After the database was closed, a null pointer exception could occur in Database.flush.

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

@@ -1149,10 +1149,12 @@ public class Database implements DataHandler {
 
     private void closeFiles() {
         try {
+            synchronized (this) {
                 if (pageStore != null) {
                     pageStore.close();
                     pageStore = null;
                 }
+            }
         } catch (DbException e) {
             traceSystem.getTrace(Trace.DATABASE).error("close", e);
         }
@@ -1674,10 +1676,10 @@ public class Database implements DataHandler {
      * Flush all pending changes to the transaction log.
      */
     public void flush() {
+        synchronized (this) {
             if (readOnly || pageStore == null) {
                 return;
             }
-        synchronized (this) {
             pageStore.flushLog();
         }
     }
@@ -1751,10 +1753,10 @@ public class Database implements DataHandler {
      * executing the SQL statement CHECKPOINT SYNC.
      */
     public void sync() {
+        synchronized (this) {
             if (readOnly || pageStore == null) {
                 return;
             }
-        synchronized (this) {
             pageStore.sync();
         }
     }
@@ -2125,10 +2127,12 @@ public class Database implements DataHandler {
      */
     public void checkpoint() {
         if (persistent) {
+            synchronized (this) {
                 if (pageStore != null) {
                     pageStore.checkpoint();
                 }
             }
+        }
         getTempFileDeleter().deleteUnused();
     }