchKeys( StorageServer *data, AddingShard* shard ) {
 			}
 		}
 
+		//FIXME: remove when we no longer support upgrades from 5.X
+		data->cx->enableLocalityLoadBalance = true;
+
 		// We have completed the fetch and write of the data, now we wait for MVCC window to pass.
 		//  As we have finished this work, we will allow more work to start...
 		shard->fetchComplete.send(Void());