Fix merged settings, check linked project alias

Author: JeremyDahlgren

server/src/main/java/org/elasticsearch/transport/LinkedProjectConfig.java

@@ -14,10 +14,12 @@
 import org.elasticsearch.core.TimeValue;
 
 import java.util.List;
+import java.util.Objects;
 import java.util.function.Predicate;
 
 import static org.elasticsearch.transport.RemoteConnectionStrategy.ConnectionStrategy;
 
+// FIXME: Refactor into a class heirarchy with subclasses for each connection strategy
 public record LinkedProjectConfig(
     ProjectId originProjectId,
     ProjectId linkedProjectId,
@@ -72,6 +74,9 @@ public Builder linkedProjectId(ProjectId linkedProjectId) {
         }
 
         public Builder linkedProjectAlias(String linkedProjectAlias) {
+            if (linkedProjectAlias == null || linkedProjectAlias.isBlank()) {
+                throw new IllegalArgumentException("linkedProjectAlias cannot be null or empty");
+            }
             this.linkedProjectAlias = linkedProjectAlias;
             return this;
         }

server/src/main/java/org/elasticsearch/transport/RemoteClusterService.java

@@ -431,7 +431,10 @@ private synchronized void updateRemoteCluster(
         boolean forceRebuild,
         ActionListener<RemoteClusterConnectionStatus> listener
     ) {
-        final var mergedSettings = Settings.builder().put(settings).put(newSettings).build();
+        if (LOCAL_CLUSTER_GROUP_KEY.equals(clusterAlias)) {
+            throw new IllegalArgumentException("remote clusters must not have the empty string as its key");
+        }
+        final var mergedSettings = Settings.builder().put(settings, false).put(newSettings, false).build();
         final var linkedProjectConfig = RemoteClusterSettings.toConfigBuilder(clusterAlias, mergedSettings)
             .originProjectId(projectId)
             .linkedProjectAlias(clusterAlias)
@@ -446,10 +449,6 @@ private synchronized void updateRemoteCluster(
     ) {
         final var projectId = config.originProjectId();
         final var clusterAlias = config.linkedProjectAlias();
-        if (LOCAL_CLUSTER_GROUP_KEY.equals(clusterAlias)) {
-            throw new IllegalArgumentException("remote clusters must not have the empty string as its key");
-        }
-
         final var connectionMap = getConnectionsMapForProject(projectId);
         RemoteClusterConnection remote = connectionMap.get(clusterAlias);
         if (RemoteConnectionStrategy.isConnectionEnabled(config) == false) {

server/src/test/java/org/elasticsearch/transport/LinkedProjectConfigTests.java

@@ -57,4 +57,9 @@ public void testBuilder() {
 
         assertThat("expect builder generated config to match the original config", builtConfig, equalTo(config));
     }
+
+    public void testLinkedProjectAliasValidation() {
+        assertThrows(IllegalArgumentException.class, () -> LinkedProjectConfig.builder().linkedProjectAlias(null).build());
+        assertThrows(IllegalArgumentException.class, () -> LinkedProjectConfig.builder().linkedProjectAlias("").build());
+    }
 }