fix(dre): [DRE-241] Prefer existing nodes in subnet when selecting be…

…st results (#734)

Co-authored-by: sa-github-api <[email protected]>

rs/decentralization/src/network.rs

@@ -3,6 +3,7 @@ use crate::subnets::unhealthy_with_nodes;
 use crate::SubnetChangeResponse;
 use actix_web::http::StatusCode;
 use actix_web::{HttpResponse, ResponseError};
+use ahash::AHashSet;
 use anyhow::anyhow;
 use ic_base_types::PrincipalId;
 use ic_management_types::{MinNakamotoCoefficients, NetworkError, NodeFeature, Status};
@@ -403,6 +404,16 @@ impl DecentralizedSubnet {
         if best_results.is_empty() {
             None
         } else {
+            // If any of the best_results nodes are already in the subnet,
+            // we should prefer them. This is because we want to keep the
+            // same nodes in the subnet if they are already there.
+            let current_nodes_set: AHashSet<_> = current_nodes.iter().collect();
+            for result in best_results {
+                if current_nodes_set.contains(&result.node) {
+                    return Some(result.clone());
+                }
+            }
+
             // We sort the current nodes by alphabetical order on their
             // PrincipalIDs to ensure consistency of the seed with the
             // same machines in the subnet