fix deadlock between OnExitDatabaseCloser.DATABASES and Engine.DATABASES

h2/src/main/org/h2/engine/OnExitDatabaseCloser.java

@@ -5,6 +5,7 @@
  */
 package org.h2.engine;
 
+import java.util.ArrayList;
 import java.util.WeakHashMap;
 
 import org.h2.message.Trace;
@@ -65,10 +66,10 @@ class OnExitDatabaseCloser extends Thread {
         }
     }
 
-    private static synchronized void onShutdown() {
+    private static void onShutdown() {
         terminated = true;
         RuntimeException root = null;
-        for (Database database : DATABASES.keySet()) {
+        for (Database database : collectDatabasesToClose()) {
             try {
                 database.close(true);
             } catch (RuntimeException e) {
@@ -94,6 +95,10 @@ class OnExitDatabaseCloser extends Thread {
         }
     }
 
+    private static synchronized Iterable<Database> collectDatabasesToClose() {
+        return new ArrayList<>(DATABASES.keySet());
+    }
+
     private OnExitDatabaseCloser() {
     }