Register inserts during shutdown instead of dropping backed up graphs

This will allow us to not interrupt existing work flows but actually wait for a good time to shutdown.
Also the catalog listener can be used for metrics in the future.

Author: FlorentinD

core/src/main/java/org/neo4j/gds/core/loading/GraphStoreCatalog.java

@@ -27,6 +27,8 @@
 import org.neo4j.gds.config.GraphProjectConfig;
 import org.neo4j.gds.utils.StringJoining;
 
+import java.util.ArrayList;
+import java.util.List;
 import java.util.Map;
 import java.util.NoSuchElementException;
 import java.util.Optional;
@@ -41,8 +43,18 @@ public final class GraphStoreCatalog {
 
     private static final ConcurrentHashMap<String, UserCatalog> userCatalogs = new ConcurrentHashMap<>();
 
+    private static final List<GraphStoreCatalogListener> listeners = new ArrayList<>();
+
     private GraphStoreCatalog() { }
 
+    public static void registerListener(GraphStoreCatalogListener listener) {
+        listeners.add(listener);
+    }
+
+    public static void unregisterListener(GraphStoreCatalogListener listener) {
+        listeners.remove(listener);
+    }
+
     public static GraphStoreWithConfig get(CatalogRequest request, String graphName) {
         var userCatalogKey = UserCatalog.UserCatalogKey.of(request.databaseName(), graphName);
         var ownCatalog = getUserCatalog(request.username());
@@ -161,6 +173,8 @@ private static void set(GraphProjectConfig config, GraphStore graphStore, boolea
             );
             return userCatalog;
         });
+
+        listeners.forEach(listener -> listener.onProject(config.username(), config.graphName()));
     }
 
     public static boolean exists(String username, String databaseName, String graphName) {

core/src/main/java/org/neo4j/gds/core/loading/GraphStoreCatalogListener.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) "Neo4j"
+ * Neo4j Sweden AB [http://neo4j.com]
+ *
+ * This file is part of Neo4j.
+ *
+ * Neo4j is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package org.neo4j.gds.core.loading;
+
+public interface GraphStoreCatalogListener {
+
+    void onProject(String user, String graphName);
+}

core/src/test/java/org/neo4j/gds/core/loading/GraphStoreCatalogTest.java

@@ -19,6 +19,7 @@
  */
 package org.neo4j.gds.core.loading;
 
+import org.apache.commons.lang3.mutable.MutableInt;
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
 import org.neo4j.gds.api.DatabaseId;
@@ -414,6 +415,28 @@ void multipleDatabaseIds() {
         assertFalse(GraphStoreCatalog.exists(USER_NAME, databaseId1, "graph0"));
     }
 
+    @Test
+    void callListeners() {
+        var listenerCalls = new MutableInt(0);
+
+        var listener = new GraphStoreCatalogListener() {
+            @Override
+            public void onProject(String user, String graphName) {
+                listenerCalls.increment();
+            }
+        };
+
+        GraphStoreCatalog.registerListener(listener);
+        assertThat(listenerCalls.intValue()).isZero();
+
+        GraphStoreCatalog.set(CONFIG, graphStore);
+        assertThat(listenerCalls.intValue()).isEqualTo(1);
+
+        GraphStoreCatalog.unregisterListener(listener);
+        GraphStoreCatalog.set(GraphProjectFromStoreConfig.emptyWithName("bob", GRAPH_NAME), graphStore);
+        assertThat(listenerCalls.intValue()).isEqualTo(1);
+    }
+
     @Test
     void shouldThrowOnMissingGraph() {
         var dummyDatabaseId = DatabaseId.from("mydatabase");